What's New in Pylint 4.0

:Release:4.0 :Date: TBA

Summary -- Release highlights

What's new in Pylint 4.0.0-dev0?

Release date: TBA

Breaking Changes

• invalid-name now distinguishes module-level constants that are assigned only once from those that are reassigned and now applies --variable-rgx to the latter. Values other than literals (lists, sets, objects) can pass against either the constant or variable regexes (e.g. "LOGGER" or "logger" but not "LoGgEr").

  Remember that --good-names or --good-names-rgxs can be provided to explicitly allow good names.

  Closes #3585 (#3585)

• Commented out code blocks such as #    bar() # TODO: remove dead code will no longer emit fixme.

  Refs #9255 (#9255)

• pyreverse Run was changed to no longer call sys.exit() in its __init__. You should now call Run(args).run() which will return the exit code instead. Having a class that always raised a SystemExit exception was considered a bug.

  Normal usage of pyreverse through the CLI will not be affected by this change.

  Refs #9689 (#9689)

• The suggestion-mode option was removed, as pylint now always emits user-friendly hints instead of false-positive error messages. You should remove it from your conf if it's defined.

  Refs #9962 (#9962)

• The async.py checker module has been renamed to async_checker.py since async is a Python keyword and cannot be imported directly. This allows for better testing and extensibility of the async checker functionality.

  Refs #10071 (#10071)

• The message-id of continue-in-finally was changed from E0116 to W0136. The warning is now emitted for every Python version since it will raise a syntax warning in Python 3.14. See PEP 765 - Disallow return/break/continue that exit a finally block.

  Refs #10480 (#10480)

New Features

• comparison-of-constants now use the unicode from the ast instead of reformatting from

  the node's values preventing some bad formatting due to utf-8 limitation. The message now use " instead of ' to better work with what the python ast returns.

  Refs #8736 (#8736)

• Enhanced pyreverse to properly distinguish between UML relationship types (association, aggregation, composition) based on object ownership semantics. Type annotations without assignment are now treated as associations, parameter assignments as aggregations, and object instantiation as compositions.

  Closes #9045 Closes #9267 (#9045)

• The fixme check can now search through docstrings as well as comments, by using check-fixme-in-docstring = true in the [tool.pylint.miscellaneous] section.

  Closes #9255 (#9255)

• The use-implicit-booleaness-not-x checks now distinguish between comparisons used in boolean contexts and those that are not, enabling them to provide more accurate refactoring suggestions.

  Closes #9353 (#9353)

• The verbose option now outputs the filenames of the files that have been checked. Previously, it only included the number of checked and skipped files.

  Closes #9357 (#9357)

• colorized reporter now colorizes messages/categories that have been configured as fail-on in red inverse. This makes it easier to quickly find the errors that are causing pylint CI job failures.

  Closes #9898 (#9898)

• Enhanced support for @property decorator in pyreverse to correctly display return types of annotated properties when generating class diagrams.

  Closes #10057 (#10057)

• Add --max-depth option to pyreverse to control diagram complexity. A depth of 0 shows only top-level packages, 1 shows one level of subpackages, etc. This helps manage visualization of large codebases by limiting the depth of displayed packages and classes.

  Refs #10077 (#10077)

• Handle deferred evaluation of annotations in Python 3.14.

  Closes #10149 (#10149)

• Enhanced pyreverse to properly detect aggregations for comprehensions (list, dict, set, generator).

  Closes #10236 (#10236)

• pyreverse: add support for colorized output when using output format mmd (MermaidJS) and html.

  Closes #10242 (#10242)

• pypy 3.11 is now officially supported.

  Refs #10287 (#10287)

• Add support for Python 3.14.

  Refs #10467 (#10467)

• Add naming styles for ParamSpec and TypeVarTuples which align with the TypeVar style.

  Refs #10541 (#10541)

New Checks

• Add match-statements checker and the following message: bare-name-capture-pattern. This will emit an error message when a name capture pattern is used in a match statement which would make the remaining patterns unreachable. This code is a SyntaxError at runtime.

  Closes #7128 (#7128)

• Add break-in-finally warning. Using break inside the finally clause will raise a syntax warning in Python 3.14. See PEP 765 - Disallow return/break/continue that exit a finally block.

  Refs #10480 (#10480)

False Positives Fixed

• Fix false positives for possibly-used-before-assignment when variables are exhaustively assigned within a match block.

  Closes #9668 (#9668)

• Fix used-before-assignment for PEP 695 type aliases and parameters.

  Closes #9815 (#9815)

• Fix a false positive for used-before-assignment when a variable defined under an if and via a named expression (walrus operator) is used later when guarded under the same if test.

  Closes #10061 (#10061)

• Fix false positives for use-implicit-booleaness-not-comparison, use-implicit-booleaness-not-comparison-to-string and use-implicit-booleaness-not-comparison-to-zero when chained comparisons are checked.

