Content from Getting Started

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How do I use the Spyder IDE?
  • How can I run Python programs?

Objectives

  • Learners can launch the Spyder IDE
  • Learners are able to use the IPython console to interact with Python
  • Learners are able to write code in the Spyder editor and run this code
  • Learners are able to save their code in a *.py file
  • Learners can use the different buttons and panels needed in the Spyder IDE

Use the Spyder IDE for editing and running Python.

  • The Anaconda package manager is an automated way to install the Spyder IDE.

  • It also installs all the extra libraries it needs to run.

  • Once you have installed Python and the Spyder IDE requirements, open a shell and type:

    PYTHON 

    $ spyder

  • This will start The Spyder IDE.

  • This environment has several useful tools we can use, which you can see in different panels in the Spyder IDE. We will look into some of them.

  • You can change the positions and sizes of these panels to your preference, as you get to know them.

Different ways of interacting with Python using Spyder

  • On the left, filling half of the screen is the editor. Here you can write and edit code, which can then be saved in a file (usually with a .py extension). We can run the code we wrote here by pressing the green ‘play’ button on top or press F5 on your keyboard.
  • On the bottom right, we find the IPython console. This is were we can talk directly to Python. It will interpret what you have typed directly when you press Enter.

Python in the console

Challenge

Go to the IPython tab at the bottom right. What happens when you type a small calculation there? For example, what happens when you type the following calculation and press enter?

PYTHON 

7 * 3

Python returns the result of the calculation.

PYTHON 

21

Python in the editor

The large panel on the left probably has some text in it that looks like this:

"""
Spyder Editor

This is a temporary script file.
"""

Write the following line below these lines and press run (the green ‘play’ button or f5). A window might pop up asking you to specify the run settings, leave the settings as they are and press ‘Run’. What happens?

PYTHON 

print('Hello world!')

In the IPython console Python tells you which files it ran and the result of this run

PYTHON 

runfile('/home/ylja/.config/spyder-py3/temp.py', wdir='/home/ylja/.config/spyder-py3')
Hello world!
  • Code written in the editor can be saved, like any other file.

Saving the code

To save the code, press ‘file’ and then ‘save as’. Now give the file a name, for example ‘mycode.py’ and save it in a directory/folder where you know how to find it. Look into your file system the way you usually do it. Is the file where you expect it to be?

Key Points

  • Use the Spyder IDE for editing and running Python
  • The IPython console can be used to interact with Python directly
  • The editor can be used to write code, run and save it

Content from Variables and Assignment

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I store data in programs?

Objectives

  • Write programs that assign values to variables and perform calculations with those values.
  • Correctly trace value changes in programs that use assignment.

Use variables to store values.

  • Variables are names for values.
  • In Python the = symbol assigns the value on the right to the name on the left.
  • The variable is created when a value is assigned to it.
  • Here, Python assigns an age to a variable age and a name in quotation marks to a variable first_name.

PYTHON 

age = 42
first_name = 'Ahmed'
  • Variable names:
    • cannot start with a digit
    • cannot contain spaces, quotation marks, or other punctuation
    • may contain an underscore (typically used to separate words in long variable names)
  • Underscores at the start like __alistairs_real_age have a special meaning so we won’t do that until we understand the convention.

Use print to display values.

  • Python has a built-in function called print that prints things as text.
  • Call the function (i.e., tell Python to run it) by using its name.
  • Provide values to the function (i.e., the things to print) in parentheses.
  • To add a string to the printout, wrap the string in single quotations.
  • The values passed to the function are called ‘arguments’

PYTHON 

print(first_name, 'is', age, 'years old')

OUTPUT 

Ahmed is 42 years old
  • print automatically puts a single space between items to separate them.
  • And wraps around to a new line at the end.

Variables must be created before they are used.

  • If a variable doesn’t exist yet, or if the name has been mis-spelled, Python reports an error.
    • Unlike some languages, which “guess” a default value.

PYTHON 

print(eye_color)

ERROR 

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-1-c1fbb4e96102> in <module>()
----> 1 print(eye_color)

NameError: name 'eye_color' is not defined
  • The last line of an error message is usually the most informative.
  • We will look at error messages in detail later.

Variables Persist Between Cells

Variables defined in one cell exist in all other cells once executed, so the relative location of cells in the notebook do not matter (i.e., cells lower down can still affect those above). Remember: Notebook cells are just a way to organize a program: as far as Python is concerned, all of the source code is one long set of instructions.

Variables can be used in calculations.

  • We can use variables in calculations just as if they were values.
    • Remember, we assigned 42 to age a few lines ago.

PYTHON 

age = age + 3
print('Age in three years:', age)

OUTPUT 

Age in three years: 45

Use an index to get a single character from a string.

  • The characters (individual letters, numbers, and so on) in a string are ordered. For example, the string ‘AB’ is not the same as ‘BA’. Because of this ordering, we can treat the string as a list of characters.
  • Each position in the string (first, second, etc.) is given a number. This number is called an index or sometimes a subscript.
  • Indices are numbered from 0 rather than 1.
  • Use the position’s index in square brackets to get the character at that position.

PYTHON 

element = 'helium'
print(element[0])

OUTPUT 

h

Use a slice to get a substring.

  • A part of a string is called a substring. A substring can be as short as a single character.
  • An item in a list is called an element. Whenever we treat a string as if it were a list, the string’s elements are its individual characters.
  • A slice is a part of a string (or, more generally, any list-like thing).
  • We take a slice by using [start:stop], where start is replaced with the index of the first element we want and stop is replaced with the index of the element just after the last element we want.
  •   Mathematically, you might say that a slice selects [start:stop].
  • The difference between stop and start is the slice’s length.
  • Taking a slice does not change the contents of the original string. Instead, the slice is a copy of part of the original string.

PYTHON 

element = 'sodium'
print(element[0:3])

OUTPUT 

sod

Use the built-in function len to find the length of a string.

PYTHON 

print(len('helium'))

OUTPUT 

6
  • Nested functions are evaluated from the inside out, just like in mathematics.

Python is case-sensitive.

  • Python thinks that upper- and lower-case letters are different, so Name and name are different variables.
  • There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.

Use meaningful variable names.

  • Python doesn’t care what you call variables as long as they obey the rules (alphanumeric characters and the underscore).

PYTHON 

flabadab = 42
ewr_422_yY = 'Ahmed'
print(ewr_422_yY, 'is', flabadab, 'years old')
  • Use meaningful variable names to help other people understand what the program does.
  • The most important “other person” is your future self.

Swapping Values

Draw a table showing the values of the variables in this program after each statement is executed. In simple terms, what do the last three lines of this program do?

PYTHON 

x = 1.0
y = 3.0
swap = x
x = y
y = swap
swap = x  #  x->1.0 y->3.0 swap->1.0
x = y     #  x->3.0 y->3.0 swap->1.0
y = swap  #  x->3.0 y->1.0 swap->1.0

These three lines exchange the values in x and y using the swap variable for temporary storage. This is a fairly common programming idiom.

Predicting Values

What is the final value of position in the program below? (Try to predict the value without running the program, then check your prediction.)

PYTHON 

initial = "left"
position = initial
initial = "right"

PYTHON 

initial = "left"  # Initial is assigned the string "left"
position = initial  # Position is assigned the variable initial, currently "left"
initial = "right"  # Initial is assigned the string "right"
print(position)

OUTPUT 

left

The last assignment to position was “left”

Can you slice integers?

If you assign a = 123, what happens if you try to get the second digit of a?

Numbers are not stored in the written representation, so they can’t be treated like strings.

PYTHON 

a = 123
print(a[1])

ERROR 

TypeError: 'int' object is not subscriptable

Choosing a Name

Which is a better variable name, m, min, or minutes? Why? Hint: think about which code you would rather inherit from someone who is leaving the library:

  1. ts = m * 60 + s
  2. tot_sec = min * 60 + sec
  3. total_seconds = minutes * 60 + seconds

minutes is better because min might mean something like “minimum” (and actually does in Python, but we haven’t seen that yet).

Slicing

What does the following program print?

PYTHON 

library_name = 'social sciences'
print('library_name[1:3] is:', library_name[1:3])
  1. What does thing[low:high] do?
  2. What does thing[low:] (without a value after the colon) do?
  3. What does thing[:high] (without a value before the colon) do?
  4. What does thing[:] (just a colon) do?
  5. What does thing[number:negative-number] do?

OUTPUT 

library_name[1:3] is: oc
  1. It will slice the string, starting at the low index and ending an element before the high index
  2. It will slice the string, starting at the low index and stopping at the end of the string
  3. It will slice the string, starting at the beginning on the string, and ending an element before the high index
  4. It will print the entire string
  5. It will slice the string, starting the number index, and ending a distance of the absolute value of negative-number elements from the end of the string

Key Points

  • Use variables to store values.
  • Use print to display values.
  • Variables persist between cells.
  • Variables must be created before they are used.
  • Variables can be used in calculations.
  • Use an index to get a single character from a string.
  • Use a slice to get a substring.
  • Use the built-in function len to find the length of a string.
  • Python is case-sensitive.
  • Use meaningful variable names.

Content from Data Types and Type Conversion

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • What kinds of data do programs store?
  • How can I convert one type to another?

Objectives

  • Explain key differences between integers and floating point numbers.
  • Explain key differences between numbers and character strings.
  • Use built-in functions to convert between integers, floating point numbers, and strings.

Every value has a type.

