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Issue #23722: improve __classcell__ compatibility

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Handling zero-argument super() in __init_subclass__ and
__set_name__ involved moving __class__ initialisation to
type.__new__. This requires cooperation from custom
metaclasses to ensure that the new __classcell__ entry
is passed along appropriately.

The initial implementation of that change resulted in abruptly
broken zero-argument super() support in metaclasses that didn't
adhere to the new requirements (such as Django's metaclass for
Model definitions).

The updated approach adopted here instead emits a deprecation
warning for those cases, and makes them work the same way they
did in Python 3.5.

This patch also improves the related class machinery documentation
to cover these details and to include more reader-friendly
cross-references and index entries.
This commit is contained in:
Nick Coghlan 2016-12-05 16:47:55 +10:00
parent 71c62e14aa
commit 19d246745d
9 changed files with 1398 additions and 1214 deletions
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3
Lib/importlib/_bootstrap_external.py
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@ -239,6 +239,7 @@ _code_type = type(_write_atomic.__code__)
# Python 3.6b1 3376 (simplify CALL_FUNCTIONs & BUILD_MAP_UNPACK_WITH_CALL)
# Python 3.6b1 3377 (set __class__ cell from type.__new__ #23722)
# Python 3.6b2 3378 (add BUILD_TUPLE_UNPACK_WITH_CALL #28257)
# Python 3.6rc1 3379 (more thorough __class__ validation #23722)
#
# MAGIC must change whenever the bytecode emitted by the compiler may no
# longer be understood by older implementations of the eval loop (usually
@ -247,7 +248,7 @@ _code_type = type(_write_atomic.__code__)
# Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
# in PC/launcher.c must also be updated.
MAGIC_NUMBER = (3378).to_bytes(2, 'little') + b'\r\n'
MAGIC_NUMBER = (3379).to_bytes(2, 'little') + b'\r\n'
_RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little') # For import.c
_PYCACHE = '__pycache__'