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Introduction to Functions in Python

What is a Function in Python?​

In Python, a function is a group of related statements that perform a specific task. Functions help break our program into smaller and modular chunks. As our program grows larger and larger, functions make it more organized and manageable. Furthermore, it avoids repetition and makes the code reusable.

Syntax of Function​

def function_name(parameters):
    """docstring"""
    statement(s)

Above shown is a function definition that consists of the following components.

  1. Keyword def marks the start of the function header.
  2. A function name to uniquely identify the function. Function naming follows the same rules of writing identifiers in Python.
  3. Parameters (arguments) through which we pass values to a function. They are optional.
  4. A colon : to mark the end of the function header.
  5. Optional documentation string (docstring) to describe what the function does.
  6. One or more valid python statements that make up the function body. Statements must have the same indentation level (usually 4 spaces).
  7. An optional return statement to return a value from the function.

How to call a function in python?​

Once we have defined a function, we can call it from another function, program or even the Python prompt. To call a function we simply type the function name with appropriate parameters.

def greet(name):
    """This function greets to
    the person passed in as
    parameter"""
    print("Hello, " + name + ". Good morning!")

greet('Paul')

The Return statement​

The return statement is used to exit a function and go back to the place from where it was called. This statement can contain an expression that gets evaluated and the value is returned. If there is no expression in the statement or the return statement itself is not present inside a function, then the function will return the None object.

def absolute_value(num):
    """This function returns the absolute
    value of the entered number"""

    if num >= 0:
        return num
    else:
        return -num

print(absolute_value(2))
print(absolute_value(-4))

Scope and Lifetime of variables​

Scope of a variable is the portion of a program where the variable is recognized. Parameters and variables defined inside a function are not visible from outside the function. Hence, they have a local scope.

The lifetime of a variable is the period throughout which the variable exists in the memory. The lifetime of variables inside a function is as long as the function executes.

They are destroyed once we return from the function. Hence, a function does not remember the value of a variable from its previous calls.

Here is an example to illustrate the scope of a variable inside a function.

def my_func():
    x = 10
    print("Value inside function:",x)

x = 20
my_func()
print("Value outside function:",x)

In the above program, we define x as a global variable and define a function my_func() to print the value of x. We then call this function. Finally, we print the value of x outside the function.

This is the basic introduction to Python functions. We learned about parameters, return statement, scope and lifetime of variables in functions. Functions make our program modular and efficient. They are reusable and make the program more readable.