  • Every value in a program has a specific type.
  • Integer (int): whole numbers like 3 or -512.
  • Floating point number (float): fractional numbers like 3.14159 or -2.5.
  • Whole numbers may also be stored as floats, e.g. 1.0, but 1.0 would still be stored as a float.
  • Character string (usually called “string”, str): text.
    • Written in either single quotes or double quotes (as long as they match).
    • The quotation marks aren’t printed using print(), but may appear when viewing a value in the Jupyter Notebook or other Python interpreter.

Use the built-in function type to find the type of a value.

  • Use the built-in function type to find out what type a value has.
  • This works on variables as well.
    • But remember: the value has the type — the variable is just a label.
    • When you change the value of a variable to a new data type, the results of print(type(your_variable)) will change accordingly.

PYTHON 

print(type(52))

OUTPUT 

<class 'int'>

PYTHON 

title = 'Biochemistry'
print(type(title))

OUTPUT 

<class 'str'>

Types control what operations (or methods) can be performed on a given value.

  • A value’s type determines what the program can do to it.

PYTHON 

print(5 - 3)

OUTPUT 

2

PYTHON 

print('hello' - 'h')

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-67f5626a1e07> in <module>()
----> 1 print('hello' - 'h')

TypeError: unsupported operand type(s) for -: 'str' and 'str'

You can use the + and * operators on strings.

  • “Adding” character strings concatenates them.

PYTHON 

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)

OUTPUT 

Ahmed Walsh
  • Multiplying a character string by an integer N creates a new string that consists of that character string repeated N times.
    • Since multiplication is repeated addition.
  • There are more ways that traditional math operators will work on other data types. There isn’t a perfect formula for figuring out what they do, so experimentation is valuable.

PYTHON 

separator = '=' * 10
print(separator)

OUTPUT 

==========

Strings have a length (but numbers don’t).

  • The built-in function len counts the number of characters in a string.

PYTHON 

print(len(full_name))

OUTPUT 

11
  • But numbers don’t have a length (not even zero).

PYTHON 

print(len(52))

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-f769e8e8097d> in <module>()
----> 1 print(len(52))

TypeError: object of type 'int' has no len()

Must convert numbers to strings or vice versa when operating on them.

  • Cannot add numbers and strings.

PYTHON 

print(1 + 'A')

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-fe4f54a023c6> in <module>()
----> 1 print(1 + '2')

TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • Not allowed because it’s ambiguous: should 1 + '2' be 3 or '12'?
  • Some types can be converted to other types by using the type name as a function.

PYTHON 

print(1 + int('2'))
print(str(1) + '2')

OUTPUT 

3
12

Can mix integers and floats freely in operations.

  • Integers and floating-point numbers can be mixed in arithmetic.
    • Python automatically converts integers to floats as needed.

PYTHON 

print('half is', 1 / 2.0)
print('three squared is', 3.0 ** 2)

OUTPUT 

half is 0.5
three squared is 9.0

Variables only change value when something is assigned to them.

  • If we make one cell in a spreadsheet depend on another, and update the latter, the former updates automatically.
  • This does not happen in programming languages.

PYTHON 

first = 1
second = 5 * first
first = 2
print('first is', first, 'and second is', second)

OUTPUT 

first is 2 and second is 5
  • The computer reads the value of first when doing the multiplication, creates a new value, and assigns it to second.
  • After that, second does not remember where it came from.

Fractions

What type of value is 3.4? How can you find out?

It is a floating-point number (often abbreviated “float”).

PYTHON 

print(type(3.4))

OUTPUT 

<class 'float'>

Automatic Type Conversion

What type of value is 3.25 + 4?

It is a float: integers are automatically converted to floats as necessary.

PYTHON 

result = 3.25 + 4
print(result, 'is', type(result))

OUTPUT 

7.25 is <class 'float'>

Choose a Type

What type of value (integer, floating point number, or character string) would you use to represent each of the following? Try to come up with more than one good answer for each problem. For example, in # 1, when would counting days with a floating point variable make more sense than using an integer?

  1. Number of days since the start of the year.
  2. Time elapsed since the start of the year.
  3. Call number of a book.
  4. Standard book loan period.
  5. Number of reference queries in a year.
  6. Average library classes taught per semester.
  1. Integer
  2. Float
  3. String
  4. Integer
  5. Integer
  6. Float

Division Types

There are three different types of division:

  1. ‘Normal’ division (aka floating-point division) is what most people may be familiar with: 5 / 2 = 2.5
  2. Floor division, which cuts out the part after the period: 5 / 2 = 2
  3. Modulo division, which only keeps the remained after division: 5 / 2 = 1

In Python 3, the / operator performs floating-point division, the // operator performs floor division, and the ‘%’ (or modulo) operator calculates the modulo division:

PYTHON 

print('5 / 3:', 5/3)
print('5 // 3:', 5//3)
print('5 % 3:', 5%3)

OUTPUT 

5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

If num_students is the number of students enrolled in a course (let say 600), and num_per_class is the number that can attend a single class (let say 42), write an expression that calculates the number of classes needed to teach everyone.

Depending on requirements it might be important to detect when the number of students per class doesn’t divide the number of students evenly. Detect it with the % operator and test if the remainder that it returns is greater than 0.

PYTHON 

num_students = 600
num_per_class = 42
num_classes = num_students // num_per_class
remainder = num_students % num_per_class

print(num_students, 'students,', num_per_class, 'per class')
print(num_classes, ' full classes, plus an extra class with only ', remainder, 'students')

OUTPUT 

600 students, 42 per class
14  full classes, plus an extra class with only  12 students

Strings to Numbers

Where reasonable, float() will convert a string to a floating point number, and int() will convert a floating point number to an integer:

PYTHON 

print("string to float:", float("3.4"))
print("float to int:", int(3.4))

OUTPUT 

string to float: 3.4
float to int: 3

Note: conversion is some times also called typecast.

If the conversion doesn’t make sense, however, an error message will occur

PYTHON 

print("string to float:", float("Hello world!"))

ERROR 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-df3b790bf0a2> in <module>()
----> 1 print("string to float:", float("Hello world!"))

ValueError: could not convert string to float: 'Hello world!'

Given this information, what do you expect the following program to do?

What does it actually do?

Why do you think it does that?

PYTHON 

print("fractional string to int:", int("3.4"))

What do you expect this program to do? It would not be so unreasonable to expect the Python int command to convert the string “3.4” to 3.4 and an additional type conversion to 3. After all, Python performs a lot of other magic - isn’t that part of its charm?

However, Python throws an error. Why? To be consistent, possibly. If you ask Python to perform two consecutive typecasts, you must convert it explicitly in code.

PYTHON 

num_as_string = "3.4"
num_as_float = float(num_as_string)
num_as_int = int(num_as_float)
print(num_as_int)

OUTPUT 

3

We could also write it in a single line like this: int(float("3.4"))

Arithmetic with Different Types

Which of the following will print 2.0? Note: there may be more than one right answer.

PYTHON 

first = 1.0
second = "1"
third = "1.1"
  1. first + float(second)
  2. float(second) + float(third)
  3. first + int(third)
  4. first + int(float(third))
  5. int(first) + int(float(third))
  6. 2.0 * second

Answer: 1 and 4.

  1. is correct
  2. gives 2.1
  3. gives an error because we cannot convert text to int directly
  4. is correct
  5. gives 2 (as an integer not as a float)
  6. gives an error because second is a string.

Key Points

  • Every value has a type.
  • Use the built-in function type to find the type of a value.
  • Types control what operations can be done on values.
  • Strings can be added and multiplied.
  • Strings have a length (but numbers don’t).
  • Must convert numbers to strings or vice versa when operating on them.
  • Can mix integers and floats freely in operations.
  • Variables only change value when something is assigned to them.

Content from Built-in Functions and Help

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I use built-in functions?
  • How can I find out what they do?
  • What kind of errors can occur in programs?

Objectives

  • Explain the purpose of functions.
  • Correctly call built-in Python functions.
  • Correctly nest calls to built-in functions.
  • Use help to display documentation for built-in functions.
  • Correctly describe situations in which SyntaxError and NameError occur.

Use comments to add documentation to programs.

PYTHON 

# This sentence isn't executed by Python.
name = 'Library Carpentry'   # Neither is this comment - anything after '#' is ignored.

A function may take zero or more arguments.

  • We have seen some functions already — now let’s take a closer look.
  • An argument is a value passed into a function.
  • Any arguments you want to pass into a function must go into the ()
    • print("I am an argument and must go here.")
  • You must always use parentheses, because this is how Python knows you are calling a function.
    • You leave them empty if you don’t want or need to pass any arguments in.
  • len takes exactly one.
  • int, str, and float create a new value from an existing one.
  • print takes zero or more.
    • print() prints a blank line.

PYTHON 

print('before')
print()
print('after')

OUTPUT 

before

after

Commonly-used built-in functions include max, min, and round.

  • Use max to find the largest value of one or more values.
  • Use min to find the smallest.
  • Both work on character strings as well as numbers.
    • “Larger” and “smaller” use (0-9, A-Z, a-z) to compare letters.
    • This means that:
      • 'a' is smaller than 'b'
      • 'A' is smaller than 'a'
      • '0' is smaller than 'a'
    • This is useful for ordering alphabetically.

PYTHON 

print(max(1, 2, 3))
print(min('a', 'b', 'c'))
print(min('a', 'A'))

OUTPUT 

3
a
A

Functions may only work for certain (combinations of) arguments.

  • max and min must be given at least one argument.
  • And they must be given things that can meaningfully be compared.

PYTHON 

print(max(1, 'a'))

ERROR 

TypeError: unorderable types: str() > int()

Functions may have default values for some arguments.

