New submission from Saumitro Dasgupta <[email protected]>:
Adding type annotations at runtime may lead to inconsistent results. Consider
the following example:
class Base:
base: int
class Alpha(Base):
pass
class Beta(Base):
foobar: int
# Case 1: This mutates Base's __annotations__
Alpha.__annotations__['injected'] = bool
assert Alpha.__annotations__ is Base.__annotations__
# Case 2: This mutates Beta's own copy of __annotations__
Beta.__annotations__['injected'] = bool
Such mutations of __annotations__ seem to be perfectly legal
(https://www.python.org/dev/peps/pep-0526/#runtime-effects-of-type-annotations).
However:
1. In case 1, this leads to the accidental mutation of Base's annotations. Not
entirely certain if that's expected, but seems undesirable.
2. There are further differences when looking at `__dict__['__annotations__']`:
for Alpha, there is no __annotations__ entry in __dict__. However, for Beta,
it's set to `{'foobar': <class 'int'>, 'injected': <class 'bool'>}`. This
discrepancy leads to further inconsistent results. In particular, when
transforming these classes to dataclasses, which specifically looks at
__dict__['__annotations__'](https://github.com/python/cpython/blob/3.8/Lib/dataclasses.py#L856).
Converting Alpha to a dataclass leads to no fields. Converting Beta to a
dataclass leads to two fields (foobar and injected). It's worth noting that
typing.get_type_hints produces reasonable results here.
----------
components: Interpreter Core, Library (Lib)
messages: 368569
nosy: Saumitro Dasgupta
priority: normal
severity: normal
status: open
title: Runtime type annotation mutation leads to inconsistent behavior
type: behavior
versions: Python 3.8
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Python tracker <[email protected]>
<https://bugs.python.org/issue40583>
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