Functions are the first step to creating more complex Python code. At a high level, they are containers of Python code that can be bundled together into a callable block. A simple model function requires a single input, performs an operation on the provided data, and returns a single output. However, this quickly becomes more complicated as functions can run without inputs or optional inputs or do not need to return an output at all.

Functions are an integral component of any programming language and have already been encountered many times in this chapter. For example, the append from list.append() is a function that requires input to add to a list. Once a function is created you can invoke it by its name and pass any required inputs.

When it comes to writing functions, more is better. It is much easier to handle and troubleshoot a bug in a program with many small functions than one big function. Smaller functions make your code more readable and make it easier to find troublesome logic. That being said, functions should contain code for a singular purpose like accessing a certain key in a registry file. There is no need to create functions for each line of code in your script.

The function syntax starts with a definition, def, followed by the name of the function, any inputs in parenthesis, and a colon. Following this format are indented lines of code that will run when the function is called. Optionally, a function may have a return statement to pass information back to the instance it was called from.

>>> def simpleFunction():
...     print 'I am a simple function'
... 
>>> simpleFunction()
I am a simple function

In the preceding example, we've created a function named simpleFunction() that takes no inputs. This function does not return anything and instead prints a string. Let's now take a look at more complicated examples.

Our first function, square(), takes one input and squares it. As this function returns a value, we catch it by assigning it to a variable when invoking the function. This variable, squared_number, will be equal to the returned value of the function. While this is a very succinct function it is very easily broken if given the wrong input. Give the square function some other data type, such as a string, and you will receive a TypeError.

>>> def square(x):
...     return x**2
... 
>>> squared_number = square(4)
>>> print squared_number
16

Our second function, even_or_odd, is slightly more advanced. This function first checks if it is passed an input that is of type integer. If not, it returns immediately, which causes the function to exit. If it is an integer, it performs some logic and prints if the integer is even or odd. Notice that when we try to give the function the string '5', not to be confused with the integer 5, it returns nothing, whereas in the square function, which lacks any input validation checks, this would have caused an error.

>>> def even_or_odd(value):
...     if isinstance(value, int):
...             if value % 2 == 0:
...                     print 'This number is even.'
...             else:
...                     print 'This number is odd.'
...     else:
...             return
... 
>>> values = [1, 3, 4, 6, '5']
>>> for value in values:
...     even_or_odd(value)
... 
This number is odd.
This number is odd.
This number is even.
This number is even.

Aspiring developers should get in the habit of writing functions. As always, functions should be well commented, to help explain their purpose. Functions will be used throughout the book, especially as we begin to develop our forensic scripts.