  • round will round off a floating-point number.
  • By default, rounds to zero decimal places.

PYTHON 

round(3.712)

OUTPUT 

4
  • We can specify the number of decimal places we want.

PYTHON 

round(3.712, 1)

OUTPUT 

3.7

Use the built-in function help to get help for a function.

  • Every built-in function has online documentation.

PYTHON 

help(round)

OUTPUT 

Help on built-in function round in module builtins:

round(...)
    round(number[, ndigits]) -> number

    Round a number to a given precision in decimal digits (default 0 digits).
    This returns an int when called with one argument, otherwise the
    same type as the number. ndigits may be negative.

Python reports a syntax error when grammar rules (that’s Python grammar, not English grammar) have been violated.

  • You’ve seen errors when you try to use a function incorrectly.
    • Can also have errors when you use punctuation incorrectly.
  • Python will run the program up until that point, but if the grammar of that line of code has produced an error, then the program will shut down with an error.

PYTHON 

# Forgot to close the quotation marks around the string.
name = 'Feng

ERROR 

SyntaxError: EOL while scanning string literal

PYTHON 

# An extra '=' in the assignment.
age = = 52

ERROR 

SyntaxError: invalid syntax
  • Look more closely at the error message:

PYTHON 

print("hello world"

ERROR 

  File "<ipython-input-6-d1cc229bf815>", line 1
    print ("hello world"
                        ^
SyntaxError: unexpected EOF while parsing
  • The message indicates a problem on first line of the input (“line 1”).
    • In this case the “ipython-input” section of the file name tells us that we are working with input into IPython.
  • The -6- part of the filename indicates that the error occurred in cell 6 of our Notebook.
  • Next is the problematic line of code, indicating the problem with a ^ pointer.

Python reports a runtime error when something goes wrong while a program is executing.

PYTHON 

age = 53
remaining = 100 - aege # mis-spelled 'age'

ERROR 

NameError: name 'aege' is not defined
  • Fix syntax errors by reading the source and runtime errors by tracing execution.

Every function returns something.

  • Every function call produces some result.
  • If the function doesn’t have a useful result to return, it usually returns the special value None.

PYTHON 

result = print('example')
print('result of print is', result)

OUTPUT 

example
result of print is None

What Happens When

  1. Explain in simple terms the order of operations in the following program: when does the addition happen, when does the subtraction happen, when is each function called, etc.
  2. What is the final value of word?

PYTHON 

word = 'blah '
word = max(min(word * 2 + 'blur ', 'aaah '), 'ping')
print(word)
ping

Spot the Difference

  1. Predict what each of the print statements in the program below will print.
  2. Does max(len(rich), poor) run or produce an error message? If it runs, does its result make any sense?

PYTHON 

rich = "gold"
poor = "tin"
print(max(rich, poor))
print(max(len(rich), len(poor)))
tin
4
TypeError: '>' not supported between instances of 'str' and 'int'

Why Not?

Why don’t max and min return None when they are given no arguments?

Both functions require an argument to execute

PYTHON 

print(max())

ERROR 

TypeError: max expected 1 arguments, got 0

Last Character of a String

If Python starts counting from zero, and len returns the number of characters in a string, what index expression will get the last character in the string name? (Note: we will see a simpler way to do this in a later episode.)

name[len(name) - 1]

Key Points

  • Use comments to add documentation to programs.
  • A function may take zero or more arguments.
  • Commonly-used built-in functions include max, min, and round.
  • Functions may only work for certain (combinations of) arguments.
  • Functions may have default values for some arguments.
  • Use the built-in function help to get help for a function.
  • Every function returns something.
  • Python reports a syntax error when it can’t understand the source of a program.
  • Python reports a runtime error when something goes wrong while a program is executing.
  • Fix syntax errors by reading the source code, and runtime errors by tracing the program’s execution.

Content from Morning Coffee

Last updated on 2023-05-08 | Edit this page

Reflection exercise

Over coffee, reflect on and discuss the following:

  • What are the different kinds of errors Python will report?
  • Did the code always produce the results you expected? If not, why?
  • Is there something we can do to prevent errors when we write code?

Content from Libraries

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I extend the capabilities of Python?
  • How can I use software that other people have written?
  • How can I find out what that software does?

Objectives

  • Explain what software libraries are and why programmers create and use them.
  • Write programs that import and use libraries from Python’s standard library.
  • Find and read documentation for standard libraries interactively (in the interpreter) and online.

Most of the power of a programming language is in its (software) libraries.

  • A (software) library is a collection of files (called modules) that contains functions for use by other programs.
    • May also contain data values (e.g., numerical constants) and other things.
    • Library’s contents are supposed to be related, but there’s no way to enforce that.
  • The Python standard library is an extensive suite of modules that comes with Python itself.
  • Many additional libraries are available from PyPI (the Python Package Index).
  • We will see later how to write new libraries.

Libraries and modules

A library is a collection of modules, but the terms are often used interchangeably, especially since many libraries only consist of a single module, so don’t worry if you mix them.

A program must import a library module before using it.

  • Use import to load a library module into a program’s memory.
  • Then refer to things from the module as module_name.thing_name.
    • Python uses . to mean “part of”.
  • Using string, one of the modules in the standard library:

PYTHON 

import string

print('The lower ascii letters are', string.ascii_lowercase)
print(string.capwords('capitalise this sentence please.'))

OUTPUT 

The lower ascii letters are abcdefghijklmnopqrstuvwxyz
Capitalise This Sentence Please.
  • You have to refer to each item with the module’s name.
    • string.capwords(ascii_lowercase) won’t work: the reference to ascii_lowercase doesn’t somehow “inherit” the function’s reference to string.

Use help to learn about the contents of a library module.

  • Works just like help for a function.

PYTHON 

help(string)

OUTPUT 

Help on module string:

NAME
    string - A collection of string constants.

MODULE REFERENCE
    https://docs.python.org/3.6/library/string
    
    The following documentation is automatically generated from the Python
    source files.  It may be incomplete, incorrect or include features that
    are considered implementation detail and may vary between Python
    implementations.  When in doubt, consult the module reference at the
    location listed above.

DESCRIPTION
    Public module variables:
    
    whitespace -- a string containing all ASCII whitespace
    ascii_lowercase -- a string containing all ASCII lowercase letters
    ascii_uppercase -- a string containing all ASCII uppercase letters
    ascii_letters -- a string containing all ASCII letters
    digits -- a string containing all ASCII decimal digits
    hexdigits -- a string containing all ASCII hexadecimal digits
    octdigits -- a string containing all ASCII octal digits
    punctuation -- a string containing all ASCII punctuation characters
    printable -- a string containing all ASCII characters considered printable

CLASSES
    builtins.object
        Formatter
        Template
⋮ ⋮ ⋮

Import specific items from a library module to shorten programs.

  • Use from ... import ... to load only specific items from a library module.
  • Then refer to them directly without library name as prefix.

PYTHON 

from string import ascii_letters

print('The ASCII letters are', ascii_letters)

OUTPUT 

The ASCII letters are abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ

Create an alias for a library module when importing it to shorten programs.

  • Use import ... as ... to give a library a short alias while importing it.
  • Then refer to items in the library using that shortened name.

PYTHON 

import string as s

print(s.capwords('capitalise this sentence again please.'))

OUTPUT 

Capitalise This Sentence Again Please.
  • Commonly used for libraries that are frequently used or have long names.
    • E.g., The pandas library is often aliased as pd.
  • But can make programs harder to understand, since readers must learn your program’s aliases.

Exploring the os Library

The os library provides a way of accessing operating system functionality.

  1. What function from the os library can you use to determine the current working directory?
  1. Using help(os) we see that we’ve got os.getcwd() which returns a string representing the current working directory.

Locating the Right Module

Given the variables year, month and day, how would you generate a date in the standard iso format:

PYTHON 

year = 2016
month = 10
day = 22
  1. Which standard library module could help you?
  2. Which function would you select from that module?
  3. Try to write a program that uses the function.

The datetime module seems like it could help you.

You could use date(year, month, date).isoformat() to convert your date:

PYTHON 

import datetime

iso_date = datetime.date(year, month, day).isoformat()
print(iso_date)

or more compactly:

PYTHON 

import datetime

print(datetime.date(year, month, day).isoformat())

When Is Help Available?

When a colleague of yours types help(os), Python reports an error:

ERROR 

NameError: name 'os' is not defined

What has your colleague forgotten to do?

Importing the os module (import os)

Importing With Aliases

  1. Fill in the blanks so that the program below prints 0123456789.
  2. Rewrite the program so that it uses import without as.
  3. Which form do you find easier to read?

PYTHON 

import string as s
numbers = ____.digits
print(____)

PYTHON 

import string as s
numbers = s.digits
print(numbers)

can be written as

PYTHON 

import string
numbers = string.digits
print(numbers)

Since you just wrote the code and are familiar with it, you might actually find the first version easier to read. But when trying to read a huge piece of code written by someone else, or when getting back to your own huge piece of code after several months, non-abbreviated names are often easier, expect where there are clear abbreviation conventions.

There Are Many Ways To Import Libraries!

Match the following print statements with the appropriate library calls

Library calls:

PYTHON 

A) from string import digits
B) import string
C) import string as s

Print commands:

PYTHON 

1. print(list(s.digits))
2. print(list(digits))
3. print(string.ascii_uppercase)

A2) Importing digits from string provides the digits methods B3) Importing string provides methods such as ascii_uppercase, but requires the string. syntax. C1) Importing string with the alias s allows s.digits

Importing Specific Items

  1. Fill in the blanks so that the program below prints 90.0.
  2. Do you find this version easier to read than preceding ones?
  3. Why wouldn’t programmers always use this form of import?

