New submission from Paul <[email protected]>:
The section "Subtyping relationships with other types" of PEP 544 states:
"A concrete type X is a subtype of protocol P if and only if X implements all
protocol members of P with compatible types. In other words, subtyping with
respect to a protocol is always structural."
This requirement is violated by the current implementation of CPython (version
3.9.2):
```
from typing import Protocol
class P(Protocol):
pm: str # no default value, but still a protocol member
class C(P):
# inherits P but does NOT implement pm, since P did not provide a default
value
pass
assert isinstance(C(), P) # violates the PEP 544 requirement cited above
C().pm # raises: AttributeError: 'C' object has no attribute 'pm'
```
----------
components: Library (Lib)
messages: 388827
nosy: paul-dest
priority: normal
severity: normal
status: open
title: Bug in isinstance(instance, cls) with cls being a protocol? (PEP 544)
type: behavior
versions: Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue43512>
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