ERRORCODES.md

List of warnings

The following warnings are currently emitted by default:

Code	Description
Y001	Names of TypeVars, ParamSpecs and TypeVarTuples in stubs should usually start with _. This makes sure you don't accidentally expose names internal to the stub.
Y002	An if test must be a simple comparison against sys.platform or sys.version_info. Stub files support simple conditionals to indicate differences between Python versions or platforms, but type checkers only understand a limited subset of Python syntax. This warning is emitted on conditionals that type checkers may not understand.
Y003	Unrecognized sys.version_info check. Similar to Y002, but adds some additional checks specific to sys.version_info comparisons.
Y004	Version comparison must use only major and minor version. Type checkers like mypy don't know about patch versions of Python (e.g. 3.4.3 versus 3.4.4), only major and minor versions (3.3 versus 3.4). Therefore, version checks in stubs should only use the major and minor versions. If new functionality was introduced in a patch version, pretend that it was there all along.
Y005	Version comparison must be against a length-n tuple.
Y006	Use only < and >= for version comparisons. Comparisons involving > and <= may produce unintuitive results when tools do use the full sys.version_info tuple.
Y007	Unrecognized sys.platform check. Platform checks should be simple string comparisons.
Y008	Unrecognized platform in a sys.platform check. To prevent you from typos, we warn if you use a platform name outside a small set of known platforms (e.g. "linux" and "win32").
Y009	Empty class or function body should contain ..., not pass. This is just a stylistic choice, but it's the one typeshed made.
Y010	Function body must contain only .... Stub files should not contain code, so function bodies should be empty.
Y011	Only simple default values (int, float, complex, bytes, str, bool, None, ..., or simple container literals) are allowed for typed function arguments. Type checkers ignore the default value, so the default value is not useful information for type-checking, but it may be useful information for other users of stubs such as IDEs. If you're writing a stub for a function that has a more complex default value, use ... instead of trying to reproduce the runtime default exactly in the stub. Also use ... for very long numbers, very long strings, very long bytes, or defaults that vary according to the machine Python is being run on.
Y012	Class body must not contain pass.
Y013	Non-empty class body must not contain ....
Y014	Only simple default values are allowed for any function arguments. A stronger version of Y011 that includes arguments without type annotations.
Y015	Only simple default values are allowed for assignments. Similar to Y011, but for assignments rather than parameter annotations.
Y016	Unions shouldn't contain duplicates, e.g. str | str is not allowed.
Y017	Stubs should not contain assignments with multiple targets or non-name targets. E.g. T, S = TypeVar("T"), TypeVar("S") is disallowed, as is foo.bar = TypeVar("T").
Y018	A private TypeVar should be used at least once in the file in which it is defined.
Y019	Certain kinds of methods should use typing_extensions.Self instead of defining custom TypeVars for their return annotation. This check currently applies for instance methods that return self, class methods that return an instance of cls, and __new__ methods.
Y020	Quoted annotations should never be used in stubs.
Y021	Docstrings should not be included in stubs.
Y022	The typing and typing_extensions modules include various aliases to stdlib objects. Use these as little as possible (e.g. prefer builtins.list over typing.List, collections.Counter over typing.Counter, etc.).
Y023	Where there is no detriment to backwards compatibility, import objects such as ClassVar and NoReturn from typing rather than typing_extensions.
Y024	Use typing.NamedTuple instead of collections.namedtuple, as it allows for more precise type inference.
Y025	Always alias collections.abc.Set when importing it, so as to avoid confusion with builtins.set. E.g. use from collections.abc import Set as AbstractSet instead of from collections.abc import Set.
Y026	Type aliases should be explicitly demarcated with typing.TypeAlias (or use a PEP-695 type statement).
Y028	Always use class-based syntax for typing.NamedTuple, instead of assignment-based syntax.
Y029	It is almost always redundant to define __str__ or __repr__ in a stub file, as the signatures are almost always identical to object.__str__ and object.__repr__.
Y030	Union expressions should never have more than one Literal member, as Literal[1] | Literal[2] is semantically identical to Literal[1, 2].
Y031	TypedDicts should use class-based syntax instead of assignment-based syntax wherever possible. (In situations where this is not possible, such as if a field is a Python keyword or an invalid identifier, this error will not be emitted.)
Y032	The second argument of an __eq__ or __ne__ method should usually be annotated with object rather than Any.
Y033	Do not use type comments (e.g. x = ... # type: int) in stubs. Always use annotations instead (e.g. x: int).
