Function in python programming

Python Functions

A function is a block of code that performs a specific task.

Suppose, you need to create a program to create a circle and color it. You can create two functions to solve this problem:

Dividing a complex problem into smaller chunks makes our program easy to understand and reuse.

Types of function

There are two types of function in Python programming:

• Standard library functions — These are built-in functions in Python that are available to use.
• User-defined functions — We can create our own functions based on our requirements.

Python Function Declaration

The syntax to declare a function is:

def function_name(arguments): # function body return

• def — keyword used to declare a function
• function_name — any name given to the function
• arguments — any value passed to function
• return (optional) — returns value from a function

def greet(): print('Hello World!')

Here, we have created a function named greet() . It simply prints the text Hello World! .

This function doesn’t have any arguments and doesn’t return any values. We will learn about arguments and return statements later in this tutorial.

Calling a Function in Python

In the above example, we have declared a function named greet() .

def greet(): print('Hello World!')

Now, to use this function, we need to call it.

Here’s how we can call the greet() function in Python.

Example: Python Function

def greet(): print('Hello World!') # call the function greet() print('Outside function')

Hello World! Outside function

In the above example, we have created a function named greet() . Here’s how the program works:

• When the function is called, the control of the program goes to the function definition.
• All codes inside the function are executed.
• The control of the program jumps to the next statement after the function call.

Python Function Arguments

As mentioned earlier, a function can also have arguments. An argument is a value that is accepted by a function. For example,

# function with two arguments def add_numbers(num1, num2): sum = num1 + num2 print('Sum: ',sum) # function with no argument def add_numbers(): # code

If we create a function with arguments, we need to pass the corresponding values while calling them. For example,

# function call with two values add_numbers(5, 4) # function call with no value add_numbers()

Here, add_numbers(5, 4) specifies that arguments num1 and num2 will get values 5 and 4 respectively.

Example 1: Python Function Arguments

# function with two arguments def add_numbers(num1, num2): sum = num1 + num2 print("Sum: ",sum) # function call with two values add_numbers(5, 4) # Output: Sum: 9

In the above example, we have created a function named add_numbers() with arguments: num1 and num2 .

We can also call the function by mentioning the argument name as:

add_numbers(num1 = 5, num2 = 4)

In Python, we call it Keyword Argument (or named argument). The code above is equivalent to

The return Statement in Python

A Python function may or may not return a value. If we want our function to return some value to a function call, we use the return statement. For example,

def add_numbers(): . return sum

Here, we are returning the variable sum to the function call.

Note: The return statement also denotes that the function has ended. Any code after return is not executed.

Example 2: Function return Type

# function definition def find_square(num): result = num * num return result # function call square = find_square(3) print('Square:',square) # Output: Square: 9

In the above example, we have created a function named find_square() . The function accepts a number and returns the square of the number.

Example 3: Add Two Numbers

# function that adds two numbers def add_numbers(num1, num2): sum = num1 + num2 return sum # calling function with two values result = add_numbers(5, 4) print('Sum: ', result) # Output: Sum: 9

Python Library Functions

In Python, standard library functions are the built-in functions that can be used directly in our program. For example,

• print() — prints the string inside the quotation marks
• sqrt() — returns the square root of a number
• pow() — returns the power of a number

These library functions are defined inside the module. And, to use them we must include the module inside our program.

For example, sqrt() is defined inside the math module.

Example 4: Python Library Function

import math # sqrt computes the square root square_root = math.sqrt(4) print("Square Root of 4 is",square_root) # pow() comptes the power power = pow(2, 3) print("2 to the power 3 is",power)

Square Root of 4 is 2.0 2 to the power 3 is 8

In the above example, we have used

• math.sqrt(4) — to compute the square root of 4
• pow(2, 3) — computes the power of a number i.e. 2 3

Here, notice the statement,

Since sqrt() is defined inside the math module, we need to include it in our program.

Benefits of Using Functions

1. Code Reusable — We can use the same function multiple times in our program which makes our code reusable. For example,

# function definition def get_square(num): return num * num for i in [1,2,3]: # function call result = get_square(i) print('Square of',i, '=',result)

Square of 1 = 1 Square of 2 = 4 Square of 3 = 9

In the above example, we have created the function named get_square() to calculate the square of a number. Here, the function is used to calculate the square of numbers from 1 to 3.

Hence, the same method is used again and again.

2. Code Readability — Functions help us break our code into chunks to make our program readable and easy to understand.

Table of Contents

• Introduction
• Types of function
• Python Function Declaration
• Calling a Function in Python
• Example: Python Function
• Python Function Arguments
• Example 1: Python Function Arguments
• The return Statement in Python
• Example 2: Function return Type
• Python Library Functions
• Example 4: Python Library Function
• Benefits of Using Functions

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Python Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

Example

Calling a Function

To call a function, use the function name followed by parenthesis:

Example

def my_function():
print(«Hello from a function»)

my_function()

Arguments

Information can be passed into functions as arguments.

Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.

The following example has a function with one argument (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

def my_function(fname):
print(fname + » Refsnes»)

my_function(«Emil»)
my_function(«Tobias»)
my_function(«Linus»)

Arguments are often shortened to args in Python documentations.

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that are passed into a function.

From a function’s perspective:

A parameter is the variable listed inside the parentheses in the function definition.

An argument is the value that is sent to the function when it is called.

Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):
print(fname + » » + lname)

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):
print(fname + » » + lname)

Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly:

Example

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):
print(«The youngest child is » + kids[2])

my_function(«Emil», «Tobias», «Linus»)

Arbitrary Arguments are often shortened to *args in Python documentations.

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):
print(«The youngest child is » + child3)

my_function(child1 = «Emil», child2 = «Tobias», child3 = «Linus»)

The phrase Keyword Arguments are often shortened to kwargs in Python documentations.

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

Example

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):
print(«His last name is » + kid[«lname»])

my_function(fname = «Tobias», lname = «Refsnes»)

Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

Example

def my_function(country = «Norway»):
print(«I am from » + country)

Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

Example

def my_function(food):
for x in food:
print(x)

fruits = [«apple», «banana», «cherry»]

Return Values

To let a function return a value, use the return statement:

Example

The pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

Example

Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself («recurse»). We use the k variable as the data, which decrements ( -1 ) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

Example

def tri_recursion(k):
if(k > 0):
result = k + tri_recursion(k — 1)
print(result)
else:
result = 0
return result

print(«\n\nRecursion Example Results»)
tri_recursion(6)

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