Python Functions (original) (raw)

**Python Functions is a block of statements that return the specific task. The idea is to put some commonly or repeatedly done tasks together and make a function so that instead of writing the same code again and again for different inputs, we can do the function calls to reuse code contained in it over and over again.

Some **Benefits of Using Functions

Python Function Declaration

The syntax to declare a function is:

Python Functions

Syntax of Python Function Declaration

Types of Functions in Python

Below are the different types of functions in Python:

**Creating a Function in Python

We can define a function in Python, using the **def keyword. We can add any type of functionalities and properties to it as we require. By the following example, we can understand how to write a function in Python. In this way we can create Python function definition by using def keyword.

Python `

A simple Python function

def fun(): print("Welcome to GFG")

`

Calling a Function in Python

After creating a function in Python we can call it by using the name of the functions Python followed by parenthesis containing parameters of that particular function. Below is the example for calling def function Python.

Python `

A simple Python function

def fun(): print("Welcome to GFG")

Driver code to call a function

fun()

`

Python Function with Parameters

If you have experience in C/C++ or Java then you must be thinking about the _return type of the function and _data type of arguments. That is possible in Python as well (specifically for Python 3.5 and above).

**Python Function Syntax with Parameters

def function_name(parameter: data_type) -> return_type:
“””Docstring”””
# body of the function
return expression

The following example uses arguments and parameters that you will learn later in this article so you can come back to it again if not understood.

Python `

def add(num1: int, num2: int) -> int: """Add two numbers""" num3 = num1 + num2

return num3

Driver code

num1, num2 = 5, 15 ans = add(num1, num2) print(f"The addition of {num1} and {num2} results {ans}.")

`

Output

The addition of 5 and 15 results 20.

**Note: The following examples are defined using syntax 1, try to convert them in syntax 2 for practice.

Python `

some more functions

def is_prime(n): if n in [2, 3]: return True if (n == 1) or (n % 2 == 0): return False r = 3 while r * r <= n: if n % r == 0: return False r += 2 return True print(is_prime(78), is_prime(79))

`

Python Function Arguments

Arguments are the values passed inside the parenthesis of the function. A function can have any number of arguments separated by a comma.

In this example, we will create a simple function in Python to check whether the number passed as an argument to the function is even or odd.

Python `

A simple Python function to check

whether x is even or odd

def evenOdd(x): if (x % 2 == 0): print("even") else: print("odd")

Driver code to call the function

evenOdd(2) evenOdd(3)

`

Types of Python Function Arguments

Python supports various types of arguments that can be passed at the time of the function call. In Python, we have the following function argument types in Python:

Let’s discuss each type in detail.

**Default Arguments

A default argument is a parameter that assumes a default value if a value is not provided in the function call for that argument. The following example illustrates Default arguments to write functions in Python.

Python `

Python program to demonstrate

default arguments

def myFun(x, y=50): print("x: ", x) print("y: ", y)

Driver code (We call myFun() with only

argument)

myFun(10)

`

Like C++ default arguments, any number of arguments in a function can have a default value. But once we have a default argument, all the arguments to its right must also have default values.

**Keyword Arguments

The idea is to allow the caller to specify the argument name with values so that the caller does not need to remember the order of parameters.

Python `

Python program to demonstrate Keyword Arguments

def student(firstname, lastname): print(firstname, lastname)

Keyword arguments

student(firstname='Geeks', lastname='Practice') student(lastname='Practice', firstname='Geeks')

`

Output

Geeks Practice Geeks Practice

Positional Arguments

We used the Position argument during the function call so that the first argument (or value) is assigned to name and the second argument (or value) is assigned to age. By changing the position, or if you forget the order of the positions, the values can be used in the wrong places, as shown in the Case-2 example below, where 27 is assigned to the name and Suraj is assigned to the age.

