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flake8-bugbear

pre-commit.ci status

A plugin for flake8 finding likely bugs and design problems in your program. Contains warnings that don't belong in pyflakes and pycodestyle:

bug·bear  (bŭg′bâr′)
n.
1. A cause of fear, anxiety, or irritation: *Overcrowding is often
   a bugbear for train commuters.*
2. A difficult or persistent problem: *"One of the major bugbears of
   traditional AI is the difficulty of programming computers to
   recognize that different but similar objects are instances of the
   same type of thing" (Jack Copeland).*
3. A fearsome imaginary creature, especially one evoked to frighten
   children.

It is felt that these lints don't belong in the main Python tools as they are very opinionated and do not have a PEP or standard behind them. Due to flake8 being designed to be extensible, the original creators of these lints believed that a plugin was the best route. This has resulted in better development velocity for contributors and adaptive deployment for flake8 users.

Installation

Install from pip with:

pip install flake8-bugbear

It will then automatically be run as part of flake8; you can check it has been picked up with:

$ flake8 --version
3.5.0 (assertive: 1.0.1, flake8-bugbear: 18.2.0, flake8-comprehensions: 1.4.1, mccabe: 0.6.1, pycodestyle: 2.3.1, pyflakes: 1.6.0) CPython 3.7.0 on Darwin

Development

If you'd like to do a PR we have development instructions here.

List of warnings

B001: Do not use bare except:, it also catches unexpected events like memory errors, interrupts, system exit, and so on. Prefer except Exception:. If you're sure what you're doing, be explicit and write except BaseException:. Disable E722 to avoid duplicate warnings.

B002: Python does not support the unary prefix increment. Writing ++n is equivalent to +(+(n)), which equals n. You meant n += 1.

B003: Assigning to os.environ doesn't clear the environment. Subprocesses are going to see outdated variables, in disagreement with the current process. Use os.environ.clear() or the env= argument to Popen.

B004: Using hasattr(x, '__call__') to test if x is callable is unreliable. If x implements custom __getattr__ or its __call__ is itself not callable, you might get misleading results. Use callable(x) for consistent results.

B005: Using .strip() with multi-character strings is misleading the reader. It looks like stripping a substring. Move your character set to a constant if this is deliberate. Use .replace() or regular expressions to remove string fragments.

B006: Do not use mutable data structures for argument defaults. They are created during function definition time. All calls to the function reuse this one instance of that data structure, persisting changes between them.

B007: Loop control variable not used within the loop body. If this is intended, start the name with an underscore.

B008: Do not perform function calls in argument defaults. The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call. If this is intended, assign the function call to a module-level variable and use that variable as a default value.

B009: Do not call getattr(x, 'attr'), instead use normal property access: x.attr. Missing a default to getattr will cause an AttributeError to be raised for non-existent properties. There is no additional safety in using getattr if you know the attribute name ahead of time.

B010: Do not call setattr(x, 'attr', val), instead use normal property access: x.attr = val. There is no additional safety in using setattr if you know the attribute name ahead of time.

B011: Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().

B012: Use of break, continue or return inside finally blocks will silence exceptions or override return values from the try or except blocks. To silence an exception, do it explicitly in the except block. To properly use a break, continue or return refactor your code so these statements are not in the finally block.

B013: A length-one tuple literal is redundant. Write except SomeError: instead of except (SomeError,):.

B014: Redundant exception types in except (Exception, TypeError):. Write except Exception:, which catches exactly the same exceptions.

B015: Pointless comparison. This comparison does nothing but waste CPU instructions. Either prepend assert or remove it.

B016: Cannot raise a literal. Did you intend to return it or raise an Exception?

B017: self.assertRaises(Exception): should be considered evil. It can lead to your test passing even if the code being tested is never executed due to a typo. Either assert for a more specific exception (builtin or custom), use assertRaisesRegex, or use the context manager form of assertRaises (with self.assertRaises(Exception) as ex:) with an assertion against the data available in ex.

B018: Found useless expression. Either assign it to a variable or remove it.

B019: Use of functools.lru_cache or functools.cache on methods can lead to memory leaks. The cache may retain instance references, preventing garbage collection.

B020: Loop control variable overrides iterable it iterates

B021: f-string used as docstring. This will be interpreted by python as a joined string rather than a docstring.

B022: No arguments passed to contextlib.suppress. No exceptions will be suppressed and therefore this context manager is redundant. N.B. this rule currently does not flag suppress calls to avoid potential false positives due to similarly named user-defined functions.

B023: Functions defined inside a loop must not use variables redefined in the loop, because late-binding closures are a classic gotcha.

Opinionated warnings

The following warnings are disabled by default because they are controversial. They may or may not apply to you, enable them explicitly in your configuration if you find them useful. Read below on how to enable.

