Flake8 Unittest Assertion Checker

flake8-assertive is a Flake8 extension that encourages using richer, more specific unittest assertions beyond just the typical assertEqual(a, b) and assertTrue(x) methods. The suggested methods perform more precise checks and provide better failure messages than the generic methods.

Original	Suggestion	Code
assertTrue(a == b)	assertEqual(a, b)	A500
assertTrue(a != b)	assertNotEqual(a, b)	A500
assertFalse(a == b)	assertNotEqual(a, b)	A500
assertFalse(a != b)	assertEqual(a, b)	A500
assertTrue(a < b)	assertLess(a, b)	A500
assertTrue(a <= b)	assertLessEqual(a, b)	A500
assertTrue(a > b)	assertGreater(a, b)	A500
assertTrue(a >= b)	assertGreaterEqual(a, b)	A500
assertTrue(a is b)	assertIs(a, b)	A501
assertTrue(a is not b)	assertIsNot(a, b)	A501
assertFalse(a is b)	assertNotIs(a, b)	A501
assertFalse(a is not b)	assertIs(a, b)	A501
assertTrue(a in b)	assertIn(a, b)	A501
assertTrue(a not in b)	assertNotIn(a, b)	A501
assertFalse(a in b)	assertNotIn(a, b)	A501
assertTrue(isinstance(a, b))	assertIsInstance(a, b)	A501
assertFalse(isinstance(a, b))	assertNotIsInstance(a, b)	A501
assertEqual(a, round(b, x))	assertAlmostEqual(a, b, x)	A501
assertAlmostEqual(a, round(b, x))	assertAlmostEqual(a, b, x)	A501
assertNotEqual(a, round(b, x))	assertNotAlmostEqual(a, b, x)	A501
assertNotAlmostEqual(a, round(b, x))	assertNotAlmostEqual(a, b, x)	A501
assertEqual(a, None)	assertIsNone(a)	A502
assertNotEqual(a, None)	assertIsNotNone(a)	A502
assertTrue(a is None)	assertIsNone(a)	A502
assertTrue(a is not None)	assertIsNotNone(a)	A502
assertFalse(a is None)	assertIsNotNone(a)	A502
assertFalse(a is not None)	assertIsNone(a)	A502
assertEqual(a, True)	assertTrue(a)	A502
assertEqual(a, False)	assertFalse(a)	A502
assertEquals(a, b)	assertEqual(a, b)	A503
assertNotEquals(a, b)	assertNotEqual(a, b)	A503
assertAlmostEquals(a, b, x)	assertAlmostEqual(a, b, x)	A503
assertNotAlmostEquals(a, b, x)	assertNotAlmostEqual(a, b, x)	A503
assertTrue(a, b)	assertTrue(a, msg=b)	A504
assertFalse(a, b)	assertFalse(a, msg=b)	A504

Note that some suggestions are normalized forms of the original, such as when a double-negative is used (assertFalse(a != b) → assertEqual(a, b)). There aren't suggestions for things like assertFalse(a > b), which may or may not be equivalent to assertLessEqual(a, b).

Installation

Install from PyPI using pip:

$ pip install flake8-assertive

The extension will be activated automatically by flake8. You can verify that it has been loaded by inspecting the flake8 --version string.

$ flake8 --version
4.0.1 (assertive: 2.1.0, ...) CPython 3.9.10 on Darwin

Error Codes

This extension adds three new error codes (using the A50 prefix):

• A500: prefer {func} for '{op}' comparisons
• A501: prefer {func} for '{op}' expressions
• A502: prefer {func} instead of comparing to {obj}
• A503: use {func} instead of the deprecated {name}
• A504: prefer the 'msg=' kwarg for {func} diagnostics

Configuration

Configuration values are specified in the [flake8] section of your config file or as command line arguments (e.g. --assertive-snakecase).

• assertive-snakecase: suggest snake_case assert method names (e.g. assert_true()) instead of the standard names (e.g. assertTrue())
• assertive-test-pattern: fnmatch pattern for identifying unittest test files (and all other files will be skipped)

Caveats

There are some specific cases when the suggestion might not match the intent of the original.

Testing the equality operator

assertEqual() won't use the == operator if the comparison has been delegated to a type-specific equalilty function. By default, this is the case for strings, sequences, lists, tuples, sets, and dicts.

If your intent is to specifically test the == operator, consider writing the assertion like this instead:

assertIs(a == b, True)

This approach has the benefit of verifying that the type's __eq__ implementation returns a boolean value. Unfortunately, it also has the downside of reporting the result of a == b on failure instead of the values of a and b.

Suggested by: Serhiy Storchaka

Alternatives

• Teyit is a Python unit test formatter that can perform similar assertion transformations.