1.2. The Python Interpreter and Idle, Part I¶

1.2.1. Your Python Folder and Python Examples¶

First you need to set up a location to store your work and the example programs from this tutorial. You can put the folder for your Python programs most anywhere you like. However, for Chapter 4, it will be important that none of the directories leading down to your Python folder contain any blanks in them, in particular your home folder.

The normal place for you to have my examples folder is under your home folder, which has the same name as your Login ID. If you are just creating a login ID, it will save some hassle in Chapter 4, if your login ID does not have a space in it. “Andy” or “anh” or AndyHarrington” would be fine for me, but

“Andy Harrington”

would bomb the server in Chapter 4. It is possible to do Chapter 4 with a folder outside your home folder, as discussed at the beginning of Chapter 4.

1.2.2. Folder For My Examples¶

Note that the examples, like this version of the tutorial, are for Python 3. There were major changes to Python in version 3.0, making it incompatible with earlier versions.

If you are using Python version 2.7 for some good reason, you should continue with the older version of the tutorial. Go to http://anh.cs.luc.edu/python/hands-on and find the links to the proper version of the tutorial and examples.

Once you have the examples.zip archive, you need to extract the files. Make a file browser window set to the directory where you put the zip file. Then:

Windows
Right click on examples.zip, select Extract All. This will create the folder examples. End up with a file browser window showing the contents of the examples folder. This will be your Python folder in later discussion.
Mac
Double click on the zip file and the expanded examples folder appears.

Warning

On Windows, files in a zip archive can be viewed while they are still in the zip archive. Modifying and adding files is not so transparent. Be sure that you unzip the archive and work from the regular directory that holds the resulting unzipped files.

1.2.3. Running A Sample Program¶

This section assumes Python 3 is already on your computer. Windows does not come with Python. (To load Python see Obtaining Python for Your Computer.)

Before getting to the individual details of Python, you will run a simple text-based sample program. Find madlib.py in your Python folder, set up in Your Python Folder and Python Examples.

Note

Different keyboards label the key for ending a line differently: Windows keyboards may use Enter while Mac keyboards use Return. The tutorial may intermix instructions to press Enter or press Return. They both are intended to mean the key for ending a line.

Options for running the program:

• If you have trouble with the following options, opening the program in Idle is discussed in Starting Idle.

• In Windows, you can display your folder contents, and double click on madlib.py to start the program.

• On a Mac, you can Control-click on madlib.py in the Finder, select Open With, and choose the Python Launcher for your Python 3 version. When you are done with the program, close the terminal window.

• In Linux you may be able to open a terminal window, change into your Python examples directory, and enter the command

python madlib.py


or possibly to distinguish Python 2 and 3:

python3 madlib.py


If neither of these work, get help.

Try the program a second time and make different responses.

1.2.4. A Sample Program, Explained¶

If you want to get right to the detailed explanations of writing your own Python, you can skip to the next section Starting Idle. If you would like an overview of a working program, even if all the explanations do not make total sense yet, read on.

Here is the text of the madlib.py program, followed by line-by-line brief explanations. Do not worry if you not totally understand the explanations! Try to get the gist now and the details later. The numbers on the right are not part of the program file. They are added for reference in the comments below:

#! /usr/bin/env python3                                  0
'''                                                      1
String Substitution for a Mad Lib                        2
Adapted from code by Kirby Urner                         3
'''                                                      4
5
storyFormat = '''                                        6
Once upon a time, deep in an ancient jungle,             7
there lived a {animal}.  This {animal}                   8
liked to eat {food}, but the jungle had                  9
very little {food} to offer.  One day, an               10
explorer found the {animal} and discovered              11
it liked {food}.  The explorer took the                 12
{animal} back to {city}, where it could                 13
eat as much {food} as it wanted.  However,              14
the {animal} became homesick, so the                    15
explorer brought it back to the jungle,                 16
leaving a large supply of {food}.                       17
18
The End                                                 19
'''                                                     20
21
def tellStory():                                        22
userPicks = dict()                                  23
story = storyFormat.format(**userPicks)             27
print(story)                                        28
29
'''Prompt for a user response using the cue string, 31
and place the cue-response pair in the dictionary.  32
'''                                                 33
prompt = 'Enter an example for ' + cue + ': '       34
response = input(prompt)                            35
dictionary[cue] = response                          36
37
tellStory()                                             38
input('Press Enter to end the program.')                39


Line By Line Explanation

#! /usr/bin/env python3                                  0


This is not technically a part of the program. It is there to tell the operating system what version of Python to choose, since the older Python 2 is incompatible with the newer Python 3. We will mostly run programs from inside the Idle programming environment, where this line is not needed. To run just by clicking on the program in an operating system window, however, the line is important if your computer also has Python 2.

'''                                                      1
String Substitution for a Mad Lib                        2
Adapted from code by Kirby Urner                         3
'''                                                      4


1-4: There is multi-line text enclosed in triple quotes. Quoted text is called a string. A string at the very beginning of a program like this is documentation for the file.

5,21,29,37: Blank lines are included for human readability to separate logical parts. The computer ignores the blank lines.

storyFormat = '''                                        6
Once upon a time, deep in an ancient jungle,             7
there lived a {animal}.  This {animal}                   8
liked to eat {food}, but the jungle had                  9
very little {food} to offer.  One day, an               10
explorer found the {animal} and discovered              11
it liked {food}.  The explorer took the                 12
{animal} back to {city}, where it could                 13
eat as much {food} as it wanted.  However,              14
the {animal} became homesick, so the                    15
explorer brought it back to the jungle,                 16
leaving a large supply of {food}.                       17
18
The End                                                 19
'''                                                     20


6: The equal sign tells the computer that this is an assignment statement. The computer will now associate the value of the expression between the triple quotes, a multi-line string, with the name on the left, storyFormat.