B901: Using return x in a generator function used to be syntactically invalid in Python 2. In Python 3 return x can be used in a generator as a return value in conjunction with yield from. Users coming from Python 2 may expect the old behavior which might lead to bugs. Use native async def coroutines or mark intentional return x usage with # noqa on the same line.

B902: Invalid first argument used for method. Use self for instance methods, and cls for class methods (which includes __new__ and __init_subclass__) or instance methods of metaclasses (detected as classes directly inheriting from type).

B903: Use collections.namedtuple (or typing.NamedTuple) for data classes that only set attributes in an __init__ method, and do nothing else. If the attributes should be mutable, define the attributes in __slots__ to save per-instance memory and to prevent accidentally creating additional attributes on instances.

B904: Within an except clause, raise exceptions with raise ... from err or raise ... from None to distinguish them from errors in exception handling. See the exception chaining tutorial for details.

B950: Line too long. This is a pragmatic equivalent of pycodestyle's E501: it considers "max-line-length" but only triggers when the value has been exceeded by more than 10%. You will no longer be forced to reformat code due to the closing parenthesis being one character too far to satisfy the linter. At the same time, if you do significantly violate the line length, you will receive a message that states what the actual limit is. This is inspired by Raymond Hettinger's "Beyond PEP 8" talk and highway patrol not stopping you if you drive < 5mph too fast. Disable E501 to avoid duplicate warnings. Like E501, this error ignores long shebangs on the first line and urls or paths that are on their own line:

#! long shebang ignored

# https://some-super-long-domain-name.com/with/some/very/long/paths
url = (
    "https://some-super-long-domain-name.com/with/some/very/long/paths"
)

How to enable opinionated warnings

To enable Bugbear's opinionated checks (B9xx), specify an --extend-select command-line option or extend-select= option in your config file (requires flake8 >=4.0):

[flake8]
max-line-length = 80
max-complexity = 12
...
extend-ignore = E501
extend-select = B950

Some of Bugbear's checks require other flake8 checks disabled - e.g. E501 must be disabled when enabling B950.

If you'd like all optional warnings to be enabled for you (future proof your config!), say B9 instead of B950. You will need flake8 >=3.2 for this feature.

For flake8 <=4.0, you will need to use the --select command-line option or select= option in your config file. For flake8 >=3.0, this option is a whitelist (checks not listed are implicitly disabled), so you have to explicitly specify all checks you want enabled (e.g. select = C,E,F,W,B,B950).

The --extend-ignore command-line option and extend-ignore= config file option require flake8 >=3.6. For older flake8 versions, the --ignore and ignore= options can be used. Using ignore will override all codes that are disabled by default from all installed linters, so you will need to specify these codes in your configuration to silence them. I think this behavior is surprising so Bugbear's opinionated warnings require explicit selection.

Note: Bugbear's enforcement of explicit opinionated warning selection is deprecated and will be removed in a future release. It is recommended to use extend-ignore and extend-select in your flake8 configuration to avoid implicitly altering selected and/or ignored codes.

Configuration

The plugin currently has one setting:

extend-immutable-calls: Specify a list of additional immutable calls. This could be useful, when using other libraries that provide more immutable calls, beside those already handled by flake8-bugbear. Calls to these method will no longer raise a B008 warning.

For example:

[flake8]
max-line-length = 80
max-complexity = 12
...
extend-immutable-calls = pathlib.Path, Path

Tests / Lints

Just run:

coverage run tests/test_bugbear.py

For linting:

pre-commit run -a

License

MIT

Change Log

22.7.1

22.6.22

  • Don't crash when select / extend_select are None (#261)
  • Ignore lambda arguments for B020 (#259)
  • Fix missing space typos in B021, B022 error messages (#257)

22.4.25

  • Ignore black formatting for b013 test case (#251)
  • B010 Fix lambda flase positive (#246)
  • B008 Fix edge case with lambda functions (#243)

22.3.23

  • B006 and B008: Detect function calls at any level of the default expression (#239)
  • B020: Fix comprehension false postives (#238)
  • Tweak B019 desc (#237)

22.3.20

  • B022: No arguments passed to contextlib.suppress (#231)
  • B021: f-string used as docstring. (#230)
  • B020: ensure loop control variable doesn't overrides iterable it iterates (#220)
  • B019: check to find cache decorators on class methods (#218)
  • Fix crash on long empty string (#223)

22.1.11

  • B018: Ignore JoinedStr (#216)
  • Build universal Python 3 wheels (#214)
  • B950: Add same special cases as E501 (#213)

21.11.29

  • B018: Disable strings from check for now (#209)

21.11.28

  • B904: ensure the raise is in the same context with the except (#191)
  • Add Option to extend the list of immutable calls (#204)
  • Update B014: binascii.Error is now treated as a subclass of ValueError (#206)
  • add simple pre-commit config (#205)
  • Test with 3.10 official
  • Add B018 check to find useless declarations (#196, #202)

