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[3.7] bpo-33134: dataclasses: use function dispatch table for hash, instead of a string lookup which then is tested with if tests. (GH-6222) #6224

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Merged
miss-islington merged 1 commit into from
Mar 25, 2018
Merged

[3.7] bpo-33134: dataclasses: use function dispatch table for hash, instead of a string lookup which then is tested with if tests. (GH-6222) #6224

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76 changes: 36 additions & 40 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -585,14 +585,24 @@ def _set_new_attribute(cls, name, value):
return False



# Decide if/how we're going to create a hash function. Key is
# (unsafe_hash, eq, frozen, does-hash-exist). Value is the action to
# take.
# Actions:
# '': Do nothing.
# 'none': Set __hash__ to None.
# 'add': Always add a generated __hash__function.
# 'exception': Raise an exception.
# take. The common case is to do nothing, so instead of providing a
# function that is a no-op, use None to signify that.

def _hash_set_none(cls, fields):
return None

def _hash_add(cls, fields):
flds = [f for f in fields if (f.compare if f.hash is None else f.hash)]
return _hash_fn(flds)

def _hash_exception(cls, fields):
# Raise an exception.
raise TypeError(f'Cannot overwrite attribute __hash__ '
f'in class {cls.__name__}')

#
# +-------------------------------------- unsafe_hash?
# | +------------------------------- eq?
Expand All @@ -602,22 +612,22 @@ def _set_new_attribute(cls, name, value):
# | | | | +------- action
# | | | | |
# v v v v v
_hash_action = {(False, False, False, False): (''),
(False, False, False, True ): (''),
(False, False, True, False): (''),
(False, False, True, True ): (''),
(False, True, False, False): ('none'),
(False, True, False, True ): (''),
(False, True, True, False): ('add'),
(False, True, True, True ): (''),
(True, False, False, False): ('add'),
(True, False, False, True ): ('exception'),
(True, False, True, False): ('add'),
(True, False, True, True ): ('exception'),
(True, True, False, False): ('add'),
(True, True, False, True ): ('exception'),
(True, True, True, False): ('add'),
(True, True, True, True ): ('exception'),
_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.
Expand Down Expand Up @@ -774,7 +784,6 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
'functools.total_ordering')

if frozen:
# XXX: Which fields are frozen? InitVar? ClassVar? hashed-only?
for fn in _frozen_get_del_attr(cls, field_list):
if _set_new_attribute(cls, fn.__name__, fn):
raise TypeError(f'Cannot overwrite attribute {fn.__name__} '
Expand All @@ -785,23 +794,10 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen):
bool(eq),
bool(frozen),
has_explicit_hash]

# No need to call _set_new_attribute here, since we already know if
# we're overwriting a __hash__ or not.
if hash_action == '':
# Do nothing.
pass
elif hash_action == 'none':
cls.__hash__ = None
elif hash_action == 'add':
flds = [f for f in field_list if (f.compare if f.hash is None else f.hash)]
cls.__hash__ = _hash_fn(flds)
elif hash_action == 'exception':
# Raise an exception.
raise TypeError(f'Cannot overwrite attribute __hash__ '
f'in class {cls.__name__}')
else:
assert False, f"can't get here: {hash_action}"
if hash_action:
# No need to call _set_new_attribute here, since by the time
# we're here the overwriting is unconditional.
cls.__hash__ = hash_action(cls, field_list)

if not getattr(cls, '__doc__'):
# Create a class doc-string.
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2018-03-24-19-34-26.bpo-33134.hbVeIX.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
When computing dataclass's __hash__, use the lookup table to contain the
function which returns the __hash__ value. This is an improvement over
looking up a string, and then testing that string to see what to do.