Python `

def nameAge(name, age): print("Hi, I am", name) print("My age is ", age)

You will get correct output because

argument is given in order

print("Case-1:") nameAge("Suraj", 27)

You will get incorrect output because

argument is not in order

print("\nCase-2:") nameAge(27, "Suraj")

`

Output

Case-1: Hi, I am Suraj My age is 27

Case-2: Hi, I am 27 My age is Suraj

Arbitrary Keyword Arguments

In Python Arbitrary Keyword Arguments, *args, and **kwargs can pass a variable number of arguments to a function using special symbols. There are two special symbols:

**Example 1: Variable length non-keywords argument

Python `

Python program to illustrate

*args for variable number of arguments

def myFun(*argv): for arg in argv: print(arg)

myFun('Hello', 'Welcome', 'to', 'GeeksforGeeks')

`

Output

Hello Welcome to GeeksforGeeks

**Example 2: Variable length keyword arguments

Python `

Python program to illustrate

*kwargs for variable number of keyword arguments

def myFun(**kwargs): for key, value in kwargs.items(): print("%s == %s" % (key, value))

Driver code

myFun(first='Geeks', mid='for', last='Geeks')

`

Output

first == Geeks mid == for last == Geeks

Docstring

The first string after the function is called the Document string or Docstring in short. This is used to describe the functionality of the function. The use of docstring in functions is optional but it is considered a good practice.

The below syntax can be used to print out the docstring of a function.

**Syntax: print(function_name.__doc__)

**Example: Adding Docstring to the function

Python `

A simple Python function to check

whether x is even or odd

def evenOdd(x): """Function to check if the number is even or odd"""

if (x % 2 == 0):
    print("even")
else:
    print("odd")

Driver code to call the function

print(evenOdd.doc)

`

Output

Function to check if the number is even or odd

Python Function within Functions

A function that is defined inside another function is known as the **inner function or **nested function. Nested functions can access variables of the enclosing scope. Inner functions are used so that they can be protected from everything happening outside the function.

Python `

Python program to

demonstrate accessing of

variables of nested functions

def f1(): s = 'I love GeeksforGeeks'

def f2():
    print(s)
    
f2()

Driver's code

f1()

`

Output

I love GeeksforGeeks

Anonymous Functions in Python

In Python, an anonymous function means that a function is without a name. As we already know the def keyword is used to define the normal functions and the lambda keyword is used to create anonymous functions.

Python `

Python code to illustrate the cube of a number

using lambda function

def cube(x): return xxx

cube_v2 = lambda x : xxx

print(cube(7)) print(cube_v2(7))

`

Recursive Functions in Python

**Recursion in Python refers to when a function calls itself. There are many instances when you have to build a recursive function to solve **Mathematical and Recursive Problems.

Using a recursive function should be done with caution, as a recursive function can become like a non-terminating loop. It is better to check your exit statement while creating a recursive function.

Python `

def factorial(n): if n == 0:
return 1 else: return n * factorial(n - 1)

print(factorial(4))

`

Here we have created a recursive function to calculate the factorial of the number. You can see the end statement for this function is when n is equal to 0.

Return Statement in Python Function

The function return statement is used to exit from a function and go back to the function caller and return the specified value or data item to the caller. **The syntax for the return statement is:

return [expression_list]

The return statement can consist of a variable, an expression, or a constant which is returned at the end of the function execution. If none of the above is present with the return statement a None object is returned.

**Example: Python Function Return Statement

Python `

def square_value(num): """This function returns the square value of the entered number""" return num**2

print(square_value(2)) print(square_value(-4))

`

**Pass by Reference and Pass by Value

One important thing to note is, in Python every variable name is a reference. When we pass a variable to a function Python, a new reference to the object is created. Parameter passing in Python is the same as reference passing in Java.

Python `

Here x is a new reference to same list lst

def myFun(x): x[0] = 20

Driver Code (Note that lst is modified

after function call.

lst = [10, 11, 12, 13, 14, 15] myFun(lst) print(lst)

`

Output

[20, 11, 12, 13, 14, 15]

When we pass a reference and change the received reference to something else, the connection between the passed and received parameters is broken. For example, consider the below program as follows:

Python `

def myFun(x):

# After below line link of x with previous
# object gets broken. A new object is assigned
# to x.
x = [20, 30, 40]

Driver Code (Note that lst is not modified

after function call.

lst = [10, 11, 12, 13, 14, 15] myFun(lst) print(lst)

`

Output

[10, 11, 12, 13, 14, 15]

Another example demonstrates that the reference link is broken if we assign a new value (inside the function).

Python `

def myFun(x):

# After below line link of x with previous
# object gets broken. A new object is assigned
# to x.
x = 20

Driver Code (Note that x is not modified

after function call.

x = 10 myFun(x) print(x)

`

**Exercise: Try to guess the output of the following code.

Python `

def swap(x, y): temp = x x = y y = temp

Driver code

x = 2 y = 3 swap(x, y) print(x) print(y)

`

Quiz: