bpo-43820: Remove an unnecessary copy of the 'namespace' parameter to make_dataclass()

Lib/dataclasses.py

@@ -1233,14 +1233,12 @@ class C(Base):
 
  if namespace is None:
  namespace = {}
- else:
- # Copy namespace since we're going to mutate it.
- namespace = namespace.copy()
 
  # While we're looking through the field names, validate that they
  # are identifiers, are not keywords, and not duplicates.
  seen = set()
- anns = {}
+ annotations = {}
+ defaults = {}
  for item in fields:
  if isinstance(item, str):
  name = item
@@ -1249,7 +1247,7 @@ class C(Base):
  name, tp, = item
  elif len(item) == 3:
  name, tp, spec = item
- namespace[name] = spec
+ defaults[name] = spec
  else:
  raise TypeError(f'Invalid field: {item!r}')
 
@@ -1261,12 +1259,19 @@ class C(Base):
  raise TypeError(f'Field name duplicated: {name!r}')
 
  seen.add(name)
- anns[name] = tp
+ annotations[name] = tp
+
+ # Update 'ns' with the user-supplied namespace plus our calculated values.
+ def exec_body_callback(ns):
+ ns.update(namespace)
+ ns.update(defaults)
+ ns['__annotations__'] = annotations
 
- namespace['__annotations__'] = anns
  # We use `types.new_class()` instead of simply `type()` to allow dynamic creation
  # of generic dataclassses.
- cls = types.new_class(cls_name, bases, {}, lambda ns: ns.update(namespace))
+ cls = types.new_class(cls_name, bases, {}, exec_body_callback)
+
+ # Apply the normal decorator.
  return dataclass(cls, init=init, repr=repr, eq=eq, order=order,
  unsafe_hash=unsafe_hash, frozen=frozen,
  match_args=match_args)

Misc/NEWS.d/next/Library/2021-04-12-18-01-10.bpo-43820.YkqYW4.rst

@@ -0,0 +1,2 @@
+Remove an unneeded copy of the namespace passed to
+dataclasses.make_dataclass().