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Functions in Python

Definition

A function in Python is a reusable block of code that performs a specific task. It can take input (parameters), process it, and return an output. Functions help avoid repetition and make code organized and modular.

Key Points

  • Defined using the def keyword
  • Can take parameters (inputs)
  • Can return values using return
  • Must be called (invoked) to execute
  • Improves readability and reusability

Example / Code

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

result = add(3, 5)
print(result)

Explanation

  • def add(a, b): → defines a function named add
  • a and b are parameters
  • return a + b → sends result back to caller
  • add(3, 5) → function call
  • Output is stored in result

Output

8

Common Mistakes

  • Forgetting to call the function
  • Missing return when a value is needed
  • Incorrect indentation
  • Using variables not defined inside the function

Short Exam Notes (very concise revision points)

  • Function = reusable code block
  • Use def to define
  • Use () to call
  • Use return to send result

Function Creation & Calling

Definition

Creating a function means defining it, and calling means executing it.

Key Points

  • Define using def
  • Call using function name with parentheses
  • Arguments are passed during call

Example / Code

def greet(name):
    print("Hello", name)

greet("Khalid")

Explanation

  • Function greet takes name
  • When called, it prints greeting

Output

Hello Khalid

Common Mistakes

  • Not passing required arguments
  • Using wrong function name

Short Exam Notes

  • Define → def
  • Call → function_name()

Arbitrary Number of Arguments (*args)

Definition

Allows a function to accept any number of positional arguments.

Key Points

  • Use *args (commonly named)
  • Stored as a tuple
  • Useful when number of inputs is unknown

Example / Code

def add(*n):
    return sum(n)

print(add(1, 2, 3, 4, 5))

Explanation

  • *n collects all values into a tuple
  • sum(n) adds all elements

Output

15

Common Mistakes

  • Forgetting *
  • Treating n as a list instead of tuple

Short Exam Notes

  • *args = multiple inputs
  • Stored as tuple

Default Parameters

Definition

Default parameters allow a function to use a predefined value if no argument is passed.

Key Points

  • Defined using =
  • Optional during function call
  • Must come after non-default parameters

Example / Code

def power(num, power=2):
    return num ** power

print(power(2, 3))
print(power(3))

Explanation

  • First call → uses power = 3
  • Second call → uses default power = 2

Output

8
9

Common Mistakes

  • Placing default before non-default parameters
  • Confusing default with fixed value

Short Exam Notes

  • Default → param=value
  • Used if argument not provided

Variable Scope

Definition

Scope determines where a variable can be accessed.

Key Points

  • Python follows LEGB rule:
    • Local
    • Enclosing
    • Global
    • Built-in

Example / Code

x = 10  # Global

def outer():
    x = 20  # Enclosing
    def inner():
        x = 30  # Local
        print(x)
    inner()

outer()

Explanation

  • Python checks variables in order: Local → Enclosing → Global → Built-in
  • inner() prints local x = 30

Output

30

Common Mistakes

  • Assuming global variable changes automatically
  • Confusing local and enclosing variables

Short Exam Notes

  • LEGB rule
  • Local > Enclosing > Global > Built-in

global Keyword

Definition

Used to modify a global variable inside a function.

Key Points

  • Without global, a new local variable is created
  • With global, original variable is modified

Example / Code

x = 90

def f1():
    global x
    x = 900

f1()
print(x)

Explanation

  • global x tells Python to use global variable
  • Value is updated globally

Output

900

Common Mistakes

  • Writing global x = 900 ❌ (invalid syntax)
  • Forgetting global when modifying

Short Exam Notes

  • global modifies global variable
  • Must be declared inside function

Classes and Objects

Definition

  • Class: Blueprint/template for creating objects
  • Object: Instance of a class

Key Points

  • Classes define properties and behaviors
  • Objects use those definitions
  • Use class keyword
  • Python uses indentation (not {})

Example / Code

class Person:
    def __init__(self, name):
        self.name = name

    def greet(self):
        print("Hello", self.name)

p1 = Person("Khalid")
p1.greet()

Explanation

  • __init__ → constructor
  • self → refers to current object
  • p1 is an object of class Person

Output

Hello Khalid

Common Mistakes

  • Using {} instead of indentation
  • Forgetting self
  • Not calling methods correctly

Short Exam Notes

  • Class = blueprint
  • Object = instance
  • __init__ = constructor
  • self required