PYTHON 

____ math import ____, ____
angle = degrees(pi / 2)
print(angle)

PYTHON 

from math import degrees, pi
angle = degrees(pi / 2)
print(angle)

Most likely you find this version easier to read since it’s less dense. The main reason not to use this form of import is to avoid name clashes. For instance, you wouldn’t import degrees this way if you also wanted to use the name degrees for a variable or function of your own. Or if you were to also import a function named degrees from another library.

Reading Error Messages

  1. Read the code below and try to identify what the errors are without running it.
  2. Run the code, and read the error message. What type of error is it?

PYTHON 

import datetime
datetime.date(2017,13,1)
  1. The date object takes arguments in the order year, month, day, so 13 is an invalid value for month.
  2. You get an error of type “ValueError”, indicating that the object received an inappropriate argument value. The additional message “month must be in 1..12” makes it clearer what the problem is.

Key Points

  • Most of the power of a programming language is in its libraries.
  • A program must import a library module in order to use it.
  • Use help to learn about the contents of a library module.
  • Import specific items from a library to shorten programs.
  • Create an alias for a library when importing it to shorten programs.

Content from Lunch

Last updated on 2023-05-08 | Edit this page

FIXME: describe what to reflect on.

Content from Lists

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I store multiple values?

Objectives

  • Explain why programs need collections of values.
  • Write programs that create flat lists, index them, slice them, and modify them through assignment and method calls.

A list stores many values in a single structure.

  • Scenario: You have set up an Arduino to do temperature measurements in a storage room for rare books.
  • Doing calculations with a hundred variables called temperature_001, temperature_002, etc., would be at least as slow as doing them by hand.
  • Use a list to store many values together.
    • Contained within square brackets [...].
    • Values separated by commas ,.
  • Use len to find out how many values are in a list.

PYTHON 

temperatures = [17.3, 17.5, 17.7, 17.5, 17.6]
print('temperatures:', temperatures)
print('length:', len(temperatures))

OUTPUT 

temperatures: [17.3, 17.5, 17.7, 17.5, 17.6]
length: 5

Use an item’s index to fetch it from a list.

  • Just like strings.

PYTHON 

print('zeroth item of temperatures:', temperatures[0])
print('fourth item of temperatures:', temperatures[4])

OUTPUT 

zeroth item of temperatures: 17.3
fourth item of temperatures: 17.6

Lists’ values can be replaced by assigning to them.

  • Use an index expression on the left of assignment to replace a value.

PYTHON 

temperatures[0] = 16.5
print('temperatures is now:', temperatures)

OUTPUT 

temperatures is now: [16.5, 17.5, 17.7, 17.5, 17.6]

Appending items to a list lengthens it.

  • Use list_name.append to add items to the end of a list.

PYTHON 

print('temperatures is initially:', temperatures)
temperatures.append(17.9)
temperatures.append(18.2)
print('temperatures has become:', temperatures)

OUTPUT 

temperatures is initially: [16.5, 17.5, 17.7, 17.5, 17.6]
temperatures has become: [16.5, 17.5, 17.7, 17.5, 17.6, 17.9, 18.2]
  • append is a method of lists.
    • Like a function, but tied to a particular object.
  • Use object_name.method_name to call methods.
    • Deliberately resembles the way we refer to things in a library.
  • We will meet other methods of lists as we go along.
    • Use help(list) for a preview.

Use del to remove items from a list entirely.

  • del list_name[index] removes an item from a list and shortens the list.
  • Not a function or a method, but a statement in the language.

PYTHON 

primes = [2, 3, 5, 7, 11]
print('primes before removing last item:', primes)
del primes[4]
print('primes after removing last item:', primes)

OUTPUT 

primes before removing last item: [2, 3, 5, 7, 11]
primes after removing last item: [2, 3, 5, 7]

The empty list contains no values.

  • Use [] on its own to represent a list that doesn’t contain any values.
    • “The zero of lists.”
  • Helpful as a starting point for collecting values (which we will see in the next episode).

Lists may contain values of different types.

  • A single list may contain numbers, strings, and anything else.

PYTHON 

goals = [1, 'Create lists.', 2, 'Extract items from lists.', 3, 'Modify lists.']

Character strings can be indexed like lists.

  • Get single characters from a character string using indexes in square brackets.

PYTHON 

element = 'carbon'
print('zeroth character:', element[0])
print('third character:', element[3])

OUTPUT 

zeroth character: c
third character: b

Character strings are immutable.

  • Cannot change the characters in a string after it has been created.
    • Immutable: cannot be changed after creation.
    • In contrast, lists are mutable: they can be modified in place.
  • Python considers the string to be a single value with parts, not a collection of values.

PYTHON 

element[0] = 'C'

ERROR 

TypeError: 'str' object does not support item assignment
  • Lists and character strings are both collections.

Indexing beyond the end of the collection is an error.

  • Python reports an IndexError if we attempt to access a value that doesn’t exist.
    • This is a kind of runtime error.
    • Cannot be detected as the code is parsed because the index might be calculated based on data.

PYTHON 

print('99th element of element is:', element[99])

OUTPUT 

IndexError: string index out of range

Fill in the Blanks

Fill in the blanks so that the program below produces the output shown.

PYTHON 

values = ____
values.____(1)
values.____(3)
values.____(5)
print('first time:', values)
values = values[____]
print('second time:', values)

OUTPUT 

first time: [1, 3, 5]
second time: [3, 5]

PYTHON 

values = []
values.append(1)
values.append(3)
values.append(5)
print('first time:', values)
values = values[1:3]
print('second time:', values)

OUTPUT 

first time [1, 3, 5]
second time [3, 5]

How Large is a Slice?

If ‘low’ and ‘high’ are both non-negative integers, how long is the list values[low:high]?

The list’s length would be equal to high - low.

From Strings to Lists and Back

Given this:

PYTHON 

print('string to list:', list('tin'))
print('list to string:', ''.join(['g', 'o', 'l', 'd']))

OUTPUT 

['t', 'i', 'n']
'gold'
  1. Explain in simple terms what list('some string') does.
  2. What does '-'.join(['x', 'y']) generate?
  1. It creates a list of the some strings characters as elements.
  2. It creates a string composed of x and y, separated by a hyphen character(-).

Working With the End

What does the following program print?

PYTHON 

element = 'helium'
print(element[-1])
  1. How does Python interpret a negative index?
  2. If a list or string has N elements, what is the most negative index that can safely be used with it, and what location does that index represent?
  3. If values is a list, what does del values[-1] do?
  4. How can you display all elements but the last one without changing values? (Hint: you will need to combine slicing and negative indexing.)

OUTPUT 

m
  1. A negative index begins at the final element.
  2. -(N) corresponds to the first index, which is the [0] index.
  3. It removes the final element of the list.
  4. You could do the following: print(values[0:-1])

Stepping Through a List

What does the following program print?

PYTHON 

element = 'fluorine'
print(element[::2])
print(element[::-1])
  1. If we write a slice as low:high:stride, what does stride do?
  2. What expression would select all of the even-numbered items from a collection?

OUTPUT 

furn
eniroulf
  1. stride indicates both the number of steps, and from which end: positive starts from first element, negative from the last element.
  2. element[1::2]

Slice Bounds

What does the following program print?

PYTHON 

element = 'lithium'
print(element[0:20])
print(element[-1:3])

OUTPUT 

lithium 
''

There is no 20th index, so the entire string is captured.
There is no element after the -1 index.

Sort and Sorted

What do these two programs print? In simple terms, explain the difference between sorted(letters) and letters.sort().

PYTHON 

# Program A
letters = list('gold')
result = sorted(letters)
print('letters is', letters, 'and result is', result)

PYTHON 

# Program B
letters = list('gold')
result = letters.sort()
print('letters is', letters, 'and result is', result)

Program A:

OUTPUT 

letters is ['g', 'o', 'l', 'd'] and result is ['d', 'g', 'l', 'o']

Program B:

OUTPUT 

letters is ['d', 'g', 'l', 'o'] and result is None

sorted(letters) returns a sorted copy of the list, while letters.sort() sorted the list in place. Thus, it was already sorted, and calling a further sort returns None.

Copying (or Not)

What do these two programs print? In simple terms, explain the difference between new = old and new = old[:].

PYTHON 

# Program A
old = list('gold')
new = old      # simple assignment
new[0] = 'D'
print('new is', new, 'and old is', old)

PYTHON 

# Program B
old = list('gold')
new = old[:]   # assigning a slice
new[0] = 'D'
print('new is', new, 'and old is', old)

Program A:

OUTPUT 

new is ['D', 'o', 'l', 'd'] and old is ['D', 'o', 'l', 'd']

Program B:

OUTPUT 

new is ['D', 'o', 'l', 'd'] and old is ['g', 'o', 'l', 'd']

new = old is assigning old to new, whereas new = old[:] is a slice assignment, which will only return a copy of old.

Key Points

  • A list stores many values in a single structure.
  • Use an item’s index to fetch it from a list.
  • Lists’ values can be replaced by assigning to them.
  • Appending items to a list lengthens it.
  • Use del to remove items from a list entirely.
  • The empty list contains no values.
  • Lists may contain values of different types.
  • Character strings can be indexed like lists.
  • Character strings are immutable.
  • Indexing beyond the end of the collection is an error.

