ENH: Make deprecate_nonkeyword_arguments alter function signature

pandas/tests/util/test_deprecate_nonkeyword_arguments.py

@@ -2,6 +2,7 @@
 Tests for the `deprecate_nonkeyword_arguments` decorator
 """
 
+import inspect
 import warnings
 
 from pandas.util._decorators import deprecate_nonkeyword_arguments
@@ -16,6 +17,10 @@ def f(a, b=0, c=0, d=0):
     return a + b + c + d
 
 
+def test_f_signature():
+    assert str(inspect.signature(f)) == "(a, b=0, *, c=0, d=0)"
+
+
 def test_one_argument():
     with tm.assert_produces_warning(None):
         assert f(19) == 19
@@ -65,6 +70,10 @@ def g(a, b=0, c=0, d=0):
         return a + b + c + d
 
 
+def test_g_signature():
+    assert str(inspect.signature(g)) == "(a, *, b=0, c=0, d=0)"
+
+
 def test_one_and_three_arguments_default_allowed_args():
     with tm.assert_produces_warning(None):
         assert g(1, b=3, c=3, d=5) == 12
@@ -93,6 +102,10 @@ def h(a=0, b=0, c=0, d=0):
     return a + b + c + d
 
 
+def test_h_signature():
+    assert str(inspect.signature(h)) == "(*, a=0, b=0, c=0, d=0)"
+
+
 def test_all_keyword_arguments():
     with tm.assert_produces_warning(None):
         assert h(a=1, b=2) == 3
@@ -122,6 +135,10 @@ def baz(self, bar=None, foobar=None):
         ...
 
 
+def test_foo_signature():
+    assert str(inspect.signature(Foo.baz)) == "(self, bar=None, *, foobar=None)"
+
+
 def test_class():
     msg = (
         r"In a future version of pandas all arguments of Foo\.baz "

pandas/util/_decorators.py

@@ -290,14 +290,29 @@ def deprecate_nonkeyword_arguments(
     """
 
     def decorate(func):
+        old_sig = inspect.signature(func)
+
         if allowed_args is not None:
             allow_args = allowed_args
         else:
-            spec = inspect.getfullargspec(func)
+            allow_args = [
+                p.name
+                for p in old_sig.parameters.values()
+                if p.kind in (p.POSITIONAL_ONLY, p.POSITIONAL_OR_KEYWORD)
+                and p.default is p.empty
+            ]
 
-            # We must have some defaults if we are deprecating default-less
-            assert spec.defaults is not None  # for mypy
-            allow_args = spec.args[: -len(spec.defaults)]
+        new_params = [
+            p.replace(kind=p.KEYWORD_ONLY)
+            if (
+                p.kind in (p.POSITIONAL_ONLY, p.POSITIONAL_OR_KEYWORD)
+                and p.name not in allow_args
+            )
+            else p
+            for p in old_sig.parameters.values()
+        ]
+        new_params.sort(key=lambda p: p.kind)
+        new_sig = old_sig.replace(parameters=new_params)
 
         num_allow_args = len(allow_args)
         msg = (
@@ -307,15 +322,15 @@ def decorate(func):
 
         @wraps(func)
         def wrapper(*args, **kwargs):
-            arguments = _format_argument_list(allow_args)
             if len(args) > num_allow_args:
                 warnings.warn(
-                    msg.format(arguments=arguments),
+                    msg.format(arguments=_format_argument_list(allow_args)),
                     FutureWarning,
                     stacklevel=find_stack_level(inspect.currentframe()),
                 )
             return func(*args, **kwargs)
 
+        wrapper.__signature__ = new_sig  # type: ignore[attr-defined]
         return wrapper
 
     return decorate