Python functions allow a scripter to create a repeatable task and have it called frequently throughout the script. When a function is part of a class or module, it means that a certain portion of the script can be called specifically from another script, also known as a module, once imported to execute a task. An additional benefit in using Python functions is the reduction of script size. An often unexpected benefit is the ability to copy functions from one script to another, speeding up development.
Remember that variables hold references to objects, so as the script is written, you are executing tests with variables that reference the value. One fact about this is that the variable can change and can still point to the original value. When a variable is passed to a function through a parameter, it is done as an alias of the original object. So, when you are writing a function, the variable name within the function will often be different—and it should be. This allows easier troubleshooting, cleaner scripts, and more accurate error control.
If you have ever written in another language, the one thing that will surprise you is that there are no curly brackets like these: {}. This is usually done to delineate where the code for a logic test or compound statement stops and begins, such as a loop, an if statement, a function, or even an entire class. Instead, Python uses the aforementioned indentation method, and the deeper the indent, the more nested the statement.
To see a difference between logic tests in Python and other languages, an example of a Perl function known as a subroutine will be shown. An equivalent Python function will also be demonstrated to showcase the differences. This will highlight how Python controls logic flows throughout a script. Feel free to try both of these scripts and see how they work.
Note
The following Python script is slightly longer than the Perl one due to the fact that a return statement was included. This is not necessary for this script, but it is a habit many scripters get into. Additionally, the print statement has been modified, as you can see, to support both version 2.X and version 3.X of Python.
Here is an example of the Perl function:
#!/usr/bin/env perl # Function in Perl sub wargames{ print "Do you want to play a game? "; print "In Perl "; } # Function call wargames();
The following function is the equivalent in Python:
#!/usr/bin/env python # Function in Python def wargames(): print("Do you want to play a game?") print("In Python") return # Function call wargames()
The output of both of these scripts can be seen in this screenshot:
Instead, in Python, curly brackets are used for dictionaries, as previously described in the Python variable section of this chapter.
In a scripting language, a comment is useful for blocking code and/or describing what it is trying to achieve. There are two types of comments in Python: single-line and multiline. Single-line comments make everything from the # sign to the end of the line a comment; it will not be interpreted. If you place code on the line and then follow it up with a comment at the end of the line, the code will still be processed. Here is an example of effective single-line comment usage:
#!/usr/bin/env python #Author: Chris Duffy #Date: 2015 x = 5 #This defines the value of the x followed by a comment
This works, but it may be easier to do the same thing using a multiline comment, as there are two lines within the preceding code are comments. Multiline comments are created by placing three quotes in each line that begins and ends the comment block. The following code shows an example of this:
""" Author: Chris Duffy Date: 2015 """