Control Flow: if, elif, else

Control flow lets your program make decisions. Instead of running every line from top to bottom, your code can choose different paths based on conditions. The if, elif, and else statements are the foundation of decision-making in Python.

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The if Statement

The simplest form of a conditional — run a block of code only if a condition is True:

age = 18

if age >= 18:
    print("You are an adult.")
    print("You can vote.")

print("This always runs.")

Output:

You are an adult.
You can vote.
This always runs.

Key syntax:

• The condition is followed by a colon :
• The body is indented (4 spaces)
• No parentheses needed around the condition (though they are allowed)

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if...else

Provide an alternative path when the condition is False:

temperature = 35

if temperature > 30:
    print("It's hot outside!")
else:
    print("The weather is pleasant.")

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if...elif...else

Use elif (short for "else if") for multiple conditions:

score = 75

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is: {grade}")  # Your grade is: C

Important: Python checks each condition from top to bottom and executes the first block whose condition is True. Once a match is found, the remaining conditions are skipped.

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Comparison Operators

These operators return True or False:

Operator	Meaning	Example
==	Equal to	5 == 5 → True
!=	Not equal to	5 != 3 → True
<	Less than	3 < 5 → True
>	Greater than	5 > 3 → True
<=	Less than or equal	5 <= 5 → True
>=	Greater than or equal	5 >= 3 → True

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Logical Operators

Combine multiple conditions with and, or, and not:

and — Both conditions must be True

age = 25
has_ticket = True

if age >= 18 and has_ticket:
    print("You can enter the concert.")

or — At least one condition must be True

day = "Saturday"

if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")

not — Reverses a boolean

is_raining = False

if not is_raining:
    print("Let's go for a walk!")

Combining Operators

age = 25
income = 50000
has_good_credit = True

if (age >= 21 and income >= 30000) or has_good_credit:
    print("Loan approved!")

Use parentheses to make the logic clear.

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Truthy and Falsy Values

In Python, every value can be evaluated as True or False:

# Falsy values — evaluate to False
if not 0:          print("0 is falsy")
if not 0.0:        print("0.0 is falsy")
if not "":         print("Empty string is falsy")
if not []:         print("Empty list is falsy")
if not {}:         print("Empty dict is falsy")
if not None:       print("None is falsy")