bpo-44015: dataclasses should allow KW_ONLY to be specified only once per class (GH-25841)
bpo-44015: Raise a TypeError if KW_ONLY is specified more than once.
diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 363d0b6..cbba320 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -930,6 +930,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
# we can.
cls_fields = []
# Get a reference to this module for the _is_kw_only() test.
+ KW_ONLY_seen = False
dataclasses = sys.modules[__name__]
for name, type in cls_annotations.items():
# See if this is a marker to change the value of kw_only.
@@ -939,6 +940,10 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
_is_kw_only))):
# Switch the default to kw_only=True, and ignore this
# annotation: it's not a real field.
+ if KW_ONLY_seen:
+ raise TypeError(f'{name!r} is KW_ONLY, but KW_ONLY '
+ 'has already been specified')
+ KW_ONLY_seen = True
kw_only = True
else:
# Otherwise it's a field of some type.