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Exception Handling

Assert Statements

Debugging assertions and their limitations

Interview: Debugging techniques — tests understanding of when to use assert vs raise, and -O flag behavior

Last Updated: June 12, 2026 • 5 min read

The assert statement is a debugging tool that tests a condition and raises AssertionError if the condition is False. Assertions should be used for internal checks that should never fail in correct code — NOT for input validation or business logic errors.

When to Use Assert

  • Verifying invariants: conditions that should always be true
  • Checking function post-conditions (return values)
  • Debugging: catching impossible states during development
  • Test code: verifying expected behavior

When NOT to Use Assert

  • Input validation (use raise ValueError instead)
  • Business logic errors (assertions can be disabled with -O flag!)
  • Data processing (assertions are not error handling)
  • Any check that MUST run in production

Critical Warning

Assertions are REMOVED when Python runs with the -O (optimize) flag. Never use assert for security checks, input validation, or any check that must run in production.

Use Cases

Verifying pre/post conditions in algorithms

Checking class invariants during development

Debugging impossible states (code that should never execute)

Test assertions in unit tests

Documenting assumptions in code (assert len(x) > 0)

Common Mistakes

Using assert for input validation (disabled with -O flag)

assert with tuple: assert (x, "msg") ALWAYS passes (tuple is truthy!)

Using assert for security checks (can be bypassed with -O)

Not understanding that AssertionError is the exception type (not ValueError)

Putting side effects in assert (they won't run with -O flag)