Coding Style

What will we cover?
  • Several new uses for comments
  • How to layout code using indentation to improve readability
  • An introduction to the use of modules for storing our programs

Comments

I've already spoken about comments in the 'More Sequences' section. However there are some other things we can do with comments and I'll enlarge on those here:

Version history information

It is good practice to create a file header at the start of each file. This should provide details such as the creation date, author, date of last change, version and a general description of the contents. Often a log of changes. This block will appear as a comment:


#############################
# Module:   Spam.py
# Author:   A.J.Gauld
# Date:     1999/09/03
# Version:  Draft 0.4
'''
This module provides a Spam class which can be
combined with any other type of Food object to create
interesting meal combinations.
'''
###############################
# Log:
# 1999/09/01    AJG - File created
# 1999/09/02    AJG - Fixed bug in pricing strategy
# 1999/09/02    AJG - Did it right this time!
# 1999/09/03    AJG - Added broiling method(cf Change Req #1234)
################################
import sys, string, food
...

Thus when you first open a file it should contain a nice summary of what the file is for, what's changed over time and who did it and when. This is particularly important if you are working on a team project and need to know who to ask about the design or the changes. There are version control tools available that can help automate the production of some of this documentation, but they are outside the scope of this tutorial.

Note that I put the description in between two sets of triple quotes. This is a Python specific trick known as a documentation string that makes the description available to Pythons built-in help function as we'll see shortly.

Commenting out redundant code

This technique is often used to isolate a faulty section of code. For example, assume a program reads some data, processes it, prints the output and then saves the results back to the data file. If the results are not what we expect it would be useful to temporarily prevent the (erroneous)data being saved back to the file and thus corrupting it. We could simply delete the relevant code but a less radical approach is simply to convert the lines into comments like so:

data = readData(datafile)
for item in data:
    results.append(calculateResult(item))
printResults(results)
######################
# Comment out till bug in calculateResult fixed
# for item in results:
#     dataFile.save(item)
######################
print 'Program terminated'

Once the fault has been fixed we can simply delete the comment markers to make the code active once more. Some editing tools, including IDLE, have menu options to comment out a selected block of code, and to uncomment it later.

Documentation strings

All languages allow you to create comments to document what a function or module does, but a few, such as Python and Smalltalk, go one stage further and allow you to document the function in a way that the language/environment can use to provide interactive help while programming. In Python this is done using the """documentation""" string style:

class Spam:
    """A meat for combining with other foods
    
    It can be used with other foods to make interesting meals.
    It comes with lots of nutrients and can be cooked using many
    different techniques"""
    
    def __init__(self):
       pass   # ie. it does nothing!
        
print Spam.__doc__

Note: We can access the documentation string by printing the special __doc__ variable. Modules, Functions and classes/methods can all have documentation strings. For example try:

import sys
print sys.__doc__

Since Python version 2.2 there is also a help() function within Python that will search for and print out any helpful documentation on a Python symbol. For example to see the help on sys.exit we can do this at the Python prompt:

>>> import sys
>>> help (sys.exit)
Help on built-in function exit:

exit(...)
    exit([status])

    Exit the interpreter by raising SystemExit(status).
    If the status is omitted or None, it defaults to zero (i.e., success).
    If the status is numeric, it will be used as the system exit status.
    If it is another kind of object, it will be printed and the system
    exit status will be one (i.e., failure).
(END)

To get out of help mode hit the letter 'q'(for quit) when you see the (END) marker. If more than one page of help is present you can hit the space bar to page through it. If you are using IDLE, or other IDE, then you likely won't see the (END) marker rather it will simply display all the text and you need to use the scroll bars to go back and read it.

Block Indentation

This is one of the most hotly debated topics in programming. It almost seems that every programmer has his/her own idea of the best way to indent code. As it turns out there have been some studies done that show that at least some factors are genuinely important beyond cosmetics - ie they actually help us understand the code better.

The reason for the debate is simple. In most programming languages the indentation is purely cosmetic, an aid to the reader. (In Python it is, in fact, needed and is essential to proper working of the program!) Thus:

< script type="text/vbscript">
For I = 1 TO 10
    MsgBox I
Next
</script>

Is exactly the same as:

< script type="text/vbscript">
For I = 1 TO 10
MsgBox I
Next
</script>

so far as the VBScript interpreter is concerned. It's just easier for us to read with indentation.

The key point is that indentation should reflect the logical structure of the code thus visually it should follow the flow of the program. To do that it helps if the blocks look like blocks thus:

XXXXXXXXXXXXXXXXXXXX
    XXXXXXXXXXXXXXXX
    XXXXXXXXXXXXXXXX
    XXXXXXXXXXXXXXXX

which reads better than:

XXXXXXXXXXXXXXXXXXXXX
  XXXXX
    XXXXXXXXXXXX
    XXXXXXXXXXXX
    XXXXXXXXXXXX
  XXXXX

because it's clearly all one block. Studies have shown significant improvements in comprehension when indenting reflects the logical block structure. In the small samples we've seen so far it may not seem important but when you start writing programs with hundreds or thousands of lines it will become much more so.