21.9.2

  • Fix crash on call in except statement in _to_name_str (#187)
  • Update B006: list, dictionary, and set comprehensions are now also disallowed (#186)

21.9.1

  • Update B008: Whitelist more immutable function calls (#173)
  • Remove Python Compatibility Warnings (#182)
  • Add B904: check for raise without from in an except clause (#181)
  • Add Python 3.10 tests to ensure we pass (#183)

21.4.3

  • Verify the element in item_context.args is of type ast.Name for b017

21.4.2

  • Add another hasattr() check to b017 visit for .func

21.4.1

  • Add B017: check for gotta-catch-em-all assertRaises(Exception)

21.3.2

  • Fix crash on tuple expansion in try/except block (#161)

21.3.1

  • Fix grammar in B015 (#150)
  • Make sure float infinity/NaN does not trigger B008 (#155)
  • Handle positional-only args in class methods (#158)

20.11.1

  • Support exception aliases properly in B014 (#129)
  • Add B015: Pointless comparison (#130)
  • Remove check for # noqa comments (#134)
  • Ignore exception classes which are not types (#135)
  • Introduce B016 to check for raising a literal. (#141)
  • Exclude types.MappingProxyType() from B008. (#144)

20.1.4

  • Ignore keywords for B009/B010

20.1.3

  • Silence B009/B010 for non-identifiers
  • State an ignore might be needed for optional B9x checks

20.1.2

  • Fix error on attributes-of-attributes in except (...): clauses

20.1.1

  • Allow continue/break within loops in finally clauses for B012
  • For B001, also check for except ():
  • Introduce B013 and B014 to check tuples in except (..., ): statements

20.1.0

  • Warn about continue/return/break in finally block (#100)
  • Removed a colon from the descriptive message in B008. (#96)

19.8.0

  • Fix .travis.yml syntax + add Python 3.8 + nightly tests
  • Fix black formatting + enforce via CI
  • Make B901 not apply to __await__ methods

19.3.0

  • allow 'mcs' for metaclass classmethod first arg (PyCharm default)
  • Introduce B011
  • Introduce B009 and B010
  • Exclude immutable calls like tuple() and frozenset() from B008
  • For B902, the first argument for metaclass class methods can be "mcs", matching the name preferred by PyCharm.

18.8.0

  • black format all .py files
  • Examine kw-only args for mutable defaults
  • Test for Python 3.7

18.2.0

  • packaging fixes

17.12.0

  • graduated to Production/Stable in trove classifiers
  • introduced B008

17.4.0

  • bugfix: Also check async functions for B006 + B902

17.3.0

  • introduced B903 (patch contributed by Martijn Pieters)
  • bugfix: B902 now enforces cls for instance methods on metaclasses and metacls for class methods on metaclasses

17.2.0

  • introduced B902
  • bugfix: opinionated warnings no longer invisible in Syntastic
  • bugfix: opinionated warnings stay visible when --select on the command-line is used with full three-digit error codes

16.12.2

  • bugfix: opinionated warnings no longer get enabled when user specifies ignore = in the configuration. Now they require explicit selection as documented above also in this case.

16.12.1

  • bugfix: B007 no longer crashes on tuple unpacking in for-loops

16.12.0

  • introduced B007
  • bugfix: remove an extra colon in error formatting that was making Bugbear errors invisible in Syntastic
  • marked as "Beta" in trove classifiers, it's been used in production for 8+ months

16.11.1

  • introduced B005
  • introduced B006
  • introduced B950

16.11.0

  • bugfix: don't raise false positives in B901 on closures within generators
  • gracefully fail on Python 2 in setup.py

16.10.0

  • introduced B004
  • introduced B901, thanks Markus!
  • update flake8 constraint to at least 3.0.0

16.9.0

  • introduced B003

16.7.1

  • bugfix: don't omit message code in B306's warning
  • change dependency on pep8 to dependency on pycodestyle, update flake8 constraint to at least 2.6.2

16.7.0

  • introduced B306

16.6.1

  • bugfix: don't crash on files with tuple unpacking in class bodies

16.6.0

  • introduced B002, B301, B302, B303, B304, and B305

16.4.2

  • packaging herp derp

16.4.1

  • bugfix: include tests in the source package (to make setup.py test work for everyone)
  • bugfix: explicitly open README.rst in UTF-8 in setup.py for systems with other default encodings

16.4.0

  • first published version
  • date-versioned

Authors

Glued together by Łukasz Langa. Multiple improvements by Markus Unterwaditzer, Martijn Pieters, Cooper Lees, and Ryan May.

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A plugin for Flake8 finding likely bugs and design problems in your program. Contains warnings that don't belong in pyflakes and pycodestyle.

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