Assertions

assert documents internal assumptions and fails loudly when they are false.

When the assertion is true, execution continues normally. The assertion documents the function's internal expectation.

Source

def average(scores):
    assert scores, "scores must not be empty"
    return sum(scores) / len(scores)

print(average([8, 10]))

Output

9.0

When the assertion is false, Python raises AssertionError. This signals a broken assumption, not a normal recovery path.

Source

try:
    average([])
except AssertionError as error:
    print(error)

Output

scores must not be empty

Notes

Use assert for internal invariants and debugging assumptions.
Use explicit exceptions for user input, files, network responses, and other expected failures.
Assertions can be disabled with Python optimization flags, so do not rely on them for security checks.

Run the complete example

9.0
scores must not be empty

Execution time appears here after you run the example.

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