Y034	Y034 detects common errors where certain methods are annotated as having a fixed return type, despite returning self at runtime. Such methods should be annotated with typing_extensions.Self. This check looks for:   1.  Any in-place BinOp dunder methods (__iadd__, __ior__, etc.) that do not return Self.   2.  __new__, __enter__ and __aenter__ methods that return the class's name unparameterised.   3.  __iter__ methods that return Iterator, even if the class inherits directly from Iterator.   4.  __aiter__ methods that return AsyncIterator, even if the class inherits directly from AsyncIterator. This check excludes methods decorated with @overload or @abstractmethod.
Y035	__all__, __match_args__ and __slots__ in a stub file should always have values, as these special variables in a .pyi file have identical semantics in a stub as at runtime. E.g. write __all__ = ["foo", "bar"] instead of __all__: list[str].
Y036	Y036 detects common errors in __exit__ and __aexit__ methods. For example, the first argument in an __exit__ method should either be annotated with object, _typeshed.Unused (a special alias for object) or type[BaseException] | None.
Y037	Use PEP 604 syntax instead of typing.Union and typing.Optional. E.g. use str | int instead of Union[str, int], and use str | None instead of Optional[str].
Y038	Use from collections.abc import Set as AbstractSet instead of from typing import AbstractSet.
Y039	Use str instead of typing.Text.
Y040	Never explicitly inherit from object, as all classes implicitly inherit from object in Python 3.
Y041	Y041 detects redundant numeric unions in the context of parameter annotations. For example, PEP 484 specifies that type checkers should allow int objects to be passed to a function, even if the function states that it accepts a float. As such, int is redundant in the union int | float in the context of a parameter annotation. In the same way, int is sometimes redundant in the union int | complex, and float is sometimes redundant in the union float | complex.
Y042	Type alias names should use CamelCase rather than snake_case.
Y043	Do not use names ending in "T" for private type aliases. (The "T" suffix implies that an object is a TypeVar.)
Y044	from __future__ import annotations has no effect in stub files, since type checkers automatically treat stubs as having those semantics.
Y045	__iter__ methods should never return Iterable[T], as they should always return some kind of iterator.
Y046	A private Protocol should be used at least once in the file in which it is defined.
Y047	A private TypeAlias should be used at least once in the file in which it is defined.
Y048	Function bodies should contain exactly one statement. (Note that if a function body includes a docstring, the docstring counts as a "statement".)
Y049	A private TypedDict should be used at least once in the file in which it is defined.
Y050	Prefer typing_extensions.Never over typing.NoReturn for argument annotations. This is a purely stylistic choice in the name of readability.
Y051	Y051 detects redundant unions between Literal types and builtin supertypes. For example, Literal[5] is redundant in the union int | Literal[5], and Literal[True] is redundant in the union Literal[True] | bool.
Y052	Y052 disallows assignments to constant values where the assignment does not have a type annotation. For example, x = 0 in the global namespace is ambiguous in a stub, as there are four different types that could be inferred for the variable x: int, Final[int], Literal[0], or Final[Literal[0]]. Enum members are excluded from this check, as are various special assignments such as __all__ and __match_args__.
Y053	Only string and bytes literals <=50 characters long are permitted.
Y054	Only numeric literals with a string representation <=10 characters long are permitted.
Y055	Unions of the form type[X] | type[Y] can be simplified to type[X | Y]. Similarly, Union[type[X], type[Y]] can be simplified to type[Union[X, Y]].
Y056	Do not call methods such as .append(), .extend() or .remove() on __all__. Different type checkers have varying levels of support for calling these methods on __all__. Use += instead, which is known to be supported by all major type checkers.
Y057	Do not use typing.ByteString or collections.abc.ByteString. These types have unclear semantics, and are deprecated; use typing_extensions.Buffer or a union such as bytes | bytearray | memoryview instead. See PEP 688 for more details.

Warnings disabled by default

The following error codes are also provided, but are disabled by default due to the risk of false-positive errors. To enable these error codes, use --extend-select={code1,code2,...} on the command line or in your flake8 configuration file.

Note that --extend-select will not work if you have --select specified on the command line or in your configuration file. We recommend only using --extend-select, never --select.

Code	Description
Y090	tuple[int] means "a tuple of length 1, in which the sole element is of type int". Consider using tuple[int, ...] instead, which means "a tuple of arbitrary (possibly 0) length, in which all elements are of type int".
Y091	Callable[<parameter_list>, None] or Callable[<parameter_list>, Any] is generally a mistake in the context of a parameter annotation for a function or method. The returned value from a callback is generally ignored at runtime, so Callable[<parameter_list>, object] is usually a better annotation in this kind of situation.