flake8-pyi's error codes can currently be divided into the following categories:
| Category | Description | Example |
|---|---|---|
| Understanding stubs | Stub files differ from .py files in many respects when it comes to how they work and how they're used. These error codes flag antipatterns that may demonstrate that a user does not fully understand the semantics or purpose of stub files. |
Y010 |
| Correctness | These error codes flag places where it looks like the stub might be incorrect in a way that would be visible for users of the stub. | Y001 |
| Redundant code | These error codes flag places where it looks like code could simply be deleted. | Y016 |
| Style | These error codes enforce a consistent, stub-specific style guide. | Y009 |
The following warnings are currently emitted by default:
| Code | Description | Code category |
|---|---|---|
| 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. |
Correctness |
| 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. |
Correctness |
| Y003 | Unrecognized sys.version_info check. Similar to Y002, but adds some additional checks specific to sys.version_info comparisons. |
Correctness |
| 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. | Correctness |
| Y005 | Version comparison must be against a length-n tuple. | Correctness |
| Y006 | Use only < and >= for version comparisons. Comparisons involving > and <= may produce unintuitive results when tools do use the full sys.version_info tuple. |
Correctness |
| Y007 | Unrecognized sys.platform check. Platform checks should be simple string comparisons. |
Correctness |
| 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"). |
Correctness |
| Y009 | Empty class or function body should contain ..., not pass. |
Style |
| Y010 | Function body must contain only .... Stub files are never executed at runtime, so function bodies should be empty. |
Understanding stubs |
| 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. |
Style |
| Y012 | Class body must not contain pass. |
Style |
| Y013 | Non-empty class body must not contain .... |
Redundant code |
| Y014 | Only simple default values are allowed for any function arguments. A stronger version of Y011 that includes arguments without type annotations. | Style |
| Y015 | Only simple default values are allowed for assignments. Similar to Y011, but for assignments rather than parameter annotations. | Style |
| Y016 | Unions shouldn't contain duplicates, e.g. str | str is not allowed. |
Redundant code |
| 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"). |
Style |
| Y018 | A private TypeVar should be used at least once in the file in which it is defined. |
Redundant code |
| 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. |
Style |
| Y020 | Quoted annotations should never be used in stubs. Since stub files are never executed at runtime, forward references can be used in any location without having to use quotes. (See also: Y044.) | Understanding stubs |
| Y021 | Docstrings should not be included in stubs. | Style |
| 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.). |
Style |
| Y023 | Where there is no detriment to backwards compatibility, import objects such as ClassVar and NoReturn from typing rather than typing_extensions. |
Style |
| Y024 | Use typing.NamedTuple instead of collections.namedtuple, as it allows for more precise type inference. |
Correctness |
| 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. |
Style |
| Y026 | Type aliases should be explicitly demarcated with typing.TypeAlias (or use a PEP-695 type statement). |
Correctness |
| Y028 | Always use class-based syntax for typing.NamedTuple, instead of assignment-based syntax. |
Correctness |
| 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__. |
Understanding stubs |
| Y030 | Union expressions should never have more than one Literal member, as Literal[1] | Literal[2] is semantically identical to Literal[1, 2]. |
Style |
| 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.) |
Style |
| Y032 | The second argument of an __eq__ or __ne__ method should usually be annotated with object rather than Any. |
Correctness |
| Y033 | Do not use type comments (e.g. x = ... # type: int) in stubs. Always use annotations instead (e.g. x: int). |
Style |
| 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. |
Correctness |
| 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]. |
Correctness |
| 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. |
Correctness |
| Y037 | Use PEP 604 syntax instead of typing(_extensions).Union and typing(_extensions).Optional. E.g. use str | int instead of Union[str, int], and use str | None instead of Optional[str]. |
Style |
