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