Hi Mark.
I've spoken with Guido, and we are willing to propose the following amendments
to PEP 634:
- Require `__match_args__` to be a tuple.
- Add new `__match_seq__` and `__match_map__` special attributes, corresponding
to new public `Py_TPFLAGS_MATCH_SEQ` and `Py_TPFLAGS_MATCH_MAP` flags for use
in `tp_flags`. When Python classes are defined with one or both of these
attributes set to a boolean value, `type.__new__` will update the flags on the
type to reflect the change (using a similar mechanism as `__slots__`
definitions). They will be inherited otherwise. For convenience,
`collections.abc.Sequence` will define `__match_seq__ = True`, and
`collections.abc.Mapping` will define `__match_map__ = True`.
Using this in Python would look like:
```
class MySeq:
__match_seq__ = True
...
class MyMap:
__match_map__ = True
...
```
Using this in C would look like:
```
PyTypeObject PyMySeq_Type = {
...
.tp_flags = Py_TPFLAGS_MATCH_SEQ | ...,
...
}
PyTypeObject PyMyMap_Type = {
...
.tp_flags = Py_TPFLAGS_MATCH_MAP | ...,
...
}
```
We believe that these changes will result in the best possible outcome:
- The new mechanism should faster than either PEP.
- The new mechanism should provide a better user experience than either PEP
when defining types in either Python *or C*.
If these amendments were made, would you be comfortable withdrawing PEP 653? We
think that if we're in agreement here, a compromise incorporating these
promising changes into the current design would be preferable to submitting yet
another large pattern matching PEP for a very busy SC to review and pronounce
before the feature freeze. I am also willing, able, and eager to implement
these changes promptly (perhaps even before the next alpha) if so.
Thanks for pushing us to make this better.
Brandt
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