Content from For Loops

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I make a program do many things?

Objectives

  • Explain what for loops are normally used for.
  • Trace the execution of a simple (unnested) loop and correctly state the values of variables in each iteration.
  • Write for loops that use the Accumulator pattern to aggregate values.

A for loop executes commands once for each value in a collection.

  • Doing calculations on the values in a list one by one is as painful as working with temperature_001, temperature_002, etc.
  • A for loop tells Python to execute some statements once for each value in a list, a character string, or some other collection.
  • “for each thing in this group, do these operations”

PYTHON 

for number in [2, 3, 5]:
    print(number)
  • This for loop is equivalent to:

PYTHON 

print(2)
print(3)
print(5)
  • And the for loop’s output is:

OUTPUT 

2
3
5

The first line of the for loop must end with a colon, and the body must be indented.

  • The colon at the end of the first line signals the start of a block of statements.
  • Python uses indentation rather than {} or begin/end to show nesting.
    • Any consistent indentation is legal, but almost everyone uses four spaces.

PYTHON 

for number in [2, 3, 5]:
print(number)

ERROR 

IndentationError: expected an indented block
  • Indentation is always meaningful in Python.

PYTHON 

firstName="Jon"
  lastName="Smith"

ERROR 

  File "<ipython-input-7-f65f2962bf9c>", line 2
    lastName="Smith"
    ^
IndentationError: unexpected indent
  • This error can be fixed by removing the extra spaces at the beginning of the second line.

A for loop is made up of a collection, a loop variable, and a body.

PYTHON 

for number in [2, 3, 5]:
    print(number)
  • The collection, [2, 3, 5], is what the loop is being run on.
  • The body, print(number), specifies what to do for each value in the collection.
  • The loop variable, number, is what changes for each iteration of the loop.
    • The “current thing”.

Loop variable names follow the normal variable name conventions.

  • Loop variables will:
    • Be created on demand during the course of each loop.
    • Persist after the loop finishes.
      • Use a new variable name to avoid overwriting a data collection you need to keep for later
    • Often be used in the course of the loop
      • So give them a meaningful name you’ll understand as the body code in your loop grows.
      • Example: for single_letter in ['A', 'B', 'C', 'D']: instead of for asdf in ['A', 'B', 'C', 'D']:

PYTHON 

for kitten in [2, 3, 5]:
    print(kitten)

The body of a loop can contain many statements.

  • But no loop should be more than a few lines long.
  • Hard for human beings to keep larger chunks of code in mind.

PYTHON 

primes = [2, 3, 5]
for p in primes:
    squared = p ** 2
    cubed = p ** 3
    print(p, squared, cubed)

OUTPUT 

2 4 8
3 9 27
5 25 125

Use range to iterate over a sequence of numbers.

  • The built-in function range produces a sequence of numbers.
    • Not a list: the numbers are produced on demand to make looping over large ranges more efficient.
  • range(N) is the numbers 0..N-1
    • Exactly the legal indices of a list or character string of length N

PYTHON 

print('a range is not a list: range(0, 3)')
for number in range(0,3):
    print(number)

OUTPUT 

a range is not a list: range(0, 3)
0
1
2

Or use range to repeat an action an arbitrary number of times.

  • You don’t actually have to use the iterable variable’s value.
  • Use this structure to simply repeat an action some number of times.
    • That number of times goes into the range function.

PYTHON 

for number in range(5):
    print("Again!")

OUTPUT 

Again!
Again!
Again!
Again!
Again!

The Accumulator pattern turns many values into one.

  • A common pattern in programs is to:
    1. Initialize an accumulator variable to zero, the empty string, or the empty list.
    2. Update the variable with values from a collection.

PYTHON 

# Sum the first 10 integers.
total = 0
for number in range(10):
   total = total + (number + 1)
print(total)

OUTPUT 

55
  • Read total = total + (number + 1) as:
    • Add 1 to the current value of the loop variable number.
    • Add that to the current value of the accumulator variable total.
    • Assign that to total, replacing the current value.
  • We have to add number + 1 because range produces 0..9, not 1..10.

Classifying Errors

Is an indentation error a syntax error or a runtime error?

It is a syntax error. The problem has to do with the placement of the code, not its logic.

Tracing Execution

Create a table showing the numbers of the lines that are executed when this program runs, and the values of the variables after each line is executed.

PYTHON 

total = 0
for char in "tin":
    total = total + 1

Reversing a String

Fill in the blanks in the program below so that it prints “nit” (the reverse of the original character string “tin”).

PYTHON 

original = "tin"
result = ____
for char in original:
    result = ____
print(result)

result is an empty string because we use it to build or accumulate on our reverse string. char is the loop variable for original. Each time through the loop char takes on one value from original. Use char with result to control the order of the string. Our loop code should look like this:

original = "tin"
result = ""
for char in original:
   result = char + result
print(result)
nit

If you were to expand out the loop the iterations would look something like this:

PYTHON 

#First loop
char = "t"
result = ""
char + result = "t"
#Second loop
char = 'i"
result = "t"
char + result = "it"
#Third loop
char = "n"
result = "it"
char + result = "nit"

Practice Accumulating

Fill in the blanks in each of the programs below to produce the indicated result.

PYTHON 

# Total length of the strings in the list: ["red", "green", "blue"] => 12
total = 0
for word in ["red", "green", "blue"]:
    ____ = ____ + len(word)
print(total)

PYTHON 

# List of word lengths: ["red", "green", "blue"] => [3, 5, 4]
lengths = ____
for word in ["red", "green", "blue"]:
    lengths.____(____)
print(lengths)

PYTHON 

# Concatenate all words: ["red", "green", "blue"] => "redgreenblue"
words = ["red", "green", "blue"]
result = ____
for ____ in ____:
    ____
print(result)

PYTHON 

# Create acronym: ["red", "green", "blue"] => "RGB"
# write the whole thing

Cumulative Sum

Reorder and properly indent the lines of code below so that they print an array with the cumulative sum of data. The result should be [1, 3, 5, 10].

PYTHON 

cumulative += [sum]
for number in data:
cumulative = []
sum += number
print(cumulative)
sum = 0
data = [1,2,2,5]

PYTHON 

data = [1,2,2,5]
cumulative = []
sum = 0
for number in data:
  sum += number
  cumulative += [sum]
print(cumulative)

OUTPUT 

[1, 3, 5, 10]

Identifying Variable Name Errors

  1. Read the code below and try to identify what the errors are without running it.
  2. Run the code and read the error message. What type of NameError do you think this is? Is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
  3. Fix the error.
  4. Repeat steps 2 and 3, until you have fixed all the errors.

PYTHON 

for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

Identifying Item Errors

  1. Read the code below and try to identify what the errors are without running it.
  2. Run the code, and read the error message. What type of error is it?
  3. Fix the error.

PYTHON 

seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[4])

It is an index error:

ERROR 

IndexError: list index out of range

The problem is that 4 points to an item that doesn’t exist in the list. Remember the first item of a list in Python is 0.
Replace seasons[4] with seasons[0], seasons[1], seasons[2] or seasons[3] to have the different items of the list printed.

Key Points

  • A for loop executes commands once for each value in a collection.
  • The first line of the for loop must end with a colon, and the body must be indented.
  • Indentation is always meaningful in Python.
  • A for loop is made up of a collection, a loop variable, and a body.
  • Loop variables can be called anything (but it is strongly advised to have a meaningful name to the looping variable).
  • The body of a loop can contain many statements.
  • Use range to iterate over a sequence of numbers.
  • The Accumulator pattern turns many values into one.

Content from Looping Over Data Sets

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I process many data sets with a single command?

Objectives

  • Be able to read and write globbing expressions that match sets of files.
  • Use glob to create lists of files.
  • Write for loops to perform operations on files given their names in a list.

Use a for loop to process files given a list of their names.

  • A filename is just a character string.
  • And lists can contain character strings.

PYTHON 

for filename in ['data/gapminder_gdp_africa.csv', 'data/gapminder_gdp_asia.csv']:
    data = pandas.read_csv(filename, index_col='country')
    print(filename, data.min())

OUTPUT 

data/gapminder_gdp_africa.csv gdpPercap_1952    298.846212
gdpPercap_1957    335.997115
gdpPercap_1962    355.203227
gdpPercap_1967    412.977514
⋮ ⋮ ⋮
gdpPercap_1997    312.188423
gdpPercap_2002    241.165877
gdpPercap_2007    277.551859
dtype: float64
data/gapminder_gdp_asia.csv gdpPercap_1952    331
gdpPercap_1957    350
gdpPercap_1962    388
gdpPercap_1967    349
⋮ ⋮ ⋮
gdpPercap_1997    415
gdpPercap_2002    611
gdpPercap_2007    944
dtype: float64

Use glob.glob to find sets of files whose names match a pattern.

  • In Unix, the term “globbing” means “matching a set of files with a pattern”.
  • The most common patterns are:
    • * meaning “match zero or more characters”
    • ? meaning “match exactly one character”
  • Python contains the glob library to provide pattern matching functionality
  • The glob library contains a function also called glob to match file patterns
  • E.g., glob.glob('*.txt') matches all files in the current directory whose names end with .txt.
  • Result is a (possibly empty) list of character strings.

PYTHON 

import glob
print('all csv files in data directory:', glob.glob('data/*.csv'))

OUTPUT 

all csv files in data directory: ['data/gapminder_all.csv', 'data/gapminder_gdp_africa.csv', \
'data/gapminder_gdp_americas.csv', 'data/gapminder_gdp_asia.csv', 'data/gapminder_gdp_europe.csv', \
'data/gapminder_gdp_oceania.csv']

PYTHON 

print('all PDB files:', glob.glob('*.pdb'))

OUTPUT 

all PDB files: []

Use glob and for to process batches of files.

