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5.1. Basics of Functions

Functions are named codeblocks that are only executed when called and perform a specific task.

Syntax

Functions are defined using the def keyword followed by name of function and any arguments or parameters the function takes. The body of function is indented inside def.

For example, the following function calculates and returns the sum of two numbers: 

def add(n1, n2):
    return n1 + n2

Inspecting above code:

  • add is the name of function.
  • n1 and n2 are known as parameters or arguments.

Calling a function

If you define a function in your program and simply run the program, you won't get any output. This is because functions have to be called in order to be used.

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2
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5
def add(n1, n2):
    return n1 + n2

result = add(5, 2)
print(result)

7

Here,

  • The function is called on line 4.
  • While calling, the value of n1 parameter is set to 5 and n2 set is to 2.
  • The function's body is executed and the value 7 is returned (as a result of line 2) which is then assigned to result variable.

Arguments

A function can take one or more parameters or arguments. An argument is essentially an input to a function that it uses while processing to generate the output.

def greet():
    print('From greet() function:')
    print('Hello, World!')

def greet_with_name(name):
    print('From greet_with_name() function:')
    print(f'Hello, {name}!')

greet()
greet_with_name('John')

From greet() function:
Hello, World!
From greet_with_name() function:
Hello, John!

In above code, there are two functions. greet function does not take any arguments while greet_with_name takes one argument: name.

More on arguments is covered in section 5.2. Arguments.

Application

Here is an example of a comparatively complex function that calculates the factorial of a number:

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def factorial(n):
    fact = 1

    for i in range(1, n + 1):
        fact = n * i

    return fact

print(factorial(3))
print(factorial(5))
print(factorial(6))

6
120
720

Note

Factorial of a number is the product of that number with the numbers below it down to 1. For example, the factorial of 3 is 3 * 2 * 1 and factorial of 5 is 5 * 4 * 3 * 2 * 1.

Note that factorial of 1 and 0 is 1, and negative and fractional numbers don't have any factorial.

In this code, line 1-7 are defining the function and line 9-11 are calling the function three times.

Note how the function is reused by calling it multiple times which is an application of function. We don't have to rewrite the factorial logic again when using it, we simply define the logic once as a function and call it whenever we want using the function name.

Another benefit of functions apart from reusability is organizing a large program in smaller functions with each function performing a specific task. This way, it is easier to read, test and debug.