Preserve the signature of partials when decorating them

src/decorator/__init__.py

@@ -38,6 +38,7 @@
 import operator
 import itertools
 import functools
+import copy
 from contextlib import _GeneratorContextManager
 from collections import namedtuple
 from inspect import Parameter, iscoroutinefunction, isgeneratorfunction
@@ -297,6 +298,8 @@ def decorate(func, caller, extras=(), kwsyntax=False):
     """
     sig = inspect_sig(func)
     if isinstance(func, functools.partial):
+        func = copy.copy(func)
+        func.__dict__ = func.__dict__.copy()
         func = functools.update_wrapper(func, func.func)
     fun: Callable[..., Any]
     if iscoroutinefunction(caller):

tests/test.py

@@ -573,6 +573,24 @@ def origin_func(x, y):
         out = partial_func("y")
         self.assertEqual(out, '<before>xy<after>')
 
+    def test_decorating_partial_does_not_change_its_signature(self):
+        def origin_func(x, y):
+            return x + y
+        original = functools.partial(origin_func, "x")
+        original.custom = "preserved"
+        signature = inspect.signature(original)
+
+        first = partial_before_after(original)
+        self.assertEqual(inspect.signature(original), signature)
+        self.assertFalse(hasattr(original, '__wrapped__'))
+        self.assertEqual(first.custom, "preserved")
+
+        second = partial_before_after(original)
+        self.assertEqual(inspect.signature(first), signature)
+        self.assertEqual(inspect.signature(second), signature)
+        self.assertEqual(first("y"), '<before>xy<after>')
+        self.assertEqual(second("z"), '<before>xz<after>')
+
 
 if PYVER >= (3, 14):
     # testing forward references in python 3.14+, see