Writing Functions

As with other languages, you can write functions in Python to avoid repeating code and to improve flow of your programs. Functions use a similar indented syntax to the loops introduced earlier, with a colon after the function name and the following lines indented. As with loops, it is considered good python style to indent with four blank space. Let’s start by defining a simple function:

def my_function():
    print "Hello World!"

This can be called with my_function().

Note

It is considered good python style to have function names all in lower case, with words separated by underscore if it improves readability.

This is a rather boring function that does not take or return any arguments. Let’s make things a little more complicated:

def add_two_numbers(x1, x2):
    total = x1 + x2
    return total

This function simply adds the two numbers that it is given as arguments. As you expect, we can pass integers, floats and strings to a function. Actually, we can pass any Python object, even functions! Consider the following function:

def print_argument(x=5):
    print x

In this example, we have defined what the default value of the x variable in the function. This is called a keyword argument. If you try and call this function without any arguments, the default will be used. If you call it with an argument, the specified value will be used.

Exercise

Write a function that prints the value of input argument multiplied by 7. If no argument is given, it should print 7. If the input argument is 6, it should print “The Answer to the Ultimate Question of Life, The Universe, and Everything.”

Click to Show/Hide Solution

def multiply_by_7(x=1):
    if x == 6:
        print ("The Answer to the Ultimate Question of Life, "
               "the Universe and Everything.")
    else:
        print x*7

Functions can also have both keyword and standard arguments:

>>> def slicer(seq, start=None, stop=None, step=None):
...    """ Implement basic Python slicing."""
...    return seq[start:stop:step]

>>> l = [1, 2, 3, 4, 5, 6]
>>> slicer()
TypeError: slicer() takes at least 1 argument (0 given)
>>> slicer(l)
[1, 2, 3, 4, 5, 6]
>>> slicer(l, stop=2)
[1, 2]
>>> slicer(l, stop=5, step=2)
[1, 3, 5]

Docstrings

You can use a single string after the first line of a function to describe what the function does and its parameters. The general convention is:

>>> def funcname(params):
...     """Concise one-line sentence describing the function.
...
...     Extended summary which can contain multiple paragraphs.
...     """
...     # function body
...     pass

Then the docstring for that function can be accessed with:

>>> help(funcname)

In IPython you can access the same documentation using funcname? (Try it now!).