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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.

Project description

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′)
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

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.


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


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 Subprocesses are going to see outdated variables, in disagreement with the current process.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.

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.

How to enable opinionated warnings

To enable these checks, specify a --select command-line option or select= option in your config file. As of Flake8 3.0, this option is a whitelist (checks not listed are being implicitly disabled), so you have to explicitly specify all checks you want enabled. For example:

max-line-length = 80
max-complexity = 12
ignore = E501
select = C,E,F,W,B,B901

Note that we’re enabling the complexity checks, the PEP8 pycodestyle errors and warnings, the pyflakes fatals and all default Bugbear checks. Finally, we’re also specifying B901 as a check that we want enabled. Some checks might need other flake8 checks disabled - e.g. E501 must be disabled for B950 to be hit.

If you’d like all optional warnings to be enabled for you (future proof your config!), say B9 instead of B901. You will need Flake8 3.2+ for this feature.

Note that pycodestyle also has a bunch of warnings that are disabled by default. Those get enabled as soon as there is an ignore = line in your configuration. I think this behavior is surprising so Bugbear’s opinionated warnings require explicit selection.


Just run:

coverage run tests/



Change Log


  • 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)


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


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


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


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


  • Fix grammar in B015 (#150)

  • Make sure float infinity/NaN does not trigger B008 (#155)

  • Handle positional-only args in class methods (#158)


  • 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)


  • Ignore keywords for B009/B010


  • Silence B009/B010 for non-identifiers

  • State an ignore might be needed for optional B9x checks


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


  • 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


  • Warn about continue/return/break in finally block (#100)

  • Removed a colon from the descriptive message in B008. (#96)


  • Fix .travis.yml syntax + add Python 3.8 + nightly tests

  • Fix black formatting + enforce via CI

  • Make B901 not apply to __await__ methods


  • 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.


  • black format all .py files

  • Examine kw-only args for mutable defaults

  • Test for Python 3.7


  • packaging fixes


  • graduated to Production/Stable in trove classifiers

  • introduced B008


  • bugfix: Also check async functions for B006 + B902


  • introduced B903 (patch contributed by Martijn Pieters)

  • bugfix: B902 now enforces cls for instance methods on metaclasses and metacls for class methods on metaclasses


  • 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


  • 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.


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


  • 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


  • introduced B005

  • introduced B006

  • introduced B950


  • bugfix: don’t raise false positives in B901 on closures within generators

  • gracefully fail on Python 2 in


  • introduced B004

  • introduced B901, thanks Markus!

  • update flake8 constraint to at least 3.0.0


  • introduced B003


  • 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


  • introduced B306


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


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


  • packaging herp derp


  • bugfix: include tests in the source package (to make test work for everyone)

  • bugfix: explicitly open README.rst in UTF-8 in for systems with other default encodings


  • first published version

  • date-versioned


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

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