Testing

Expresses the programmer's expectation that two values are equal.

Unit testing: asserting function output matches expected value; Loop invariants: checking cumulative sum stays non-negative; Prototype code: guarding against invalid arguments

Python codebases, especially test suites (unittest, pytest) and prototype scripts.

assert actual == expected

assert ... == ...

actual: expression, expected: expression

• optional message string
• custom error message

• self.assertEqual(actual
• expected) in unittest
• assert actual is expected in pytest (assert rewriting)

Using assert for runtime checks in production code (can be disabled with -O), relying on side effects inside assert, omitting helpful messages.

if actual != expected: raise AssertionError('...') – more explicit but less concise.

Coming from Java: assert statements are disabled by default unless enabled with a flag; in C/C++ assert may abort the program.

• assertion
• testing
• defensive programming

Assertions are removed when Python runs with the -O (optimize) flag, so they should not be used for validating user input or essential logic.

Detects regressions early, documents intended behavior, and prevents silent logic errors during development.

Result should equal expected.

Assert statements are removed when Python is run with the -O optimization flag, so they should not be used for runtime checks that must always execute.

Use a testing framework's assertion methods, e.g., `self.assertEqual(result, expected)` in unittest or plain `assert result == expected` with pytest's rich introspection.