  • Helps a lot if the files are named and stored systematically and consistently so that simple patterns will find the right data.

PYTHON 

for filename in glob.glob('data/*.csv'):
    data = pandas.read_csv(filename)
    print(filename, data['gdpPercap_1952'].min())

OUTPUT 

data/gapminder_all.csv 298.8462121
data/gapminder_gdp_africa.csv 298.8462121
data/gapminder_gdp_americas.csv 1397.717137
data/gapminder_gdp_asia.csv 331.0
data/gapminder_gdp_europe.csv 973.5331948
data/gapminder_gdp_oceania.csv 10039.59564
  • This includes all data, as well as per-region data.
  • Use a more specific pattern in the exercises to exclude the whole data set.
  • But note that the minimum of the entire data set is also the minimum of one of the data sets, which is a nice check on correctness.

Determining Matches

Which of these files is not matched by the expression glob.glob('data/*as*.csv')?

  1. data/gapminder_gdp_africa.csv
  2. data/gapminder_gdp_americas.csv
  3. data/gapminder_gdp_asia.csv
  4. 1 and 2 are not matched.

##Solution

1 is not matched by the regular expresion.

Minimum File Size

Modify this program so that it prints the number of records in the file that has the fewest records.

PYTHON 

import pandas
fewest = ____
for filename in glob.glob('data/*.csv'):
    dataframe = pandas.____(filename)
    fewest = min(____, dataframe.shape[0]) 
print('smallest file has', fewest, 'records')

Notice that the shape method returns a tuple with the number of rows and columns of the data frame.

##Solution

PYTHON 

import pandas
fewest = 0
for filename in glob.glob('data/*.csv'):
    dataframe = pandas.read_csv(filename)
    fewest = min(fewest , dataframe.shape[0]) 
print('smallest file has', fewest, 'records')

Comparing Data

Write a program that reads in the regional data sets and plots the average GDP per capita for each region over time in a single chart.

Key Points

  • Use a for loop to process files given a list of their names.
  • Use glob.glob to find sets of files whose names match a pattern.
  • Use glob and for to process batches of files.

Content from Writing Functions

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I create my own functions?

Objectives

  • Explain and identify the difference between function definition and function call.
  • Write a function that takes a small, fixed number of arguments and produces a single result.

Break programs down into functions to make them easier to understand.

  • Human beings can only keep a few items in working memory at a time.
  • Understand larger/more complicated ideas by understanding and combining pieces.
    • Components in a machine.
    • Lemmas when proving theorems.
  • Functions serve the same purpose in programs.
    • Encapsulate complexity so that we can treat it as a single “thing”.
  • Also enables re-use.
    • Write one time, use many times.

Define a function using def with a name, parameters, and a block of code.

  • Begin the definition of a new function with def.
  • Followed by the name of the function.
    • Must obey the same rules as variable names.
  • Then parameters in parentheses.
    • Empty parentheses if the function doesn’t take any inputs.
    • We will discuss this in detail in a moment.
  • Then a colon.
  • Then an indented block of code.

PYTHON 

def print_greeting():
    print('Hello!')

Defining a function does not run it.

  • Defining a function does not run it.
    • Like assigning a value to a variable.
  • Must call the function to execute the code it contains.
  • The commands for the function are read and stored after the def block, but not actually executed until the function is called later on.
    • Imagine getting a recipe card and keeping it in your kitchen. You can cook it anytime, but you haven’t completed any of the steps until you start that cooking process.
    • This means that Python won’t complain about problems until you call the function. More specifically, just because the definition of a function runs without error doesn’t mean that there won’t be errors when it executes later.

PYTHON 

print_greeting()

OUTPUT 

Hello!

Arguments in call are matched to parameters in definition.

  • Functions are most useful when they can operate on different data.
  • Specify parameters when defining a function.
    • These become variables when the function is executed.
    • Are assigned the arguments in the call (i.e., the values passed to the function).

PYTHON 

def print_date(year, month, day):
    joined = str(year) + '/' + str(month) + '/' + str(day)
    print(joined)

print_date(1871, 3, 19)

OUTPUT 

1871/3/19
  • Via Twitter: () contains the ingredients for the function while the body contains the recipe.

Functions may return a result to their caller using return.

  • Use return ... to give a value back to the caller.
  • May occur anywhere in the function.
  • But functions are easier to understand if return occurs:
    • At the start to handle special cases.
    • At the very end, with a final result.

PYTHON 

def average(values):
    if len(values) == 0:
        return None
    return sum(values) / len(values)

PYTHON 

a = average([1, 3, 4])
print('average of actual values:', a)

OUTPUT 

2.6666666666666665

PYTHON 

print('average of empty list:', average([]))

OUTPUT 

None

PYTHON 

result = print_date(1871, 3, 19)
print('result of call is:', result)

OUTPUT 

1871/3/19
result of call is: None

Definition and Use

What does the following program print?

PYTHON 

def report(pressure):
    print('pressure is', pressure)

report(22.5)

OUTPUT 

pressure is 22.5

Order of Operations

The example above:

PYTHON 

result = print_date(1871, 3, 19)
print('result of call is:', result)

printed:

OUTPUT 

1871/3/19
result of call is: None

Explain why the two lines of output appeared in the order they did.

Each line of Python code is executed in order, regardless of whether that line calls out to a function, which may call out to other functions, or a variable assignment. In this case, the second line call to print will not execute until the result of print_date is complete in the first line.

Encapsulation

Fill in the blanks to create a function that takes a single filename as an argument, loads the data in the file named by the argument, and returns the minimum value in that data.

PYTHON 

import pandas

def min_in_data(____):
    data = ____
    return ____

PYTHON 

import pandas

def min_in_data(filename):
    data = pandas.read_csv(filename)
    return data.min()

Find the First

Fill in the blanks to create a function that takes a list of numbers as an argument and returns the first negative value in the list. What does your function do if the list is empty?

PYTHON 

def first_negative(values):
    for v in ____:
        if ____:
            return ____

PYTHON 

def first_negative(values):
    for v in values:
        if v < 0:
            return v

Calling by Name

What does this short program print?

PYTHON 

def print_date(year, month, day):
    joined = str(year) + '/' + str(month) + '/' + str(day)
    print(joined)

print_date(day=1, month=2, year=2003)
  1. When have you seen a function call like this before?
  2. When and why is it useful to call functions this way? {: .python}

The program prints:

OUTPUT 

2003/2/1

It is useful to call a function with named arguments to ensure that the values of each argument are assigned to the intended argument in the function. This allows the order of arguments to be specified independently of how they are defined in the function itself.

Encapsulate of If/Print Block

The code below will run on a label-printer for chicken eggs. A digital scale will report a chicken egg mass (in grams) to the computer and then the computer will print a label.

Please re-write the code so that the if-block is folded into a function.

PYTHON 

 import random
 for i in range(10):

    # simulating the mass of a chicken egg
    # the (random) mass will be 70 +/- 20 grams
    mass=70+20.0*(2.0*random.random()-1.0)

    print(mass)
   
    #egg sizing machinery prints a label
    if(mass>=85):
       print("jumbo")
    elif(mass>=70):
       print("large")
    elif(mass<70 and mass>=55):
       print("medium")
    else:
       print("small")

The simplified program follows. What function definition will make it functional?

PYTHON 

 # revised version
 import random
 for i in range(10):

    # simulating the mass of a chicken egg
    # the (random) mass will be 70 +/- 20 grams
    mass=70+20.0*(2.0*random.random()-1.0)

    print(mass,print_egg_label(mass))
  1. Create a function definition for print_egg_label() that will work with the revised program above. Note, the function’s return value will be significant. Sample output might be 71.23 large.
  2. A dirty egg might have a mass of more than 90 grams, and a spoiled or broken egg will probably have a mass that’s less than 50 grams. Modify your print_egg_label() function to account for these error conditions. Sample output could be 25 too light, probably spoiled.

PYTHON 

def print_egg_label(mass):
  if(mass>=90):
     print(mass, "dirty")
  elif(mass>=85):
     print(mass, "jumbo")
  elif(mass>=70):
     print(mass, "large")
  elif(mass<70 and mass>=55):
     print(mass, "medium")
  else:
     print(mass, "too light, probably spoiled")

Encapsulating Data Analysis

Assume that the following code has been executed:

PYTHON 

import pandas

df = pandas.read_csv('gapminder_gdp_asia.csv', index_col=0)
japan = df.ix['Japan']
  1. Complete the statements below to obtain the average GDP for Japan across the years reported for the 1980s.

PYTHON 

year = 1983
gdp_decade = 'gdpPercap_' + str(year // ____)
avg = (japan.ix[gdp_decade + ___] + japan.ix[gdp_decade + ___]) / 2
  1. Abstract the code above into a single function.

PYTHON 

def avg_gdp_in_decade(country, continent, year):
    df = pd.read_csv('gapminder_gdp_'+___+'.csv',delimiter=',',index_col=0)
    ____
    ____
    ____
    return avg
  1. How would you generalize this function if you did not know beforehand which specific years occurred as columns in the data? For instance, what if we also had data from years ending in 1 and 9 for each decade? (Hint: use the columns to filter out the ones that correspond to the decade, instead of enumerating them in the code.)

