pylint-dev/pylint v4.1.0 on GitHub

What's new in Pylint 4.1.0?

Release date: 2026-09-29

Startup is about 25% faster thanks to lazy imports. The import checker caches
its isort configuration, which makes pylint about 17% faster on ansible.
Finding the files to lint with --recursive=y no longer walks ignored
directories such as .venv or node_modules, which took seconds on large
trees. The duplicate-code checker and symilar also received optimizations
that result in considerable performance improvements and memory use reduction
on larger codebases. For example, pandas analysis went from 20 min to 55 s and
pylint does not get OOM-killed when analyzing cpython anymore.

Python 3.15 support progresses: the unpacking in comprehensions added by
PEP 798 no longer raises false positives, and the standard library
deprecations of Python 3.15 are followed.

For CI, there is a new built-in junit output format
(--output-format=junit), the NO_COLOR and FORCE_COLOR environment
variables are respected, and the files to lint can now be set with the
files option in the configuration file.

New checks: looping-through-iterator, impossible-comparison,
chained-comparison-all-equal and
using-comprehension-unpacking-in-unsupported-version.

Running with --jobs no longer duplicates the messages of extensions or
ignores extensions enabled in the configuration.

Plugin authors: the confidence parameter can no longer be None, except
in is_message_enabled, and the MSG_STATE_* constants are deprecated in
favor of the MessageDisableReason enum.

The required astroid version is now 4.3.2. See the astroid changelog for additional fixes, features, and performance improvements applicable to pylint.

Breaking Changes

The confidence parameter is no longer nullable on any APIs except for
is_message_enabled (where confidence=None means "don't filter by
confidence"). The default became interfaces.UNDEFINED (an immutable value);
the behavior is unchanged unless you were passing an explicit None. This
avoids a runtime check in multiple function to set the default value conditionally.
The constants.MSG_STATE_* integer were replaced by a MessageDisableReason enum.
The old names remain as deprecated aliases pointing at the enum members.
MessageDisableReason is an IntEnum so existing code comparing the return of
_get_message_state_scope to the literal 0 / 1 / 2 keeps working.
Refs #11018

New Features

Add support for ignore-pattern-in-long-lines to allow ignoring specific parts of a line when checking line length.
Refs #3352
Support for the NO_COLOR and FORCE_COLOR environment variables has been added.
When running pylint, the reporter that writes to stdout is switched between text
and colorized according to the requested mode.
The order is: NO_COLOR > FORCE_COLOR > --output-format=....
Closes #3995
The dict-init-mutate message now includes a suggested dictionary literal showing how to combine the initialization and subsequent mutations into a single statement.
Closes #7819
Add a built-in junit output format (--output-format=junit) that produces JUnit-compatible XML output for CI/CD integration with Jenkins, Azure DevOps, GitLab CI, and GitHub Actions.
Closes #9143
Trailing pragmas understood by other common tooling (# type: ignore,
# pyright: ignore, # noqa, # pragma: no cover and
# pragma: no branch) are no longer counted toward the line length, so a line
is not flagged as line-too-long solely because of such a pragma. This mirrors
the existing behaviour for Pylint's own # pylint: pragmas.
Closes #10172
pyreverse: add --no-signatures to show method names without parameter lists or return type annotations in class diagrams.
Closes #10772
Add support for --known-first-party similar to --known-third-party.
Refs #10803

New Checks

Added a new checker looping-through-iterator (W4801) to detect when an iterator from an outer scope is consumed in a nested loop, which can lead to the iterator being unexpectedly exhausted.
Refs #2996
Add new checks impossible-comparison and chained-comparison-all-equal.
impossible-comparison flags boolean conditions whose chain of numeric
comparisons is logically contradictory and can never be true (for example
a > b and b > a). chained-comparison-all-equal flags boolean conditions
whose operands form a cycle of weak inequalities (<= or >=) and can be
simplified to a chain of equalities (for example a >= b and b >= a is
equivalent to a == b).
Closes #5814
Add using-comprehension-unpacking-in-unsupported-version (W2607), emitted when
the code uses the unpacking in comprehensions added by PEP 798 while
py-version still includes a Python version that cannot compile it.
Refs #10982

