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Allowing typing.NoReturn as a return hint for __init__ methods #10342
Description
Feature
Allow typing.NoReturn as a return hint for __init__ methods, instead of constraining it to just None.
Pitch
The following is valid Python code:
class A:
def __init__(self):
raise RuntimeError>>> A()
Traceback (most recent call last):
...
RuntimeErrorMany projects using Boost.Python will employ its option to define classes exported to Python which aren't meant to be instantiated directly in Python, but rather via helper functions/methods, and attempts to instantiate these classes directly in Python will look like the above. These are created by defining a boost::python::class with boost::python::no_init, and interfacing with the Python API typically looks like the following:
>>> import MyBoostPythonModule # A binary library
>>>
>>> # The final ancestor is `builtins.object`, just like standard Python classes.
>>> # Verify that this is `True`
>>> MyBoostPythonModule.MyNoInitClass.__mro__
(MyBoostPythonModule.MyNoInitClass, Boost.Python.instance, object)
>>> MyBoostPythonModule.MyNoInitClass.__mro__[-1] is object
True
>>>
>>> # Bad attempt at instantiation
>>> # See https://github.com/boostorg/python/blob/f5d14ef15e98838b3cf18692d46c481287ed50d0/src/object/class.cpp#L656-L674
>>> MyBoostPythonModule.MyNoInitClass()
Traceback (most recent call last):
...
RuntimeError: This class cannot be instantiated from Python
>>>
>>> # Example instance-getting function
>>> type(MyBoostPythonModule.GetMyNoInitClass).__mro__
(Boost.Python.function, object)
>>> MyBoostPythonModule.GetMyNoInitClass() # OK
<MyBoostPythonModule.MyNoInitClass at 0x7fc56b2f25d0>Having the ability to define stub classes which look like
from typing import NoReturn
class A:
def __init__(self) -> NoReturn: ...and to be able to use mypy to detect bad attempts at creating objects would greatly aid bug detection.
Activity
[-]Allowing `typing.NoReturn` as the return hint for `__init__` methods[/-][+]Allowing `typing.NoReturn` as a return hint for `__init__` methods[/+]JelleZijlstra commentedon Apr 20, 2021
This also recently came up in python/typeshed#5137 (comment). Annotating
__init__asNoReturnseems like a reasonable way to indicate that a class cannot be instantiated.AlexWaygood commentedon Aug 25, 2022
Implemented in #13480 🎉