I believe you are saying:

1) `isinstance(X(), P)` succeeds when it should fail, because X doesn't have `protocol_member` defined.

2) Protocols are not ABCs at runtime when PEP 544 says they should be.

I'm not one of the authors of PEP 544, so I can't give insight into why things are so.

For 1:

As noted by Ken Jin in the bug you linked, PEP 544 is concerned with Protocols as a static type checking concept, not a runtime concept. One reason is that this is impossible in some cases, such as due to type erasure. For example, even though `complex` implements `__float__`, it does not implement the SupportsFloat protocol, because complex's `__float__` signature differs from the SupportsFloat signature. This cannot be checked at runtime, because the type annotations are erased.

Your example, with `P.protocol_member`, shows another case. Because `protocol_member` has no default value, it is never stored in `P`. (i.e. `hasattr(P, 'protocol_member') is False.) Therefore it cannot be checked at runtime. Static type checkers don't drop annotated values like this, so a static type checker can evaluate `isinstance(X(), P)` correctly.

As PEP 544 says:
"All structural subtyping checks will be performed by static type checkers, such as mypy [mypy]. No additional support for protocol validation will be provided at runtime."

That said, I am a little confused as to why `isinstance(X(), P)` succeeds at all, because PEP 544 says "The default semantics is that isinstance() and issubclass() fail for protocol types." P should need to be decorated with `@runtime_checkable` for this to work.

For 2:

Protocols aren't literally instances of abc.ABCMeta, but they're still abstract base classes: they can't be instantiated and they define an interface that subclasses should implement. Saying they aren't ABCs at runtime is splitting hairs.

-- Teddy

On Thu, Mar 18, 2021 at 1:25 PM Paul Dest <paul.dest@web.de> wrote:

I hope to have been directed to the right place for my question from https://bugs.python.org/issue43512
I'm completely new to the Python dev process. Therefore, I ask for your indulgence if my approach is not in line with the process.

I would like to collect opinions (especially from authors of PEP 544) if the CPython behavior described below should be regarded as a violation of PEP 544 or at least as a behavior that should be improved.

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."

In contrast to that, the behavior of CPython 3.9.2 for protocols with protocol members without default values is as follows:
```
from typing import Protocol

class P(Protocol):
protocol_member: str # no default value, but still a protocol member

class X(P):
# inherits P but does NOT implement protocol_member, since P did not provide a default value
pass

assert isinstance(X(), P) # violates the PEP 544 requirement cited above

X().protocol_member # raises: AttributeError: 'X' object has no attribute 'protocol_member'
```

In this regard, it was argued that "At runtime, protocol classes will be simple ABCs." (PEP 544)

But unfortunately, this is currently not the case. Actually, there is an extra metaclass for protocols, solely to provide an __instancecheck__.
https://github.com/python/cpython/blob/3.9/Lib/typing.py#L1096

```
class _ProtocolMeta(ABCMeta):
# This metaclass is really unfortunate and exists only because of
# the lack of __instancehook__.
def __instancecheck__(cls, instance):
# We need this method for situations where attributes are
# assigned in __init__.
if ((not getattr(cls, '_is_protocol', False) or
_is_callable_members_only(cls)) and
issubclass(instance.__class__, cls)):
return True
if cls._is_protocol:
if all(hasattr(instance, attr) and
# All *methods* can be blocked by setting them to None.
(not callable(getattr(cls, attr, None)) or
getattr(instance, attr) is not None)
for attr in _get_protocol_attrs(cls)):
return True
return super().__instancecheck__(instance)
```

I am inclined to assess the behavior described above as a violation of PEP 544 and an incomplete implementation of `_ProtocolMeta`.
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