False Positives Fixed

Fix a false positive for unnecessary-negation (C0117) when negating a
comparison between dict views (dict.keys()/dict.items()), which like
set/frozenset support only a partial (subset/superset) ordering, so
not a.items() <= b.items() is not equivalent to a.items() > b.items().
Closes #3668
Fix false positives for attribute-defined-outside-init where __init__
(etc.) uses a helper method to create attributes.
Closes #5214
Fix a false positive for consider-using-generator and use-a-generator
with an asynchronous list comprehension. Turning it into a generator expression
would create an asynchronous generator, which those functions cannot consume.
Closes #7271
Fixed a false positive assigning-non-slot when assigning to an inherited
descriptor through an instance returned by a method annotated with the
subclass. This regressed in pylint 3.0.0.
Fixed by upgrading astroid to 4.1.0.
Closes #8053
Fixed a false positive no-member when a classmethod annotated with
typing.Self is overridden in a subclass and its result is used directly,
as in Subclass.build().only_on_subclass(). The return type is now narrowed
to the subclass rather than the base class. This regressed in pylint 3.0.0.
Fixed by upgrading astroid to 4.1.0.
Closes #9159
Avoid emitting deprecated-class for imports inside a recognized
sys.version_info guard.
Closes #9533
Fix a false positive for inconsistent-return-statements when an instance
method annotated with NoReturn (or Never) is called via the class
rather than an instance (e.g. MyClass.raise_method(obj)). The unbound
method form is now recognised as never returning, matching the existing
behaviour for the bound-method form.
Closes #9692
Fix used-before-assignment false positive for names bound in only some arms of an if/elif/else chain.
Closes #9879
Fix a false positive for useless-parent-delegation when a method overrides a
method of a C-level parent whose signature cannot be inspected, such as
Exception.__init__. Because Exception.__init__ accepts *args, an
override taking only self narrows the accepted arguments and is not useless.
Overrides of object.__init__ are still reported.
Closes #9994
Fixed a false positive no-name-in-module when a module is imported with
an alias that shadows its base module and a function named format is
called on the alias.
Fixed by upgrading astroid to 4.3.1.
Closes #10193
Fixed a false positive unsubscriptable-object on instances of a generic class defining __class_getitem__.
Closes #10360
Fixed a false positive unexpected-keyword-argument when passing dtype
to numpy.concatenate().
Fixed by upgrading astroid to 4.3.1.
Closes #10548
Fix a false positive for unexpected-keyword-arg for dataclasses
using generic type aliases (PEP 695).
Closes #10703
Fix false positive unreachable when calling a function with @overload where one signature returns NoReturn.
Closes #10785
Fix a false positive for too-many-function-args for dataclasses
using generic type aliases (PEP 695).
Closes #10788
Fix a false positive relative-beyond-top-level error when linting specific files in namespace packages in parallel mode by augmenting sys.path before loading plugins and expanding files consistently for parallel workers.
Closes #10794

Fix # pylint: enable inside a try block leaking into the except
handler. For example in the following code, no-member is no longer
incorrectly re-enabled in the except block:

class Basket:
    # pylint: disable=no-member
    def pick(self):
        try:
            # pylint: enable=no-member
            print(self.apple)  # no-member emitted here
        except KeyError:
            print(self.banana)  # no-member NOT emitted here (correct)

Requires astroid 4.2.

Refs #10933

Fix false positives for the Python 3.15 syntax added by PEP 798, unpacking in
comprehensions:
- star-needs-assignment-target (E0114) was emitted for the unpacked element
  of a comprehension, e.g. [*sub for sub in lists].
- consider-using-dict-comprehension (R1717) was emitted for
  dict([*pairs for pairs in nested]), which flattens its argument and is
  therefore not equivalent to a key/value dict comprehension.
Refs #10982
Fix access-member-before-definition false positive for bare type annotations
(self.x: Type) that don't assign a value.
Refs #11015
Fix a false positive for assignment-from-no-return when the called function's
body ends in an unconditional raise, such as pathlib.Path.readlink() on
platforms without symlink support.
Closes #11114
Fix a false positive for protected-access when a protected member is
accessed through self.__class__, which is now treated like type(self).
Closes #11160
Treat typing.NoReturn and typing.Never the same as NoReturn / Never when deciding that a call never returns.
Closes #11271
Fix a false positive for unreachable on the statement following an
instantiation of _sitebuiltins.Quitter (the class of the exit and
quit builtins). Only calling the instance terminates, not creating it.
Closes #11310
Fixed a false positive unbalanced-tuple-unpacking when unpacking the args
of an exception.
Fixed by upgrading astroid to 4.3.2.
Closes #11312
Fixed a false positive for unnecessary-semicolon when an f-string ending in ; is continued onto the next line with a backslash on Python 3.12+.
Closes #11444

