Hi all,
Tl;dr - As mentioned in issue https://bugs.python.org/issue5945,
PySequence_Check and PyMapping_Check should be deprecated and
discontinued. The reason for this is that they do not provide accurate
results and can lead to unexpected behavior. (PyMapping_Check for a list
will return True, and PySequence_Check for a mapping that doesn't subclass
dict will also return true)
This proposal is to have a process to deprecate those and introduce new functions instead.
Thanks to the latest changes in the C-API, we now have flags set for classes that are sequences or mapping - those were created for the pattern matching functionality.
This lets us have a path for clear and accurate determination of sequence or mapping.
The use cases for the change are as follows:
C-API code that wishes to use the PySequence/PyMapping API and to guarantee correct behavior needs to check it is indeed a valid sequence/mapping
Static types casting from ambiguous “PyObject” to “PySequence” & “PyMapping” types as provided by PyO3 ( https://pyo3.rs/v0.15.1/conversions/tables.html#argument-types)
The correct way to determine right now if an object is a sequence, is to
check its Py_TPFLAGS_SEQUENCE flag and check if it’s a string, bytes or
bytearray as they are the only sequences not having TPFLAGS_SEQUENCE per
specification. This is cumbersome for most non-expert developers not aware
of the nuances in the C-API. Having a C-API that provides this check out of
the box can ease it for most.
PyMapping_Check and PySequence_Check are still widely used and are
**part of the stable ABI** even though core developers recommend avoiding
those. This is very odd to stabilize and is an API that should be
deprecated.
Our suggested change is to add two new functions: PyMapping_CheckNew &
PySequence_CheckNew. They will be implemented using the flags check and
the str/bytes/bytearray check. Those functions will be 100% accurate and
follow the existing documentations.
We will do the following process to improve C-API:
Add these functions. 2.
Add a deprecation notice on PyMapping_Check and PySequence_Check and
remove all built-in usage of it. We will recommend the use of the new
functions.
3.
Drop the old functions. 4.
Alias the new functions to PyMapping_Check and PySequence_Check and
add a deprecation notice of PyMapping_CheckNew and PySequence_CheckNew.
5.
Rename PyMapping_CheckNew and PySequence_CheckNew to Mapping_Check
and PySequence_Check.
The timeline will probably be long as it’s part of stable ABI, but in our opinion, the change is for the better.
Caveats:
Sequences and Mappings will match the typing.Sequence and typing.Mapping abcs. Duck typing is further “discouraged”. We do not think of it as an issue since Python already has idioms and static type checkers that discourage this sort of duck typing.
C extensions skipping multiple minor Python versions will use the new code without their knowledge, which is not backwards compatible. This is an issue with any removal of a stable ABI or language function, and we consider it acceptable.
Thanks to Bar Harel for helping me with the proposal.
This is was initially discussed in this bpo: https://bugs.python.org/issue46376
Best regards,
Aviram