Variable Names

The variable names we have used so far have been fairly meaningless, mainly because they had no meaning but simply illustrated techniques. In general it's much better if your variable names reflect what you want them to represent. For example in our times table exercise we used 'multiplier' as the variable to indicate which table we were printing. That is much more meaningful than simply 'm' - which would have worked just as well and been less typing.

Its a trade-off between comprehensibility and effort. Generally the best choice is to go for short but meaningful names. Too long a name becomes confusing and is difficult to get right consistently(for example I could have used the_table_we_are_printing instead of multiplier but it's far too long and not really much clearer.

Saving Your Programs

While the Python interactive interpreter prompt (>>>) is very useful for trying out ideas quickly, it loses all you type the minute you exit. In the longer term we want to be able to write programs and then run them over and over again. To do this in Python we create a text file with an extension .py (this is a convention only, you could use anything you like. But it's a good idea to stick with convention in my opinion...). You can then run your programs from an Operating System command prompt by typing:

$ python spam.py

Where spam.py is the name of your Python program file and the $ is the operating system prompt.

The other advantage of using files to store the programs is that you can edit mistakes without having to retype the whole fragment or, in IDLE, cursor all the way up past the errors to reselect the code. IDLE supports having a file open for editing and running it from the 'Edit|Run module' menu.

From now on I won't normally be showing the >>> prompt in examples, I'll assume you are creating the programs in a separate file and running them either within IDLE or from a command prompt (my personal favourite).

Note for Windows users

Under Windows you can set up a file association for files ending .py within Explorer. This will allow you to run Python programs by simply double clicking the file's icon. This should already have been done by the installer. You can check by finding some .py files and trying to run them. If they start (even with a Python error message) it's set up. The problem you will likely run into at this point is that the files will run in a DOS box and then immediately close, so fast you scarcely even see them! There are a couple of options:

  • The first way is simplest and involves putting the following line of code at the end of each program:

    raw_input("Hit ENTER to quit")

    Which simply displays the message and waits for the user to hit the ENTER or Return key. We will discuss raw_input() in a future topic.

  • The second technique uses the Windows Explorer settings. The procedure is fairly standard but may vary according to the version of Windows you have. I will describe Windows XP Home.
    First select a .py file and go to the Tools->Folder Options menu item. In the dialog box select the File Types tab. Scroll down till you find the PY file type and click on it to select it. Click the Advanced button at the bottom. In the new dialog select open from the list and click Edit... In the new dialog you should see the Application... line say something like:

    E:\PYTHON22\python.exe "%1" %*

    Edit it to add a -i after the python.exe, like this:

    E:\PYTHON22\python.exe -i "%1" %*

    Now close all the dialogs.

    This will stop Python from exiting at the end of your program and leave you at the >>> prompt where you can inspect variable values etc, or just exit manually. (An alternative trick is to add a new option called Test alongside Open. This allows you to Right Click in explorer and choose open to run the program and it close automatically and choose Test to run the program finishing in Python. The choice is yours.)


Note for Unix users

The first line of a Python script file should contain the sequence #! followed by the full path of python on your system. You can find that by typing, at your shell prompt:

$ which python

On my system the line looks like:

#! /usr/local/bin/python

This will allow you to run the file without calling Python at the same time (after you set it to be executable via chmod - but you knew that already I'm sure!):

$ spam.py

You can use an even more convenient trick on most modern Unix systems (including all Linux distros) which replaces the path information with /usr/bin/env/python, like this:

#! /usr/bin/env/python

That will find where Python is in your path automatically. The only snag is where you may have two or more different versions of Python installed and the script will only work with one of them (maybe it uses a brand new language feature, say), in that case you will be better with the full path technique.

This #! line doesn't do any harm under Windows/Mac either, since it just looks like a comment, so those users can put it in too, if their code is likely to ever be run on a unix box.

VBScript & JavaScript

You VBScript and JavaScript users can ignore the above, you've already been saving your programs as files, it's the only way to get them to work!

Points to remember
  • Comments can be used to temporarily prevent code from executing, which is useful when testing or 'debugging' code.
  • Comments can be used to provide an explanatory header with version history of type file.
  • Documentation strings can be used to provide run-time information about a module and the objects within it.
  • Indentation of blocks of code helps the reader see clearly the logical structure of the code.
  • By typing a python program into a file instead of at the Python '>>>' prompt the program can be saved and run on demand by typing $ python progname.py at the command prompt or by double clicking the filename within an Explorer window on Windows.

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