A function is a block of reusable code that performs a specific task. Functions help organize code, reduce repetition, and improve readability. In Python, functions can have parameters, return values, and even be nested or anonymous.

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1. Defining and Calling a Function

Syntax:

def function_name():
    # Code block

Example:

def greet():
    print("Hello, welcome to Python!")

greet()  # Calling the function

Output:

Hello, welcome to Python!

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2. Function with Parameters

Parameters allow a function to accept input values.

Syntax:

def function_name(param1, param2):
    # Code block

Example:

def greet(name):
    print(f"Hello, {name}!")

greet("Alice")
greet("Bob")

Output:

Hello, Alice!
Hello, Bob!

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3. Function with Return Value

A function can return a value using the return statement.

Syntax:

pythonCopyEditdef function_name():
    return value

Example:

def add(a, b):
    return a + b

result = add(5, 3)
print("Sum:", result)

Output:

Sum: 8

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4. Default Parameter Values

If a parameter is not provided, the function uses a default value.

Example:

def greet(name="Guest"):
    print(f"Hello, {name}!")

greet()          # Uses default value
greet("Charlie") # Uses provided value

Output:

Hello, Guest!
Hello, Charlie!

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5. Keyword Arguments

Keyword arguments allow specifying parameters in any order.

Example:

def person_info(name, age):
    print(f"Name: {name}, Age: {age}")

person_info(age=25, name="John")

Output:

Name: John, Age: 25

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6. Variable-Length Arguments (*args and **kwargs)

*args – Multiple Positional Arguments

Allows passing multiple values as a tuple.

def total_sum(*numbers):
    return sum(numbers)

print(total_sum(1, 2, 3, 4))

Output:

10

**kwargs – Multiple Keyword Arguments

Allows passing multiple named arguments as a dictionary.

def print_info(**info):
    for key, value in info.items():
        print(f"{key}: {value}")

print_info(name="Alice", age=30, city="New York")

Output:

name: Alice
age: 30
city: New York

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7. Lambda (Anonymous) Functions

A lambda function is a small, single-line function without a name.

Syntax:

lambda arguments: expression

Example:

square = lambda x: x * x
print(square(5))

Output:

25

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8. Nested Functions

A function inside another function.

Example:

def outer_function():
    def inner_function():
        print("Inner function executed")
    inner_function()

outer_function()

Output:

Inner function executed

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9. Recursive Functions

A function that calls itself.

Example: Factorial Calculation

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

print(factorial(5))

Output:

120

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10. Function Scope and global Keyword

Variables inside functions are local unless declared global.

Example:

x = 10  # Global variable

def modify():
    global x
    x = 20  # Modify global variable

modify()
print(x)  # Prints 20

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Conclusion

• Functions help in code reusability and organization.
• They can have parameters, return values, and default arguments.
• *args handles multiple positional arguments, while **kwargs handles keyword arguments.
• Lambda functions are used for short, simple functions.
• Recursion enables solving problems like factorial calculation.