7-20: These lines contain the body of the string and the ending triple quotes. This storyFormat string contains some special symbols making it a format string, unlike the string in lines 1-4. The storyFormat string will be used later to provide a format into which substitutions are made. The parts of the string enclosed in braces are places a substitute string will be inserted later. The substituted string will come from a custom dictionary that will contain the user’s definitions of these words. The words in the braces: {animal}, {food}, {city}, indicate that animal, food, and city are words in a dictionary. This custom dictionary will be created in the program and contain the user’s definitions of these words. These user’s definitions will be substituted later in the format string where each {...} is currently.

def tellStory():                                        22
userPicks = dict()                                  23
story = storyFormat.format(**userPicks)             27
print(story)                                        28


22: def is short for definition of a function. This line is the heading of a definition, which makes the name tellStory becomes defined as a short way to refer to the sequence of statements that start indented on line 23, and continue through line 28.

23: The equal sign tells the computer that this is another assignment statement. The computer will now associate the name userPicks with a new empty dictionary created by the Python code dict().

24-26: addPick is the name for a function defined by the sequence of instructions on lines 29-31, used to add another definition to a dictionary, based on the user’s input. The result of these three lines is to add definitions for each of the three words animal, food, and city to the dictionary called userPicks.

27: Assign the name story to a string formed by substituting into storyFormat using definitions from the dictionary userPicks, to give the user’s customized story.

28: This is where all the work becomes visible: Print the story string to the screen.

def addPick(cue, dictionary):                           30
'''Prompt for a user response using the cue string, 31
and place the cue-response pair in the dictionary.  32
'''                                                 33
prompt = 'Enter an example for ' + cue + ': '       34
response = input(prompt)                            35
dictionary[cue] = response                          36


30: This line is the heading of a definition, which gives the name addPick as a short way to refer to the sequence of statements indented on line 34-36. The name addPick is followed by two words in parenthesis, cue and dictionary. These two words are associated with an actual cue word and dictionary given when this definition is invoked in lines 24-26.

31-33: A documentation comment for the addPick definition.

34: The plus sign here is used to concatenate parts of the string assigned to the name prompt. The current value of cue is placed into the string.

35: The right-hand-side of this equal sign causes an interaction with the user, to input text from the keyboard: The prompt string is printed to the computer screen, and the computer waits for the user to enter a line of text. That line of text then becomes a string inside the program. This string is assigned to the name response.

36: The left-hand-side of the equal sign is a reference to the definition of the cue word in the dictionary. The whole line ends up making the definition of the current cue word become the response typed by the user.

tellStory()                                             38
input('Press Enter to end the program.')                39


38: The definition of tellStory above does not make the computer do anything besides remember what the instruction tellStory means. It is only in this line, with the name, tellStory, followed by parentheses, that the whole sequence of remembered instructions are actually carried out.

39: This line is only here to accommodate running the program in Windows by double clicking on its file icon. Without this line, the story would be displayed and then the program would end, and Windows would make it immediately disappear from the screen! This line forces the program to continue being displayed until there is another response from the user, and meanwhile the user may look at the output from tellStory.

1.2.5. Starting Idle¶

The program that translates Python instructions and then executes them is the Python interpreter.

This interpreter is embedded in a number of larger programs that make it particularly easy to develop Python programs. Such a programming environment is Idle, and it is a part of the standard distribution of Python.

It is possible to open the Idle app directly:

• Start a search:

Windows: Press the Windows key or go to the start menu.

Mac: open Spotlight (Command-spacebar).

• Enter Idle in the search field. Select the Idle app.

If you have Linux:

The approach depends on the installation. In Ubuntu, you should find Idle in the Programming section of the Applications menu. You are better starting idle from a terminal, with the current directory being your Python folder. You may need a special name set up to distinguish idle for versions 2 and 3, for instance idle3 for version 3.X.

We will discuss later that if you want to edit files, this is not the best way to start Idle, since the associated folder is not the one you want, but it works for now.

1.2.6. Windows in Idle¶

Idle has several parts you may choose to display, each with its own window. Depending on the configuration, Idle can start up showing either of two windows, an Edit Window or a Python Shell Window. You are likely to first see an Edit window, whose top left corner looks something like in Windows:

For more on the Edit Window, see The Idle Editor and Execution.

If you see this Edit Window with its Run menu on top, go to the Run menu and choose PYTHON SHELL to open a Python Shell Window for now. Then you may close the Edit Window. For now we will just be using the Python Shell.

Either initially, or after explicitly opening it, you should now see the Python Shell window, with a menu like the following, though the text may be slightly different:

In the Shell the last line should look like

>>>


The >>> is the prompt, telling you Idle is waiting for you to type something. Continuing on the same line enter

6+3


Be sure to end with the Enter key. After the Shell responds, you should see something like

>>> 6+3
9
>>>


The shell evaluates the line you entered, and prints the result. You see Python does arithmetic. At the end you see a further prompt >>> where you can enter your next line.... The result line, showing 9, that is produced by the computer, does not start with >>>.

When we get to whole programs, we will use The Idle Editor and Execution.