Python User Defined Functions
Last Updated :
12 Jul, 2025
A User-Defined Function (UDF) is a function created by the user to perform specific tasks in a program. Unlike built-in functions provided by a programming language, UDFs allow for customization and code reusability, improving program structure and efficiency.
Example:
Python
# function defination
def fun( x ):
if (x % 2 == 0):
print("even")
else:
print("odd")
fun(2) # function calling
Explanation: fun(x) checks if a number is even or odd using x % 2. If divisible by 2, it prints "even", otherwise "odd".
Syntax of User defined Functions
Function Defination:
def function_name(parameters):
# Function body
return result
Function call:
function_name(arguments)
Types of Users defined functions
1. Parameterized Functions
These functions accept parameters (arguments) to process and return results dynamically. Parameters allow for flexibility, enabling the function to handle different inputs each time it is called.
Python
# function definition
def fun(name):
print("Hello,", name)
# function call
fun("shakshi")
Explanation: fun(name) prints a greeting with the provided name. When called with "shakshi", it outputs "Hello, shakshi".
2. Functions with default arguments
A function can have default values assigned to its parameters. If no argument is provided when calling the function, it takes the default value.
Python
# function definition:
def fun(x, y=50):
print("x:", x)
print("y:", y)
# function call
fun(10)
Explanation: fun(x, y=50) takes two parameters, where y has a default value of 50. It prints the values of x and y. When called with fun(10), it uses 10 for x and the default value 50 for y .
3. Keyword argument functions
Function arguments can be passed using keywords to improve code readability. This ensures the correct mapping of values to parameters, regardless of their order.
Python
# function definition
def fun(name, age):
print(name, "is", age, "years old.")
# function call
fun(age=21, name="shakshi")
Outputshakshi is 21 years old.
Explanation: fun(name, age) prints the name and age with the message "is [age] years old." It is called using keyword arguments (age=21, name="shakshi"), which assigns the values explicitly to name and age.
4. Variable length argument functions
When the number of arguments is unknown, a function can accept multiple arguments using *args (for non-keyword arguments) or **kwargs (for keyword arguments).
Python
# function defination
def fun(*args):
for arg in args:
print(arg)
# function calling
fun("Python", "Java", "C++")
Explanation: fun(*args) uses a variable-length argument list, allowing it to accept any number of arguments. It then iterates through each argument and prints it.
5. Functions with Return value
A function can return a value using the return statement. This allows the function to send back a result for further computation.
Python
# function defination
def fun(num):
return num * num
# function calling
res = fun(5)
print(res)
Explanation: fun(num) takes a number num as input and returns its square (num * num). When called with fun(5), it calculates 5 * 5 and stores the result in res.
6. Lambda functions
A lambda function is an anonymous (nameless) function that is defined in a single line using the lambda keyword. It is used for short, simple operations where defining a full function is unnecessary.
Python
# lambda function definition:
res = lambda x: x * x
print(res(4))
Explanation: lambda x: x * x takes one input x and returns its square (x * x). When res(4) is called, it computes 4 * 4, resulting in 16 .
Pass by reference or Pass by value in Python
Python handles argument passing by reference for mutable objects (e.g., lists, dictionaries) and by value for immutable objects (e.g., integers, strings, tuples). If an immutable object is modified inside a function, a new object is created, whereas changes to mutable objects reflect outside the function.
Example : Pass by reference (Immutable data types)
Python
# function definition
def fun(x):
print("Value received:", x, "id:", id(x))
# driver code
x = 12
print("Value passed:", x, "id:", id(x))
# function call
fun(x)
OutputValue passed: 12 id: 140027364606256
Value received: 12 id: 140027364606256
Explanation: fun(x) prints the value of x and its memory identifier using id(). In the driver code, x is assigned the value 12 and before calling the function, both the value and id of x are printed. When the function is called, it prints the same information for x .
Example: Pass by Reference (Mutable Data Types)
Python
# Function definition
def fun(a):
a[0] = 100 # Changing the first element of the list
print("Inside function - lst:", a)
# Driver code
a = [1, 2, 3] # List is mutable
print("Before function call - lst:", a)
# Function call
fun(a)
print("After function call - lst:",a) # List is modified outside the function
OutputBefore function call - lst: [1, 2, 3]
Inside function - lst: [100, 2, 3]
After function call - lst: [100, 2, 3]
Explanation: fun(a) modifies the first element of the passed list ([0] = 100). Since lists are mutable in Python, the change is reflected outside the function. Before the call, a is [1, 2, 3], and after the call, it becomes [100, 2, 3].
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