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bpo-38366: dataclasses: generate the _hash_action table more cleanly #16547

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bpo-38366: dataclasses: generate the _hash_action table more cleanly #16547

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62 changes: 34 additions & 28 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
import keyword
import builtins
import functools
import itertools
import _thread


Expand Down Expand Up @@ -762,34 +763,39 @@ def _hash_exception(cls, fields):
raise TypeError(f'Cannot overwrite attribute __hash__ '
f'in class {cls.__name__}')

#
# +-------------------------------------- unsafe_hash?
# | +------------------------------- eq?
# | | +------------------------ frozen?
# | | | +---------------- has-explicit-hash?
# | | | |
# | | | | +------- action
# | | | | |
# v v v v v
_hash_action = {(False, False, False, False): None,
(False, False, False, True ): None,
(False, False, True, False): None,
(False, False, True, True ): None,
(False, True, False, False): _hash_set_none,
(False, True, False, True ): None,
(False, True, True, False): _hash_add,
(False, True, True, True ): None,
(True, False, False, False): _hash_add,
(True, False, False, True ): _hash_exception,
(True, False, True, False): _hash_add,
(True, False, True, True ): _hash_exception,
(True, True, False, False): _hash_add,
(True, True, False, True ): _hash_exception,
(True, True, True, False): _hash_add,
(True, True, True, True ): _hash_exception,
}
# See https://bugs.python.org/issue32929#msg312829 for an if-statement
# version of this table.
def _gen_hash_action():
tab = {}
for state in itertools.product((False, True), repeat=4):
unsafe_hash, has_explicit_hash, eq, frozen = state

if unsafe_hash:
# If there's already a __hash__, raise TypeError, otherwise add __hash__.
if has_explicit_hash:
hash_action = _hash_exception
else:
hash_action = _hash_add
else:
# unsafe_hash is False (the default).
if has_explicit_hash:
# There's already a __hash__, don't overwrite it.
hash_action = None
else:
if eq and frozen:
# It's frozen and we added __eq__, generate __hash__.
hash_action = _hash_add
elif eq and not frozen:
# It's not frozen but has __eq__, make it unhashable.
# This is the default if no params to @dataclass.
hash_action = _hash_set_none
else:
# There's no __eq__, use the base class __hash__.
hash_action = None

tab[state] = hash_action

return tab

_hash_action = _gen_hash_action()


def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
Expand Down