assert Statement¶

The assert statement is a debugging aid that tests a condition and raises AssertionError if the condition is False. It helps verify assumptions during development.

Basic Syntax¶

assert condition, message

condition: Expression that should evaluate to True
message: Optional error message if assertion fails

x = 10
assert x > 0, "x must be positive"  # Passes silently

y = -5
assert y > 0, "y must be positive"  # Raises AssertionError

Simple Assertions¶

Assertions without a message raise a bare AssertionError.

x = 5
assert x == 5      # Passes
assert x == 10     # AssertionError

Assertions with Messages¶

Provide context when the assertion fails.

def divide(a, b):
    assert b != 0, "Division by zero is not allowed"
    return a / b

result = divide(10, 2)
print(result)  # 5.0

result = divide(10, 0)  # AssertionError: Division by zero is not allowed

Practical Example¶

Validate function inputs during development.

def calculate_average(numbers):
    assert len(numbers) > 0, "List cannot be empty"
    assert all(isinstance(n, (int, float)) for n in numbers), \
        "All elements must be numbers"
    return sum(numbers) / len(numbers)

print(calculate_average([1, 2, 3, 4, 5]))  # 3.0
print(calculate_average([]))  # AssertionError: List cannot be empty

Assert vs Exception Handling¶

Use assert for debugging, not for production error handling.

# Good: assert for internal consistency checks
def process_data(data):
    assert data is not None, "Bug: data should never be None here"
    # ... processing logic

# Bad: assert for user input validation
def get_age(age):
    assert age > 0  # Don't do this!
    return age

# Good: exceptions for user input validation
def get_age(age):
    if age <= 0:
        raise ValueError("Age must be positive")
    return age

When to Use assert¶

Do use assert for:

Internal consistency checks
Verifying programmer assumptions
Development and testing
Documenting invariants

Don't use assert for:

Validating user input
Checking conditions that might legitimately fail
Production error handling
Security checks

Assertions Can Be Disabled¶

Assertions are removed when Python runs with optimization (-O flag).

python -O script.py  # Assertions are ignored

This is why you should never use assert for critical checks.

# WRONG: Security check with assert
assert user.is_authenticated, "User must be logged in"

# CORRECT: Proper security check
if not user.is_authenticated:
    raise PermissionError("User must be logged in")

Multiple Conditions¶

Check multiple conditions in one assertion.

def set_coordinates(x, y):
    assert 0 <= x <= 100 and 0 <= y <= 100, \
        f"Coordinates ({x}, {y}) out of bounds"
    print(f"Position set to ({x}, {y})")

set_coordinates(50, 50)   # Position set to (50, 50)
set_coordinates(150, 50)  # AssertionError: Coordinates (150, 50) out of bounds