| Y038 | Use from collections.abc import Set as AbstractSet instead of from typing import AbstractSet or from typing_extensions import AbstractSet. |
Style |
| Y039 | Use str instead of typing.Text or typing_extensions.Text. |
Style |
| Y040 | Never explicitly inherit from object, as all classes implicitly inherit from object in Python 3. |
Style |
| 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. |
Style |
| Y042 | Type alias names should use CamelCase rather than snake_case. | Style |
| Y043 | Do not use names ending in "T" for private type aliases. (The "T" suffix implies that an object is a TypeVar.) |
Style |
| Y044 | from __future__ import annotations has no effect in stub files, since type checkers automatically treat stubs as having those semantics. (See also: Y020.) |
Understanding stubs |
| Y045 | __iter__ methods should never return Iterable[T], as they should always return some kind of iterator. |
Correctness |
| Y046 | A private Protocol should be used at least once in the file in which it is defined. |
Redundant code |
| Y047 | A private TypeAlias should be used at least once in the file in which it is defined. |
Redundant code |
| Y048 | Function bodies should contain exactly one statement. This is because stub files are never executed at runtime, so any more than one statement would be redundant. (Note that if a function body includes a docstring, the docstring counts as a "statement".) | Understanding stubs |
| Y049 | A private TypedDict should be used at least once in the file in which it is defined. |
Redundant code |
| Y050 | Prefer typing_extensions.Never over typing.NoReturn for argument annotations. |
Style |
| 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. |
Redundant code |
| 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__. |
Correctness |
| Y053 | Only string and bytes literals <=50 characters long are permitted. | Style |
| Y054 | Only numeric literals with a string representation <=10 characters long are permitted. | Style |
| 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]]. |
Style |
| 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. |
Correctness |
| 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. |
Correctness |
| Y058 | Use Iterator rather than Generator as the return value for simple __iter__ methods, and AsyncIterator rather than AsyncGenerator as the return value for simple __aiter__ methods. Using (Async)Iterator for these methods is simpler and more elegant, and reflects the fact that the precise kind of iterator returned from an __iter__ method is usually an implementation detail that could change at any time, and should not be relied upon. |
Style |
| Y059 | Generic[] should always be the last base class, if it is present in a class's bases tuple. At runtime, if Generic[] is not the final class in a the bases tuple, this can cause the class creation to fail. In a stub file, however, this rule is enforced purely for stylistic consistency. |
Style |
| Y060 | Redundant inheritance from Generic[]. For example, class Foo(Iterable[_T], Generic[_T]): ... can be written more simply as class Foo(Iterable[_T]): ....To avoid false-positive errors, and to avoid complexity in the implementation, this check is deliberately conservative: it only flags classes where all subscripted bases have identical code inside their subscript slices. |
Style |
| Y061 | Do not use None inside a Literal[] slice. For example, use Literal["foo"] | None instead of Literal["foo", None]. While both are legal according to PEP 586, the former is preferred for stylistic consistency. Note that this warning is not emitted if Y062 is emitted for the same Literal[] slice. For example, Literal[None, None, True, True] only causes Y062 to be emitted. |
Style |
| Y062 | Literal[] slices shouldn't contain duplicates, e.g. Literal[True, True] is not allowed. |
Redundant code |
| Y063 | Use PEP 570 syntax (e.g. def foo(x: int, /) -> None: ...) to denote positional-only arguments, rather than the older Python 3.7-compatible syntax described in PEP 484 (def foo(__x: int) -> None: ..., etc.). |
Style |
| Y064 | Use simpler syntax to define final literal types. For example, use x: Final = 42 instead of x: Final[Literal[42]]. |
Style |
| Y065 | Don't use bare Incomplete in argument and return annotations. Instead, leave them unannotated. Omitting an annotation entirely from a function will cause some type checkers to view the parameter or return type as "untyped"; this may result in stricter type-checking on code that makes use of the stubbed function. |
Style |
| Y066 | When using if/else with sys.version_info, put the code for new Python versions first. |
Style |
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 | Code category |
|---|---|---|
| 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". |
Correctness |