diff --git a/plum/dispatcher.py b/plum/dispatcher.py
index bf018c39..0f2b5adc 100644
--- a/plum/dispatcher.py
+++ b/plum/dispatcher.py
@@ -1,4 +1,4 @@
-from typing import Any, Callable, Dict, Optional, Tuple, TypeVar, Union
+from typing import Any, Dict, Optional, Tuple, TypeVar, Union
 
 from .function import Function
 from .overload import get_overloads
diff --git a/plum/function.py b/plum/function.py
index 15b4b9c6..fd53c65a 100644
--- a/plum/function.py
+++ b/plum/function.py
@@ -3,6 +3,7 @@
 from functools import wraps
 from types import MethodType
 from typing import Any, Callable, List, Optional, Tuple, TypeVar, Union
+from copy import copy
 
 from .resolver import AmbiguousLookupError, NotFoundLookupError, Resolver
 from .signature import Signature, append_default_args, extract_signature
@@ -281,7 +282,7 @@ def _resolve_pending_registrations(self) -> None:
             else:
                 # Ensure that the implementation is `f`, but make a copy before
                 # mutating.
-                signature = signature.__copy__()
+                signature = copy(signature)
                 signature.implementation = f
 
             # Ensure that the implementation has the right name, because this name
diff --git a/plum/promotion.py b/plum/promotion.py
index c34f8ae3..344f67dc 100644
--- a/plum/promotion.py
+++ b/plum/promotion.py
@@ -126,7 +126,7 @@ def rule(t1: type1, t2: type2):
     # If the types are the same, the method will get overwritten.
 
     @_promotion_rule.dispatch
-    def rule(t1: type2, t2: type1):
+    def rule(t1: type2, t2: type1):  # noqa F811
         return type_to
 
 
@@ -161,11 +161,11 @@ def _promote_types(t0, t1):
 
 
 @_dispatch
-def promote(obj: object):
+def promote(obj: object):  # noqa F811
     # Promote should always return a tuple to avoid edge cases.
     return (obj,)
 
 
 @_dispatch
-def promote():
+def promote():  # noqa F811
     return ()
diff --git a/plum/signature.py b/plum/signature.py
index 0f6c97df..5486725a 100644
--- a/plum/signature.py
+++ b/plum/signature.py
@@ -1,7 +1,9 @@
+from typing import Callable, List, Tuple, Optional
+
+import typing
 import inspect
 import operator
-import typing
-from typing import Callable, List, Optional, Tuple
+from copy import copy
 
 import beartype.door
 from beartype.peps import resolve_pep563
@@ -286,15 +288,15 @@ def append_default_args(signature: Signature, f: Callable) -> List[Signature]:
         if p.kind == p.VAR_POSITIONAL:
             continue
 
-        copy = signatures[-1].__copy__()
+        signature_copy = copy(signatures[-1])
 
         # As specified over, these additional signatures should never have variable
         # arguments.
-        copy.varargs = Missing
+        signature_copy.varargs = Missing
 
         # Remove the last positional argument.
-        copy.types = copy.types[:-1]
+        signature_copy.types = signature_copy.types[:-1]
 
-        signatures.append(copy)
+        signatures.append(signature_copy)
 
     return signatures
diff --git a/plum/util.py b/plum/util.py
index fbce29cf..4c9629a6 100644
--- a/plum/util.py
+++ b/plum/util.py
@@ -1,7 +1,7 @@
 import abc
 import sys
 import typing
-from typing import Callable, List
+from typing import List
 
 if sys.version_info.minor <= 8:  # pragma: specific no cover 3.9 3.10 3.11
     from typing import Callable