False Negatives Fixed

missing-param-doc and missing-type-doc no longer false-negative on
NumPy-style parameters whose type line includes a default value, e.g.
number : int, default 0. Any text after the colon on the type line is
now accepted as the type, matching the NumPy style guide.
Closes #6211
The docparams extension now emits multiple-constructor-doc when
constructor parameters are documented in both the class docstring and the
constructor docstring, even when the constructor method is skipped by
no-docstring-rgx.
Closes #6692
chained-comparison is now emitted for additional simplifiable patterns
(e.g. a > 1 and a > 10) and its message now includes the suggested
simplification.
Refs #7611
Fix a false negative for abstract-method where a concrete subclass
inheriting from an abstract class (without redeclaring abc.ABC or
ABCMeta) was treated as abstract and silently exempted from the check.
A class is now only considered abstract when it opts in explicitly, via
direct abc.ABC inheritance, metaclass=ABCMeta, an
@abstractmethod defined on the class, or being a Protocol.
Closes #7950
no-value-for-parameter is now emitted when a call unpacks a dictionary literal
with ** and that dictionary does not provide a required argument.
Closes #8785
attribute-defined-outside-init now reports attributes assigned with
setattr(self, "name", value) outside defining methods.
It no longer reports attributes assigned normally when a defining method of the
class or of a parent initializes them with setattr.
Closes #9798
superfluous-parens (C0325) no longer false-negatives on a single
parenthesised literal after the in keyword, e.g. x in ("foo"). The
parentheses around a single string or number literal are now reported as
superfluous, while a tuple (x in ("foo",)) or a larger expression
(x in ("foo" + bar)) is still left untouched.
Closes #9878
comparison-with-itself now detects repeated attribute chains such as
object.attribute == object.attribute.
Closes #10713
not-an-iterable and not-a-mapping are now also emitted for the value
unpacked by PEP 798 comprehension unpacking, e.g. [*number for number in numbers] or {**number for number in numbers}.
Refs #10982
Fix a false negative in unnecessary-negation (C0117): not (a is not b) and not (a not in b) are now flagged (they simplify to a is b and a in b), consistent with the existing handling of is / in.
Closes #11140
Emit arguments-differ when an overridden special method takes a different
number of parameters. Only renamed parameters and removed variadics stay
exempt, and the constructor family (__new__, __init__,
__init_subclass__ and __post_init__) is still fully ignored.
Closes #11295
access-member-before-definition is now also emitted when __init__ calls a
method that reads an instance attribute which __init__ only assigns after the call.
Closes #11338
Fix a false negative for unspecified-encoding and bad-open-mode when the
mode of an open call is a parameter of the enclosing function that has a
default value. The default is now used to check the call, as a literal mode
would be. Calls with such a mode stopped being reported in pylint 4.0.8.
Refs #11415

Other Bug Fixes

# pylint: disable comments at the beginning of an else block (or on
the line just above the else keyword) now suppress messages in that block
instead of being ignored.
Fixed by upgrading astroid to 4.3.1.
Closes #872
dangerous-default-value now detects mutable default values in typing.NamedTuple field definitions.
Closes #3716
Repeated --output-format options now write reports to every requested file instead of only the last one.
Closes #8147
Fixed a crash when defining a functional namedtuple with a field name
that changes under NFKC normalization, like "µ" (MICRO SIGN).
Fixed by upgrading astroid to 4.3.1.
Closes #8746
Fix a crash in pyreverse when a requested class cannot be inferred.
Closes #9797
Fix enabling checks from extensions which are disabled by default if multiple jobs are used.
Closes #10037
Fixed an AstroidBuildingError crash when inheriting from a generic
dataclass that rebinds __init__ in __init_subclass__.
Fixed by upgrading astroid to 4.3.1.
Closes #10519
wrong-import-position now exempts try, if, with, and match blocks from marking the import boundary. Fixed async def not being detected as an import boundary. Pragma on non-import lines now suppresses following imports until the next non-import.
Closes #10589
Fix duplicate messages for extension checks if multiple jobs are used.
Refs #10642
Fix an issue where discovery can miss a similarly named directory if a shorter named directory is processed first.
Closes #10969
Follow the standard library deprecations of Python 3.15.
Refs #10982
Fixed inflated message occurrence counts in the final Messages report when
running pylint in parallel mode with --jobs greater than 1.
Closes #10996
Fix a crash in consider-using-dict-items when the for loop or comprehension target is an attribute or a subscript (e.g. for self.key in d) rather than a simple variable name.
Closes #11173
nan-comparison now also recognizes math.nan, numpy.nan, Decimal("nan")
and any name or attribute that pylint can infer to a NaN constant, such as a module
level constant defined as math.nan. Only numpy.NaN -- removed in numpy 2.0 --
and float("nan") were detected before. Infinities are still not reported, as
comparing against them is meaningful.
Refs #11219
Fix a crash in the comparison checker when a NaN comparison operand is a call to a name that cannot be inferred, such as 1 == b('nan').
Closes #11224
Fix a crash in invalid-class-object and assigning-non-slot when __class__ is assigned outside a simple assignment (e.g. for obj.__class__ in classes:).
Closes #11267
Avoid a fatal astroid-error in invalid-name,
stop-iteration-return, assigning-non-slot and
redefined-slots-in-subclass for classes with duplicate or inconsistent
bases, which leave the class without an MRO to walk.
Refs #11272
collections.abc.Callable and collections.abc.Buffer no longer count towards too-many-ancestors. Every other abstract base class in collections.abc was already ignored, so a class deriving from Callable was charged for an ancestor while an otherwise identical class deriving from Iterable was not.
Refs #11358
Fix import-private-name depending on the order of the checked files: type
annotations are now collected for each module instead of only for the first
module checked that contains an import. This removes a false positive on imports
used only as annotations and a false negative on private imports used at runtime.
Closes #11466

