Change Y034 rule such that it does not emit errors in protocols (#541)

Closes #539.

---------

Co-authored-by: Sebastian Rittau <sebastian.rittau@zfutura.de>

Author: jonathandung

CHANGELOG.md

@@ -10,6 +10,7 @@ flake8-pyi uses Calendar Versioning (CalVer).
   avoid false positives in stub files. This monkey patch has been removed.
   Instead, we now recommend to disable F821 when running flake8 on stub files.
 * Remove the now unnecessary `--no-pyi-aware-file-checker` option.
+* Y034 no longer triggers in protocol class definitions.
 
 ### New Error Codes
 

ERRORCODES.md

@@ -47,7 +47,7 @@ The following warnings are currently emitted by default:
 | <a id="Y031" href="#Y031">Y031</a> | `TypedDict`s 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
 | <a id="Y032" href="#Y032">Y032</a> | The second argument of an `__eq__` or `__ne__` method should usually be annotated with `object` rather than `Any`. | Correctness
 | <a id="Y033" href="#Y033">Y033</a> | Do not use type comments (e.g. `x = ... # type: int`) in stubs. Always use annotations instead (e.g. `x: int`). | Style
-| <a id="Y034" href="#Y034">Y034</a> | 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:<br><br>&nbsp;&nbsp;**1.**&nbsp;&nbsp;Any in-place BinOp dunder methods (`__iadd__`, `__ior__`, etc.) that do not return `Self`.<br>&nbsp;&nbsp;**2.**&nbsp;&nbsp;`__new__`, `__enter__` and `__aenter__` methods that return the class's name unparameterised.<br>&nbsp;&nbsp;**3.**&nbsp;&nbsp;`__iter__` methods that return `Iterator`, even if the class inherits directly from `Iterator`.<br>&nbsp;&nbsp;**4.**&nbsp;&nbsp;`__aiter__` methods that return `AsyncIterator`, even if the class inherits directly from `AsyncIterator`.<br><br>This check excludes methods decorated with `@overload` or `@abstractmethod`. | Correctness
+| <a id="Y034" href="#Y034">Y034</a> | 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:<br><br>&nbsp;&nbsp;**1.**&nbsp;&nbsp;Any in-place BinOp dunder methods (`__iadd__`, `__ior__`, etc.) that do not return `Self`.<br>&nbsp;&nbsp;**2.**&nbsp;&nbsp;`__new__`, `__enter__` and `__aenter__` methods that return the class's name unparameterised.<br>&nbsp;&nbsp;**3.**&nbsp;&nbsp;`__iter__` methods that return `Iterator`, even if the class inherits directly from `Iterator`.<br>&nbsp;&nbsp;**4.**&nbsp;&nbsp;`__aiter__` methods that return `AsyncIterator`, even if the class inherits directly from `AsyncIterator`.<br><br>Note that this check excludes methods decorated with `@overload` or `@abstractmethod`, and those in the bodies of protocol classes. | Correctness
 | <a id="Y035" href="#Y035">Y035</a> | `__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
 | <a id="Y036" href="#Y036">Y036</a> | 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
 | <a id="Y037" href="#Y037">Y037</a> | 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

flake8_pyi/visitor.py

@@ -366,6 +366,10 @@ def _has_bad_hardcoded_returns(
     if class_ctx.is_metaclass:
         return False
 
+    # Return false if in a protocol (class inheriting from typing.Protocol); see #539
+    if class_ctx.is_protocol_class:
+        return False
+
     # Much too complex for our purposes to worry
     # about overloaded functions or abstractmethods
     if any(

tests/classdefs.pyi

@@ -89,6 +89,9 @@ class InvalidButPluginDoesNotCrash:
     def __enter__() -> InvalidButPluginDoesNotCrash: ...
     async def __aenter__() -> InvalidButPluginDoesNotCrash: ...
 
+class ProtocolsAreExemptedFromY034(typing.Protocol):
+    def __iadd__(self, other: Self, /) -> object: ...
+
 class BadIterator1(Iterator[int]):
     def __iter__(self) -> Iterator[int]: ...  # Y034 "__iter__" methods in classes like "BadIterator1" usually return "self" at runtime. Consider using "typing_extensions.Self" in "BadIterator1.__iter__", e.g. "def __iter__(self) -> Self: ..."