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Conditional Statements

Conditional statements allow your program to make decisions and run different blocks of code depending on whether a condition is true or false.

1. if, elif, and else

  • if: Executes code if the condition is True.
  • elif (else if): Checks another condition if the previous one was False.
  • else: Catches everything not caught by preceding conditions.
score = 85

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

2. Nested if Statements

You can place if statements inside another if statement to build complex decision trees.

x = 41

if x > 10:
    print("Above ten,")
    if x > 20:
        print("and also above 20!")

3. Ternary (One-Line) Conditional Expressions

A ternary expression allows you to quickly evaluate a condition and assign a value on a single line.

# Syntax: value_if_true if condition else value_if_false
age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # Output: adult

4. The match Statement (Python 3.10+)

Introduced in Python 3.10, the match statement provides structural pattern matching, which acts like a modern version of switch-case statements in other languages. The underscore _ acts as the wildcard/default case.

status_code = 404

match status_code:
    case 200:
        print("Success")
    case 404:
        print("Not Found")
    case 500:
        print("Server Error")
    case _:
        print("Unknown Status")

Common Mistakes

Forgetting Truthy and Falsy Rules

In Python, you do not always need to check if x == True: or if len(x) > 0:. Python evaluates empty collections and zero values as False (Falsy) and non-empty values as True (Truthy).

The following values evaluate to Falsy:

  • None
  • False
  • 0 (and 0.0, 0j)
  • Empty sequences/mappings: "", [], (), {}, set()
# Common Beginner Mistake:
username = ""
if len(username) == 0:
    print("No username entered")

# Idiomatic Python Way:
if not username:
    print("No username entered")

Further Reading