Control Flow (if, elif, else)¶

Python uses standard control flow statements (if, elif, else) to make decisions based on boolean conditions.

Python Indentation Rule¶

Unlike C, Java, or JavaScript, which use curly braces {} to delimit blocks of code, Python uses indentation (whitespace).

• PEP 8 standard recommendation is 4 spaces per indentation level.
• Do not mix tabs and spaces.
• Statements that start a block (if, for, def, class) always end with a colon :.

score = 85

if score >= 50:
    # Indented 4 spaces: inside the if block
    print("Exam Passed")
    print("Congratulations!")

print("This line always executes because it is unindented.")

The if, elif, and else Statements¶

Syntax¶

if condition1:
    # statement 1
elif condition2:
    # statement 2
else:
    # fallback statement

marks = 78

if marks >= 90:
    grade = "A"
elif marks >= 80:
    grade = "B"
elif marks >= 70:
    grade = "C"
else:
    grade = "F"

print("Assigned Grade:", grade)

Comparison: Value Equality (==) vs Identity (is)¶

• == compares the values of two objects.
• is checks whether two variables point to the exact same memory location (id(a) == id(b)).

list1 = [1, 2, 3]
list2 = [1, 2, 3]

print(list1 == list2)  # True  (same values)
print(list1 is list2)  # False (different objects in memory)

# Comparing with None: Always use 'is' or 'is not'
value = None
if value is None:
    print("Value is None")

Truth Value Testing (Truthy and Falsy)¶

In Python, any object can be tested for truth value:

Falsy Values¶

The following objects evaluate to False in conditions: - Constants: None, False - Numeric zeros: 0, 0.0, 0j - Empty sequences and collections: "", (), [], {}, set()

All other objects evaluate to True (Truthy).

items = []

# Idiomatic check for empty list
if not items:
    print("The list is empty.")

Conditional Expression (Ternary Operator)¶

Assign a value conditionally in a single readable line:

Syntax¶

value_if_true if condition else value_if_false

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)  # Adult

Pattern Matching (match / case in Python 3.10+)¶

Python 3.10 introduced structural pattern matching:

def check_status(code: int) -> str:
    match code:
        case 200:
            return "OK"
        case 301 | 302:
            return "Redirect"
        case 404:
            return "Not Found"
        case 500:
            return "Server Error"
        case _:
            return "Unknown Code"

print(check_status(404))  # Not Found