Python — If Else

The if statement

Python's if statement runs code only when a condition is true:

age = 25

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

The condition must evaluate to a truthy or falsy value. No parentheses needed around the condition.

The if-else statement

temperature = 5

if temperature > 30:
    print("It's hot outside.")
else:
    print("It's not that hot.")

One of the two blocks always executes. Indentation defines which code belongs to which branch.

elif chains

score = 78

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

Python evaluates conditions top to bottom and runs the first matching block. The rest are skipped. No limit to how many elif branches you can have.

Ternary expression

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

Format: value_if_true if condition else value_if_false. Use it for simple assignments. For complex logic, use a regular if-else block.

Nested conditions

has_ticket = True
age = 16

if has_ticket:
    if age >= 18:
        print("Welcome to the show.")
    else:
        print("You need a guardian.")
else:
    print("Please buy a ticket first.")

Nesting works but becomes hard to read beyond two levels. Combine conditions with and/or when possible:

if has_ticket and age >= 18:
    print("Welcome!")
elif has_ticket:
    print("Need a guardian.")
else:
    print("Buy a ticket.")

Truthy and falsy values

Python treats certain values as false in conditions:

# Falsy values
if 0:        print("This won't print")
if "":       print("This won't print")
if []:       print("This won't print")
if {}:       print("This won't print")
if None:     print("This won't print")

# Truthy values
if 1:        print("This prints")
if "hello":  print("This prints")
if [1, 2]:   print("This prints")
if {"a": 1}: print("This prints")

This means you can write clean boolean checks:

# Instead of:
if len(items) > 0:
    process(items)

# Write:
if items:
    process(items)

The match statement (Python 3.10+)

day = "Monday"

match day:
    case "Saturday" | "Sunday":
        print("Weekend")
    case "Monday":
        print("Start of the week")
    case "Friday":
        print("TGIF!")
    case _:
        print("Regular day")

match is Python's structural pattern matching. It handles multiple patterns cleanly and can destructure data:

point = (1, 0)

match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"On x-axis at {x}")
    case (0, y):
        print(f"On y-axis at {y}")
    case (x, y):
        print(f"At ({x}, {y})")

Chained comparisons

Python allows chaining comparisons — unusual but useful:

x = 5

# Chained comparison
if 1 < x < 10:
    print("Between 1 and 10")

# Multiple conditions
if 0 <= x <= 100 and x % 2 == 0:
    print("Even number between 0 and 100")

Short-circuit evaluation

Python evaluates and and or lazily:

# and — stops at first False
result = False and expensive_function()  # never calls the function

# or — stops at first True
result = True or expensive_function()   # never calls the function

# Practical use — safe attribute access
name = user and user.get("name") or "Anonymous"

Common mistakes

Forgetting the colon

if age >= 18    # SyntaxError: expected ':'
    print("Adult")

Every if, elif, else, for, while, def, and class line must end with a colon.

Comparing with = instead of ==

x = 5
# if x = 5:    # SyntaxError: invalid syntax
if x == 5:     # correct

Python catches this — assignment is a statement, not an expression.

Mutating a list while iterating

# Bad — modifies the list while iterating
numbers = [1, 2, 3, 4, 5]
for n in numbers:
    if n % 2 == 0:
        numbers.remove(n)  # unpredictable behavior!

# Good — create a new list
numbers = [1, 2, 3, 4, 5]
evens = [n for n in numbers if n % 2 != 0]

Combining conditions

age = 25
has_id = True
is_vip = False

# AND — all must be true
if age >= 18 and has_id:
    print("Entry allowed")

# OR — at least one must be true
if is_vip or age >= 65:
    print("Priority lane")

# NOT — flips the boolean
if not is_vip:
    print("Standard lane")

if-elif-else as an expression

score = 85

# As an expression
result = (
    "A" if score >= 90 else
    "B" if score >= 80 else
    "C" if score >= 70 else
    "D" if score >= 60 else
    "F"
)
print(result)  # B

Nested ternaries can be hard to read — use sparingly.

Mini Practice

1. Write a program that classifies a number as positive, negative, or zero
2. Create a temperature converter that recommends clothing based on the temperature
3. Use a ternary expression to assign "even" or "odd" to a variable
4. Write a grade calculator with if-elif-else
5. Use match-case to categorize a day of the week as "Weekday", "Weekend", or "Invalid"

Next: loops — repeating actions →