  Closes #10065 (#10065)

• Fix false positive for missing-raises-doc and missing-yield-doc when the method length is less than docstring-min-length.

  Refs #10104 (#10104)

• Comparisons between two calls to type() won't raise an unidiomatic-typecheck warning anymore, consistent with the behavior applied only for == previously.

  Closes #10161 (#10161)

• Fix a false positive for invalid-getnewargs-ex-returned when the tuple or dict has been assigned to a name.

  Closes #10208 (#10208)

• Remove getopt and optparse from the list of deprecated modules.

  Closes #10211 (#10211)

• Fix a false positive for unused-variable when multiple except handlers bind the same name under a try block.

  Closes #10426 (#10426)

False Negatives Fixed

• Fix false negative for used-before-assignment when a TYPE_CHECKING import is used as a type annotation prior to erroneous usage.

  Refs #8893 (#8893)

• Match cases are now counted as edges in the McCabe graph and will increase the complexity accordingly.

  Refs #9667 (#9667)

• Check module-level constants with type annotations for invalid-name. Remember to adjust const-naming-style or const-rgx to your liking.

  Closes #9770 (#9770)

• We now raise a logging-too-few-args for format string with no interpolation arguments at all (i.e. for something like logging.debug("Awaiting process %s") or logging.debug("Awaiting process {pid}")). Previously we did not raise for such case.

  Closes #9999 (#9999)

• Fix false negative for used-before-assignment when a function is defined inside a TYPE_CHECKING guard block and used later.

  Closes #10028 (#10028)

• Fix a false negative for possibly-used-before-assignment when a variable is conditionally defined and later assigned to a type-annotated variable.

  Closes #10421 (#10421)

• Fix false negative for deprecated-module when a __import__ method is used instead of import sentence.

  Refs #10453 (#10453)

• Fix false-negative for used-before-assignment with from __future__ import annotations in function definitions.

  Refs #10482 (#10482)

• Count match cases for too-many-branches check.

  Refs #10542 (#10542)

Other Bug Fixes

• When displaying unicode with surrogates (or other potential UnicodeEncodeError), pylint will now display a '?' character (using encode(encoding="utf-8", errors="replace")) instead of crashing. The functional tests classes are also updated to handle this case.

  Closes #8736. (#8736)

• Fixes "skipped files" count calculation; the previous method was displaying an arbitrary number.

  Closes #10073 (#10073)

• Fixed conditional import x.y causing false positive possibly-used-before-assignment.

  Closes #10081 (#10081)

• Fixes a crash that occurred when pylint was run in a container on a host with cgroupsv2 and restrictions on CPU usage.

  Closes #10103 (#10103)

• Fixed a crash when importing a class decorator that did not exist with the same name than a class attribute after the class definition.

  Closes #10105 (#10105)

• Fix a crash when something besides a class is found in an except handler.

  Closes #10106 (#10106)

• Fixed raising invalid-name when using camelCase for private methods with two leading underscores.

  Closes #10189 (#10189)

• Relaxed the requirements for isort so pylint can benefit from isort 6.

  Closes #10203 (#10203)

• Fixed unidiomatic-typecheck only checking left-hand side.

  Closes #10217 (#10217)

• Fix a crash caused by malformed format strings when using .format with keyword arguments.

  Closes #10282 (#10282)

• Using a slice as a class decorator now raise a 'not-callable' message instead of crashing pylint. A lot of checks that dealt with decorators (too many to list) are now shortcut if the decorator can't immediately be inferred to a function or class definition.

  Closes #10334 (#10334)

• Fix a bug in Pyreverse where aggregations and associations were included in diagrams regardless of the selected --filter-mode (such as PUB_ONLY, ALL, etc.).

  Closes #10373 (#10373)

• Fix double underscores erroneously rendering as bold in pyreverse's Mermaid output.

  Closes #10402 (#10402)

• Fixed crash in 'unnecessary-list-index-lookup' when starting an enumeration using minus the length of an iterable inside a dict comprehension when the len call was only made in this dict comprehension, and not elsewhere. Also changed the approach, to use inference in all cases but the simple ones, so we don't have to fix crashes one by one for arbitrarily complex expressions in enumerate.

  Closes #10510 (#10510)

Other Changes

• Upload release assets to PyPI via Trusted Publishing.

  Closes #10256 (#10256)

• The algorithm used for no-member suggestions is now more efficient and cut the calculation when the distance score is already above the threshold.

  Refs #10277 (#10277)

• Remove support for launching pylint with Python 3.9. Code that supports Python 3.9 can still be linted with the --py-version=3.9 setting.

  Refs #10405 (#10405)

Internal Changes

• Modified test framework to allow for different test output for different Python versions.

  Refs #10382 (#10382)