Conditionals (if / elif / else)

Making decisions in your program with if, elif, and else statements.

Overview

Conditionals allow a program to take different paths depending on whether conditions are true or false. Python's conditional syntax uses three keywords: if, elif (else-if), and else. A condition is any expression that evaluates to a truthy or falsy value. Python does not require the condition to be a strict True/False boolean an empty string, 0, None, or an empty list are all falsy; non-empty strings, non-zero numbers, and populated collections are all truthy. Python does not have a switch/case statement in versions before 3.10, but the match statement (introduced in 3.10) fills that role for structural pattern matching. For older versions, chains of if/elif are the standard approach. The ternary (one-line conditional) expression `value_if_true if condition else value_if_false` is compact and Pythonic when the logic is simple.

Key Points

•if block runs when the condition is truthy; elif adds alternative conditions; else is the fallback
•Indentation defines the block every line in the if block must be indented equally
•Conditions can be any truthy/falsy expression, not just explicit True/False
•Ternary: `x = "yes" if condition else "no"` one-liner for simple branching
•Comparison operators: ==, !=, <, >, <=, >= all return True or False
•Combine conditions with and, or, not e.g. `if age >= 18 and has_id:`

Examples

Basic if / elif / else

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"Score {score} → Grade {grade}")  # Score 75 → Grade C

Truthy/falsy values in conditions

name = ""
if name:
    print(f"Hello, {name}!")
else:
    print("No name provided.")   # runs because "" is falsy

items = [1, 2, 3]
if items:
    print(f"You have {len(items)} items.")  # runs list is non-empty

Ternary expression

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

# Can also be used inline
print("even" if 42 % 2 == 0 else "odd")  # even

Common Mistakes

Missing colon after the condition: `if x > 5` needs a colon → `if x > 5:`
Using = instead of == in a condition: `if x = 5:` is a SyntaxError
Forgetting that elif and else must align with their if inconsistent indentation causes errors

Note

When you have many elif chains checking the same variable against different values, consider if a dictionary lookup might be cleaner: `grades = {90: "A", 80: "B"}` though if/elif is perfectly fine for most cases.

Conditionals (if / elif / else)

Overview

Key Points

Examples

Common Mistakes

Note

Exercises

Knowledge Check

What is the output of the following code? ```python x = 0 if x: print("truthy") else: print("falsy") ```