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Python Functions : def statement

This is an on-going series of notes on writing functions in Python. Most of these posts are based on Mark Lutz’s excellent book Learning Python (5th Edition)

A function is a single group of statements that makes our Python code more reusable and structured! So there is every reason to learn how to write functions effectively and to know the various ways in which functions can be created in Python.

The def statement

The most common way to create functions in Python is using def. The def statement simply creates the function object and assigns it to the name written after def. For example,

def average(a,b):
    Computes the average of a and b and returns it
    return (a+b)/2.0
print average(3,4)
>>> 3.5

The name average points to the block of code that computes the average of the two numbers. However, we can do all sorts of things to a function’s name. For example, we can

  • store it in a list

#call the function from the list and print the result
print python_functions[0](6,5)
>>> 5.5
  • assign it to another name and use the name to refer to it in the future
print average_of_two_numbers(9,8)
>>> 8.5

def is executable

… which means that any function you write will not exist until the Python interpreter reads the def statement that first creates the function. We can easily test this by writing two functions and calling the second function from the first function.

def print_geometric_series():
    Prints a geometric series in a nice way
    :return None:
    for integer in geometric_series:
        print integer


def compute_geometric_series(initial, quotient, n):
    Returns a geometric series of length n
    :param initial:
    :param quotient:
    :param n:
    return [initial*quotient**x for x in range(n)]
>>>NameError: global name 'compute_geometric_series' is not defined

As we can see from the NameError that Python throws, the function compute_geometric_series does not exist at the time when print_geometric_series calls it. If you are coming from a Java background, this may seem bizarre, because of course in Java, the order in which the methods appear in the file doesnot matter.