Other Changes

Clarify how to choose the Python interpreter and py-version when linting a
project that supports multiple Python versions.
Closes #5038
You can now set the files option in configuration files and on the command line.
Passing files without the --files flag is still supported. This allows to set
files to files = my_source_directory and invoking pylint with only
the pylint command similar to how other CLI tools allow to do so.
The help message can always be invoked with pylint -h or pylint --help.
Closes #5701
Removed messages (such as print-statement or apply-builtin) now have
their own page in the documentation, with a link to the change that removed
them. They are also listed in the messages overview alongside renamed messages.
Closes #6670
Documentation for options defined by Run, such as --errors-only and
--init-hook, is now generated alongside checker configuration options.
Closes #6938
Document that the wrong-import-order (C0411) classification of imports as
third-party vs first-party depends on the current working directory and
recommend known-first-party as the deterministic workaround.
Closes #8801
Clarify related no-else-* messages so they say that only the first elif
after the reported branch should change. Expand the no-else-return
documentation to explain later branches and when retaining an elif chain
can better communicate an exhaustive decision.
Closes #9274
assignment-from-no-return now names the callable that does not return anything and,
for functions listed in the new known-side-effects-only-functions option, hints at
the equivalent function to use instead (e.g. reversed(...) for reverse()).
Closes #10383

Internal Changes

Add assertDoesNotAddMessages to CheckerTestCase to assert that
specific messages are not emitted, while allowing other messages to be
present. This complements assertNoMessages which asserts that no
messages at all are emitted.
Refs #9598
The primer now pairs residual messages — first by (symbol, path, obj) and then by
exact source location — and reports altered messages as a single changed entry with
a compact diff, rather than as a separate removal + addition. The location-based pass
also catches symbol renames at the same code position (e.g. used-before-assignment
→ possibly-used-before-assignment). When several messages are eligible, the one
closest to the original line wins, so pairs never cross. New messages are classified
into fixed false positives (useless-suppression), astroid-error fatal errors,
and the rest. astroid-error messages are excluded from pairing (their text embeds
a unique crash-report path) so persistent crashes keep raising the prominent warning.
Truncated comments are now cut at a line break and keep their code fences and
<details> blocks closed.
Refs #10914
The primer's project cache key is now derived from the commits pinned in
packages_to_prime.json instead of the remote branch tips. main and PR primer
runs now share the same project cache and lint files in the same on-disk order,
removing spurious diffs from primer comments (message positions and astroid inference
results depend on the order in which modules are linted).
Closes #11192

Performance Improvements

Lazily import isort, dill, multiprocessing/concurrent.futures,
and tomlkit so they are only loaded when actually needed.
This reduces startup time by ~25% (e.g. --version: 91 => 67 ms,
--help: 176 => 133 ms, single-file lint: 272 => 226 ms).
Closes #2866
Sped up the duplicate-code checker. When run inside pylint the
checker now reuses the already-parsed AST instead of re-parsing every
file like it has to do when launched via symilar, and it uses a
rolling hash window with caching across file pairs. Additionally, a
quadratic blow-up in the hash-matching phase is avoided by switching
algorithm at a threshold, which previously caused the checker to hang
on files with many repeated lines.
Speedup scales with codebase size from 1.5x on small projects
(~10k lines), to 20x on large ones (500k+ lines). Memory usage also
drops 12-27%. Codebases that previously hung or were OOM-killed could
now complete.
Refs #10881
Skip isort classification in the import checker when no import-ordering message is enabled,
and cache the isort configuration so it is built once instead of once per import statement.
Skipping the isort processing become a negligible improvement once the caching is applied.
pylint became ~~17% faster on ansible (~~=4500 imports) even with isort enabled.
Refs #10886, #2866, #10637
Finding the files to lint with --recursive=y is faster. Directories matching
ignore, ignore-patterns or ignore-paths (such as .venv, .git
or node_modules) are no longer walked, and neither are the packages already
found. In a checkout of pylint with its virtual environment, this step is about
three times faster when .git, .tox and .venv are ignored, and about
twenty times faster when a large ignored tree is present.
Directories and files are now also visited in sorted order, so the order in
which files are linted no longer depends on the file system. The order of
messages can change once for projects whose file system listed directories in
another order.
Closes #11005