Python Functions is a block of statements that does a specific task. The idea is to put some commonly or repeatedly done task 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.
Benefits of Using Functions
- Code Reuse
- Reduced code length
- Increased redability of code
Python Function Declaration
The syntax to declare a function is:
Syntax of Python Function DeclarationTypes of Functions in Python
Below are the different types of functions in Python:
- Built-in library function: These are Standard functions in Python that are available to use.
- User-defined function: We can create our own functions based on our requirements.
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.
What is def ?
The def keyword stands for define. It is used to create a user-defined function. It marks the beginning of a function block and allows you to group a set of statements so they can be reused when the function is called.
Syntax:
def function_name(parameters):
# function body
Explanation:
- def: Starts the function definition.
- function_name: Name of the function.
- parameters: Inputs passed to the function (inside ()), optional.
- : : Indicates the start of the function body.
- Indented code: The function body that runs when called.
Example: Let’s understand this with a simple example. Here, we define a function using def that prints a welcome message when called.
Python
def fun():
print("Welcome to GFG")
For more information, refer to this article: Python def Keyword
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
def fun():
print("Welcome to GFG")
# Driver code to call a function
fun()
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.
Syntax for functions with arguments:
def function_name(parameter: data_type) -> return_type:
"""Docstring"""
# body of the function
return expression
data_type and return_type are optional in function declaration, meaning the same function can also be written as:
def function_name(parameter) :
"""Docstring"""
# body of the function
return expression
Let's understand this with an 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
def evenOdd(x: int) ->str:
if (x % 2 == 0):
return "Even"
else:
return "Odd"
print(evenOdd(16))
print(evenOdd(7))
The above function can also be declared without type_hints, like this:
Python
def evenOdd(x):
if (x % 2 == 0):
return "Even"
else:
return "Odd"
print(evenOdd(16))
print(evenOdd(7))
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:
- Default argument
- Keyword arguments (named arguments)
- Positional arguments
- Arbitrary arguments (variable-length arguments *args and **kwargs)
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
def myFun(x, y=50):
print("x: ", x)
print("y: ", y)
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
def student(fname, lname):
print(fname, lname)
student(fname='Geeks', lname='Practice')
student(lname='Practice', fname='Geeks')
OutputGeeks 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)
print("Case-1:")
nameAge("Suraj", 27)
print("\nCase-2:")
nameAge(27, "Suraj")
OutputCase-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:
- *args in Python (Non-Keyword Arguments)
- **kwargs in Python (Keyword Arguments)
Example 1: Variable length non-keywords argument
Python
def myFun(*argv):
for arg in argv:
print(arg)
myFun('Hello', 'Welcome', 'to', 'GeeksforGeeks')
OutputHello
Welcome
to
GeeksforGeeks
Example 2: Variable length keyword arguments
Python
def myFun(**kwargs):
for key, value in kwargs.items():
print("%s == %s" % (key, value))
myFun(first='Geeks', mid='for', last='Geeks')
Outputfirst == 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
def evenOdd(x):
"""Function to check if the number is even or odd"""
if (x % 2 == 0):
print("even")
else:
print("odd")
print(evenOdd.__doc__)
OutputFunction 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
def f1():
s = 'I love GeeksforGeeks'
def f2():
print(s)
f2()
f1()
OutputI 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
def cube(x): return x*x*x # without lambda
cube_l = lambda x : x*x*x # with lambda
print(cube(7))
print(cube_l(7))
Return Statement in Python Function
The return statement in Python is used to exit a function and send a value back to the caller. It can return any data type, and if multiple values are separated by commas, they are automatically packed into a tuple. If no value is specified, the function returns None by default.
Syntax:
return [expression]
Explanation:
- return: Ends the function and optionally sends a value to the caller.
- [expression]: Optional value to return, defaults to None if omitted.
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))
For more information, refer to this article: Python return statement
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):
x = [20, 30, 40]
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):
x = 20
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
x = 2
y = 3
swap(x, y)
print(x)
print(y)
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. It calls itself until a base case (n==0) is met.
Quiz:
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