PYTHON 

year = 1983
gdp_decade = 'gdpPercap_' + str(year // 10)
avg = (japan.ix[gdp_decade + '2'] + japan.ix[gdp_decade + '7']) / 2

PYTHON 

def avg_gdp_in_decade(country, continent, year):
    df = pd.read_csv('gapminder_gdp_' + continent + '.csv', index_col=0)
    c = df.ix[country]
    gdp_decade = 'gdpPercap_' + str(year // 10)
    avg = (c.ix[gdp_decade + '2'] + c.ix[gdp_decade + '7'])/2
    return avg
  1. We need to loop over the reported years to obtain the average for the relevant ones in the data.

PYTHON 

def avg_gdp_in_decade(country, continent, year):
    df = pd.read_csv('gapminder_gdp_' + continent + '.csv', index_col=0)
    c = df.ix[country] 
    gdp_decade = 'gdpPercap_' + str(year // 10)
    total = 0.0
    num_years = 0
    for yr_header in c.index: # c's index contains reported years
        if yr_header.startswith(gdp_decade):
            total = total + c.ix[yr_header]
            num_years = num_years + 1
    return total/num_years

Key Points

  • Break programs down into functions to make them easier to understand.
  • Define a function using def with a name, parameters, and a block of code.
  • Defining a function does not run it.
  • Arguments in call are matched to parameters in definition.
  • Functions may return a result to their caller using return.

Content from Variable Scope

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How do function calls actually work?
  • How can I determine where errors occurred?

Objectives

  • Identify local and global variables.
  • Identify parameters as local variables.
  • Read a traceback and determine the file, function, and line number on which the error occurred, the type of error, and the error message.

The scope of a variable is the part of a program that can ‘see’ that variable.

  • There are only so many sensible names for variables.
  • People using functions shouldn’t have to worry about what variable names the author of the function used.
  • People writing functions shouldn’t have to worry about what variable names the function’s caller uses.
  • The part of a program in which a variable is visible is called its scope.

PYTHON 

pressure = 103.9

def adjust(t):
    temperature = t * 1.43 / pressure
    return temperature
  • pressure is a global variable.
    • Defined outside any particular function.
    • Visible everywhere.
  • t and temperature are local variables in adjust.
    • Defined in the function.
    • Not visible in the main program.
    • Remember: a function parameter is a variable that is automatically assigned a value when the function is called.

PYTHON 

print('adjusted:', adjust(0.9))
print('temperature after call:', temperature)

OUTPUT 

adjusted: 0.01238691049085659

ERROR 

Traceback (most recent call last):
  File "/Users/swcarpentry/foo.py", line 8, in <module>
    print('temperature after call:', temperature)
NameError: name 'temperature' is not defined

Local and Global Variable Use

Trace the values of all variables in this program as it is executed. (Use ‘—’ as the value of variables before and after they exist.)

PYTHON 

limit = 100

def clip(value):
    return min(max(0.0, value), limit)

value = -22.5
print(clip(value))

PYTHON 

# limit = ---
# value = ---

limit = 100   

def clip(value):  
  return min(max(0.0, value), limit)

# limit = 100
# value = ---

value = -22.5    # value = -22.5, limit = 100

print(clip(value))   # result is 0.0

# value = -22.5
# limit = 100

Identifying Syntax Errors

  1. Read the code below and try to identify what the errors are without running it.
  2. Run the code and read the error message. Is it a SyntaxError or an IndentationError?
  3. Fix the error.
  4. Repeat steps 2 and 3 until you have fixed all the errors.

PYTHON 

def another_function
  print("Syntax errors are annoying.")
   print("But at least Python tells us about them!")
  print("So they are usually not too hard to fix.")

There are missing parentheses and colon (): after the function call, and the print messages don’t appear aligned via whitespace

ERROR 

   File "<stdin>", line 1
def another_function
                   ^
SyntaxError: invalid syntax

ERROR 

  File "<stdin>", line 1
  print("Syntax errors are annoying.")
^
IndentationError: unexpected indent

ERROR 

  File "<stdin>", line 1
  print("But at least Python tells us about them!")
^
IndentationError: unexpected indent

Working function:

PYTHON 

def another_function():
    print("Syntax errors are annoying.")
    print("But at least Python tells us about them!")
    print("So they are usually not too hard to fix.")

Reading Error Messages

Read the traceback below, and identify the following:

  1. How many levels does the traceback have?
  2. What is the file name where the error occurred?
  3. What is the function name where the error occurred?
  4. On which line number in this function did the error occur?
  5. What is the type of error?
  6. What is the error message?

ERROR 

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-2-e4c4cbafeeb5> in <module>()
      1 import errors_02
----> 2 errors_02.print_friday_message()

/Users/ghopper/thesis/code/errors_02.py in print_friday_message()
     13
     14 def print_friday_message():
---> 15     print_message("Friday")

/Users/ghopper/thesis/code/errors_02.py in print_message(day)
      9         "sunday": "Aw, the weekend is almost over."
     10     }
---> 11     print(messages[day])
     12
     13

KeyError: 'Friday'
  1. 3 levels, since there are 3 arrows
  2. The file is errors_02.py
  3. The function is print_message()
  4. Line 11
  5. It is a KeyError
  6. There isn’t really a message; you’re supposed to infer that Friday is not a key in messages.

Key Points

  • The scope of a variable is the part of a program that can ‘see’ that variable.

Content from Afternoon Coffee

Last updated on 2023-05-08 | Edit this page

FIXME: describe what to reflect on.

Content from Conditionals

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can programs do different things for different data?

Objectives

  • Correctly write programs that use if and else statements and simple Boolean expressions (without logical operators).
  • Trace the execution of unnested conditionals and conditionals inside loops.

Use if statements to control whether or not a block of code is executed.

  • An if statement (more properly called a conditional statement) controls whether some block of code is executed or not.
  • Structure is similar to a for statement:
    • First line opens with if and ends with a colon
    • Body containing one or more statements is indented (usually by 4 spaces)

PYTHON 

mass = 3.54
if mass > 3.0:
    print(mass, 'is larger')

mass = 2.07
if mass > 3.0:
    print (mass, 'is larger')

OUTPUT 

3.54 is larger

Conditionals are often used inside loops.

  • Not much point using a conditional when we know the value (as above).
  • But useful when we have a collection to process.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for m in masses:
    if m > 3.0:
        print(m, 'is larger')

OUTPUT 

3.54 is larger
9.22 is larger

Use else to execute a block of code when an if condition is not true.

  • else can be used following an if.
  • Allows us to specify an alternative to execute when the if branch isn’t taken.

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for mass in masses:
    if mass > 3.0:
        print(mass, 'is larger')
    else:
        print(mass, 'is smaller')

OUTPUT 

3.54 is larger
2.07 is smaller
9.22 is larger
1.86 is smaller
1.71 is smaller

Use elif to specify additional tests.

  • May want to provide several alternative choices, each with its own test.
  • Use elif (short for “else if”) and a condition to specify these.
  • Always associated with an if.
  • Must come before the else (which is the “catch all”).

PYTHON 

masses = [3.54, 2.07, 9.22, 1.86, 1.71]
for mass in masses:
    if mass > 9.0:
        print(mass, 'is HUGE')
    elif mass > 3.0:
        print(mass, 'is larger')
    else:
        print(mass, 'is smaller')

OUTPUT 

3.54 is larger
2.07 is smaller
9.22 is HUGE
1.86 is smaller
1.71 is smaller

Conditions are tested once, in order.

  • Python steps through the branches of the conditional in order, testing each in turn.
  • So ordering matters.

PYTHON 

grade = 85
if grade >= 70:
    print('grade is C')
elif grade >= 80:
    print('grade is B')
elif grade >= 90:
    print('grade is A')

OUTPUT 

grade is C
  • Does not automatically go back and re-evaluate if values change.

PYTHON 

velocity = 10.0
if velocity > 20.0:
    print('moving too fast')
else:
    print('adjusting velocity')
    velocity = 50.0

OUTPUT 

adjusting velocity
  • Often use conditionals in a loop to “evolve” the values of variables.

PYTHON 

velocity = 10.0
for i in range(5): # execute the loop 5 times
    print(i, ':', velocity)
    if velocity > 20.0:
        print('moving too fast')
        velocity = velocity - 5.0
    else:
        print('moving too slow')
        velocity = velocity + 10.0
print('final velocity:', velocity)

OUTPUT 

0 : 10.0
moving too slow
1 : 20.0
moving too slow
2 : 30.0
moving too fast
3 : 25.0
moving too fast
4 : 20.0
moving too slow
final velocity: 30.0

Create a table showing variables’ values to trace a program’s execution.

i 0 . 1 . 2 . 3 . 4 .
velocity 10.0 20.0 . 30.0 . 25.0 . 20.0 . 30.0
  • The program must have a print statement outside the body of the loop to show the final value of velocity, since its value is updated by the last iteration of the loop.

Compound Relations Usingand,or, and Parentheses

Often, you want some combination of things to be true. You can combine relations within a conditional using and and or. Continuing the example above, suppose you have

PYTHON 

mass     = [ 3.54,  2.07,  9.22,  1.86,  1.71]
velocity = [10.00, 20.00, 30.00, 25.00, 20.00]

i = 0
for i in range(5):
    if mass[i] > 5 and velocity[i] > 20:
        print("Fast heavy object.  Duck!")
    elif mass[i] > 2 and mass[i] <= 5 and velocity[i] <= 20:
        print("Normal traffic")
    elif mass[i] <= 2 and velocity <= 20:
        print("Slow light object.  Ignore it")
    else:
        print("Whoa!  Something is up with the data.  Check it")

Just like with arithmetic, you can and should use parentheses whenever there is possible ambiguity. A good general rule is to always use parentheses when mixing and and or in the same condition. That is, instead of:

PYTHON 

if mass[i] <= 2 or mass[i] >= 5 and velocity[i] > 20:

write one of these:

PYTHON 

if (mass[i] <= 2 or mass[i] >= 5) and velocity[i] > 20:
if mass[i] <= 2 or (mass[i] >= 5 and velocity[i] > 20):

so it is perfectly clear to a reader (and to Python) what you really mean.

Tracing Execution

What does this program print?

pressure = 71.9
if pressure > 50.0:
    pressure = 25.0
elif pressure <= 50.0:
    pressure = 0.0
print(pressure)

{: python}

OUTPUT 

25.0

Trimming Values

Fill in the blanks so that this program creates a new list containing zeroes where the original list’s values were negative and ones where the original list’s values were positive.

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = ____
for value in original:
    if ____:
        result.append(0)
    else:
        ____
print(result)

OUTPUT 

[0, 1, 1, 1, 0, 1]

PYTHON 

original = [-1.5, 0.2, 0.4, 0.0, -1.3, 0.4]
result = []
for value in original:
   if value < 0:
       result.append(0)
   else:
      result.append(1)
print(result)

Processing Small Files

Modify this program so that it only processes files with fewer than 50 records.

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
    contents = pandas.read_csv(filename)
    ____:
        print(filename, len(contents))

PYTHON 

import glob
import pandas
for filename in glob.glob('data/*.csv'):
   contents = pandas.read_csv(filename)
   if len(contents) < 50:
       print(filename, len(contents))

Initializing

Modify this program so that it finds the largest and smallest values in the list no matter what the range of values originally is.

What are the advantages and disadvantages of using this method to find the range of the data?

PYTHON 

values = [...some test data...]
smallest, largest = None, None
for v in values:
    if ____:
        smallest, largest = v, v
    ____:
        smallest = min(____, v)
        largest = max(____, v)
print(smallest, largest)

PYTHON 

values = [1, 3, 4, 5, 10]
smallest, largest = None, None
for v in values:
   if largest is None:
       smallest, largest = v, v
   else:
       smallest = min(smallest, v)
       largest = max(largest, v)
print(smallest, largest)

Using Functions With Conditionals in Pandas

Functions will often contain conditionals. Here is a short example that will indicate which quartile the argument is in based on hand-coded values for the quartile cut points.

PYTHON 

def calculate_life_quartile(exp):
    if exp < 58.41:
        # This observation is in the first quartile
        return 1
    elif exp >= 58.41 and exp < 67.05:
        # This observation is in the second quartile
       return 2
    elif exp >= 67.05 and exp < 71.70:
        # This observation is in the third quartile
       return 3
    elif exp >= 71.70:
        # This observation is in the fourth quartile
       return 4
    else:
        # This observation has bad data
       return None

calculate_life_quartile(62.5)

OUTPUT 

2

That function would typically be used within a for loop, but Pandas has a different, more efficient way of doing the same thing, and that is by applying a function to a dataframe or a portion of a dataframe. Here is an example, using the definition above.

PYTHON 

data = pd.read_csv('Americas-data.csv')
data['life_qrtl'] = data['lifeExp'].apply(calculate_life_quartile)

There is a lot in that second line, so let’s take it piece by piece. On the right side of the = we start with data['lifeExp'], which is the column in the dataframe called data labeled lifExp. We use the apply() to do what it says, apply the calculate_life_quartile to the value of this column for every row in the dataframe.

Key Points

  • Use if statements to control whether or not a block of code is executed.
  • Conditionals are often used inside loops.
  • Use else to execute a block of code when an if condition is not true.
  • Use elif to specify additional tests.
  • Conditions are tested once, in order.
  • Create a table showing variables’ values to trace a program’s execution.

Content from Programming Style

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How can I make my programs more readable?
  • How do most programmers format their code?
  • How can programs check their own operation?

Objectives

  • Provide sound justifications for basic rules of coding style.
  • Refactor one-page programs to make them more readable and justify the changes.
  • Use Python community coding standards (PEP-8).

Follow standard Python style in your code.

  • PEP8: a style guide for Python that discusses topics such as how you should name variables, how you should use indentation in your code, how you should structure your import statements, etc. Adhering to PEP8 makes it easier for other Python developers to read and understand your code, and to understand what their contributions should look like. The PEP8 application and Python library can check your code for compliance with PEP8.

Use assertions to check for internal errors.

Assertions are powerful method for making sure that the context in which your code is executing is as you expect.

PYTHON 

def calc_bulk_density(mass, volume):
    '''Return dry bulk density = powder mass / powder volume.'''
    assert volume > 0
    return mass / volume

If the assertion is False, the Python interpreter raises an AssertionError runtime exception. The source code for the expression that failed will be displayed as part of the error message. To ignore assertions in your code run the interpreter with the ‘-O’ (optimize) switch. Assertions should contain only basic checks and never change the state of the program. For example, an assertion should never contain an assignment.

Use docstrings to provide online help.

  • If the first thing in a function is a character string that is not assigned to a variable, Python attaches it to the function as the online help.
  • Called a docstring (short for “documentation string”).

PYTHON 

def average(values):
    "Return average of values, or None if no values are supplied."

    if len(values) == 0:
        return None
    return sum(values) / average(values)

help(average)

OUTPUT 

Help on function average in module __main__:

average(values)
    Return average of values, or None if no values are supplied.

Multiline Strings

Often use multiline strings for documentation. These start and end with three quote characters (either single or double) and end with three matching characters.

PYTHON 

"""This string spans
multiple lines.

Blank lines are allowed."""

What Will Be Shown?

Highlight the lines in the code below that will be available as online help. Are there lines that should be made available, but won’t be? Will any lines produce a syntax error or a runtime error?

PYTHON 

"Find maximum edit distance between multiple sequences."
# This finds the maximum distance between all sequences.

def overall_max(sequences):
    '''Determine overall maximum edit distance.'''

    highest = 0
    for left in sequences:
        for right in sequences:
            '''Avoid checking sequence against itself.'''
            if left != right:
                this = edit_distance(left, right)
                highest = max(highest, this)

    # Report.
    return highest

These two lines will show up in online help:

PYTHON 

 '''Determine overall maximum edit distance.'''
 '''Avoid checking sequence against itself.'''

This line will not be made available because it is not in the docstring format: "Find maximum edit distance between multiple sequences." Docstrings should reside within the function definition, using three single quotes.

There is one syntax error:

PYTHON 

Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
File "<module2>", line 11, in overall_max
NameError: global name 'edit_distance' is not defined

The edit_distance function has not been defined:

PYTHON 

 this = edit_distance(left, right)

Document This

Turn the comment on the following function into a docstring and check that help displays it properly.

PYTHON 

def middle(a, b, c):
    # Return the middle value of three.
    # Assumes the values can actually be compared.
    values = [a, b, c]
    values.sort()
    return values[1]

Change the # to ''' to change to docstring:

PYTHON 

 def middle(a, b, c):
 ''' Return the middle value of three.
 Assumes the values can actually be compared.
 '''
 values = [a, b, c]
 values.sort()
 return values[1]

Calling help on the middle function now shows the docstring:

PYTHON 

help (middle)
Help on function middle in module __main__:
middle(a, b, c)
   Return the middle value of three.
   Assumes the values can actually be compared.

Clean Up This Code

  1. Read this short program and try to predict what it does.
  2. Run it: how accurate was your prediction?
  3. Refactor the program to make it more readable. Remember to run it after each change to ensure its behavior hasn’t changed.
  4. Compare your rewrite with your neighbor’s. What did you do the same? What did you do differently, and why?

PYTHON 

import sys
n = int(sys.argv[1])
s = sys.argv[2]
print(s)
i = 0
while i < n:
    # print('at', j)
    new = ''
    for j in range(len(s)):
        left = j-1
        right = (j+1)%len(s)
        if s[left]==s[right]: new += '-'
        else: new += '*'
    s=''.join(new)
    print(s)
    i += 1

Here’s one solution.

PYTHON 

def string_machine(input_string, iterations):
    """
    Takes input_string and generates a new string with -'s and *'s
    corresponding to characters that have identical adjacent characters
    or not, respectively.  Iterates through this procedure with the resultant
    strings for the supplied number of iterations.
    """
    print(input_string)
    old = input_string
    for i in range(iterations):
        new = ''
        # iterate through characters in previous string
        for j in range(len(s)):
            left = j-1
            right = (j+1)%len(s) # ensure right index wraps around
            if old[left]==old[right]:
                new += '-'
            else:
                new += '*'
        print(new)
        # store new string as old
        old = new

string_machine('et cetera', 10)

Key Points

  • Follow standard Python style in your code.
  • Use docstrings to provide online help.

Content from Wrap-Up

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • What have we learned?
  • What else is out there and where do I find it?

Objectives

  • Name and locate scientific Python community sites for software, workshops, and help.

Leslie Lamport once said, “Writing is nature’s way of showing you how sloppy your thinking is.” The same is true of programming: many things that seem obvious when we’re thinking about them turn out to be anything but when we have to explain them precisely.

Python supports a large community within and outwith research.

Key Points

  • Python supports a large community within and outwith research.

Content from Feedback

Last updated on 2023-05-08 | Edit this page

Overview

Questions

  • How did the class go?

Objectives

  • Gather feedback on the class

Gather feedback from participants.

Key Points

  • We are constantly seeking to improve this course.