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Introduction to Functions in Python

A function in Python is a block of reusable code that performs a specific task.
Instead of writing the same code repeatedly, you write it once inside a function and “call” it whenever needed.

Functions help to make programs:

✔ Modular
✔ Reusable
✔ Easy to read
✔ Easy to test
✔ Easy to debug

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What is a Function?

A function is a group of statements that perform a particular operation and return a result.

Python provides two types of functions:

1. Built-in Functions (predefined)
2. User-defined Functions (created by the programmer)

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1. Built-in Functions

Python comes with many built-in functions such as:

• print()
• input()
• len()
• type()
• sum()
• max()
• min()
• range()

Example:

print(len("Python"))

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2. User-defined Functions

These are functions created by the user using the def keyword.

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Why Use Functions?

✔ Code reusability

Write once → use many times.

✔ Better organization

Break a large program into smaller parts.

✔ Testing and debugging

Functions isolate logic → easier to test.

✔ Avoid repetition

Removes duplicate code.

✔ Increases readability

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Defining a Function

A user-defined function is created using the def keyword.

Syntax:

def function_name(parameters):
    statements
    return value

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Example: Simple function

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

Calling the function:

greet()

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Function with Parameters (Arguments)

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

add(10, 20)      # Output: 30

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Function with Return Statement

A function can return a value using the return keyword.

def square(x):
    return x * x

result = square(5)
print(result)

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Types of Function Arguments

Python supports different types of arguments:

1. Required arguments
2. Default arguments
3. Keyword arguments
4. Variable-length arguments (*args, **kwargs)

We can go deeper if you want.

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Flow of Function Execution

1. Function is defined
2. Program reaches function call
3. Control shifts to function body
4. Function executes
5. Function returns (if return is used)
6. Control goes back to calling statement

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Docstring (Function Documentation)

A docstring explains what a function does.

def greet():
    """This function prints a greeting message."""
    print("Hello!")

print(greet.__doc__)

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Lambda Functions (Anonymous Functions)

Small, one-line functions created using lambda.

square = lambda x: x * x
print(square(6))     # 36

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Example Programs Using Functions

Example 1: Check even or odd

def check(num):
    if num % 2 == 0:
        return "Even"
    else:
        return "Odd"

print(check(7))

Example 2: Calculate factorial

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

print(factorial(5))

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When Should We Use Functions?

Use a function when:

• A piece of code is used multiple times
• You want a modular program
• You want to group related logic
• You want to return results from computation

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Conclusion

Functions are a fundamental part of Python programming.
They make programs:

✔ simpler
✔ modular
✔ efficient
✔ reusable

Understanding functions is essential before learning:

▶ Modules
▶ Object-Oriented Programming
▶ Data structures
▶ Recursion

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