bpo-33134: dataclasses: use function dispatch table for hash, instead of a string lookup which then is tested with if tests. (GH-6222)
* Change _hash_action to be a function table lookup, instead of a list
of strings which is then tested with if statements.
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 4425408..8ccc4c8 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -585,14 +585,24 @@
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?
@@ -602,22 +612,22 @@
# | | | | +------- 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.
@@ -774,7 +784,6 @@
'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__} '
@@ -785,23 +794,10 @@
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.