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Functions are a set of related statements that perform a task. They are defined outside of the main block of code. When required, functions can be used or ‘called’ in a code. Defining a function to perform a task helps avoid redundancy and improves the readability of the script.

Function Definition Syntax

Functions are defined following several conventions. The following is specific to python.

Firstly, the function must be defined. The name of the function is specified after def. To use the function, you may call the name of the function and supply an argument that the function acts on.

Example 1: The function ‘double’ is defined. It takes a number, multiplies the number by 2 and returns the output. The function is applied on the variable ‘x’.

def double(n):
n = n * 2
return n

x = 2
x = double(x)
print ('x:',x)

Example 2: In this example, the function accepts two arguments, ‘s’ and ‘exclaim’. The first input is a string and the second is a boolean. Whatever the first input is will be printed three time. The second input can have the values of 0 or 1. If the second input is 1, then the 3 exclamation marks will be printed. If the second input is 0, then no exclamation marks will be printed.

def repeat(s, exclaim):
Returns the string 's' repeated 3 times.
If exclaim is true, add exclamation marks.
result = s + s + s # can also use "s * 3" which is faster (Why?)
if exclaim:
    result = result + '!!!'
    return result


Notice also how the lines that make up the function or if-statement in both examples are grouped by all having the same level of indentation.

The def keyword defines the function with its parameters. The function name is followed by a set of parentheses, which define how the input should look like. The code within the function is indented. The first line of a function can be a documentation string (“docstring”) that describes what the function does. The docstring can be a single line, or a multi-line description as in the example above. (Yes, those are “triple quotes,” a feature unique to Python!) Variables defined in the function are local to that function, so the “result” in the above function is separate from a “result” variable in another function. The return statement can take an argument, in which case that is the value returned to the caller.