bpo-37163: Deprecate passing argument obj of dataclasses.replace() by keyword. (pythonGH-13877)

Author: serhiy-storchaka

Lib/dataclasses.py

@@ -1206,7 +1206,7 @@ class C(Base):
  unsafe_hash=unsafe_hash, frozen=frozen)
 
 
-def replace(obj, **changes):
+def replace(*args, **changes):
  """Return a new object replacing specified fields with new values.
 
  This is especially useful for frozen classes. Example usage:
@@ -1220,6 +1220,17 @@ class C:
  c1 = replace(c, x=3)
  assert c1.x == 3 and c1.y == 2
  """
+ if len(args) > 1:
+ raise TypeError(f'replace() takes 1 positional argument but {len(args)} were given')
+ if args:
+ obj, = args
+ elif 'obj' in changes:
+ obj = changes.pop('obj')
+ import warnings
+ warnings.warn("Passing 'obj' as keyword argument is deprecated",
+ DeprecationWarning, stacklevel=2)
+ else:
+ raise TypeError("replace() missing 1 required positional argument: 'obj'")
 
  # We're going to mutate 'changes', but that's okay because it's a
  # new dict, even if called with 'replace(obj, **my_changes)'.
@@ -1255,3 +1266,4 @@ class C:
  # changes that aren't fields, this will correctly raise a
  # TypeError.
  return obj.__class__(**changes)
+replace.__text_signature__ = '(obj, /, **kwargs)'

Lib/test/test_dataclasses.py

@@ -3075,6 +3075,13 @@ class C:
  self.assertEqual(c1.x, 3)
  self.assertEqual(c1.y, 2)
 
+ self.assertRaises(TypeError, replace)
+ self.assertRaises(TypeError, replace, c, c)
+ with self.assertWarns(DeprecationWarning):
+ c1 = replace(obj=c, x=3)
+ self.assertEqual(c1.x, 3)
+ self.assertEqual(c1.y, 2)
+
  def test_frozen(self):
  @dataclass(frozen=True)
  class C:

Misc/NEWS.d/next/Library/2019-06-07-08-18-05.bpo-37163.36JkUh.rst

@@ -0,0 +1,2 @@
+Deprecated passing ``obj`` argument of :func:`dataclasses.replace` as
+keyword argument.