[issue45483] pure Python class that has been registered as a `collections.abc.Sequence` can't be recgnized by the match statement without the `_abc` module

[Raymond Hettinger]

Change by Raymond Hettinger :


--
nosy: +rhettinger

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[issue45483] pure Python class that has been registered as a `collections.abc.Sequence` can't be recgnized by the match statement without the `_abc` module

[GalaxySnail]

New submission from GalaxySnail :

Pure Python class that has been registered as a `collections.abc.Sequence` 
can't be recgnized by the match statement without the `_abc` module.

For example:

```
>>> from test.support.import_helper import import_fresh_module

>>> collections_abc_with_c_abc = import_fresh_module(
... "collections.abc", fresh=["_collections_abc", "abc", "_abc"])

>>> class MyList:
... def __init__(self, iterable):
... self.list = list(iterable)
... def __len__(self):
... return len(self.list)
... def __getitem__(self, item):
... return self.list[item]
...
>>> collections_abc_with_c_abc.Sequence.register(MyList)

>>> match MyList(range(3, 10)):
... case [x, *_]:
... print(x)
... case _:
... print("not a sequence")
...
3

>>> collections_abc_with_py_abc = import_fresh_module(
... "collections.abc", fresh=["_collections_abc", "abc"], blocked=["_abc"])
>>> class MyList:
... def __init__(self, iterable):
... self.list = list(iterable)
... def __len__(self):
... return len(self.list)
... def __getitem__(self, item):
... return self.list[item]
...
>>> collections_abc_with_py_abc.Sequence.register(MyList)

>>> match MyList(range(3, 10)):
... case [x, *_]:
... print(x)
... case _:
... print("not a sequence")
...
not a sequence
```

It seems to be caused by 
https://github.com/python/cpython/commit/069e81ab3da46c441335ca762c4333b7bd91861d
 , only `tp_flags` are checked in the `MATCH_SEQUENCE` opcode. `Mapping` has 
the same problem.

--
messages: 404011
nosy: GalaxySnail
priority: normal
severity: normal
status: open
title: pure Python class that has been registered as a 
`collections.abc.Sequence` can't be recgnized by the match statement without 
the `_abc` module
type: behavior
versions: Python 3.10, Python 3.11

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