Fixed instrumentation mutating function annotations on py < 3.9

Fixes #335.

docs/versionhistory.rst

@@ -7,6 +7,8 @@ This library adheres to `Semantic Versioning 2.0 <https://semver.org/#semantic-v
 
 - Fixed imports guarded by ``if TYPE_CHECKING:`` when used with subscripts
   (``SomeType[...]``) being replaced with ``Any[...]`` instead of just ``Any``
+- Fixed instrumentation inadvertently mutating a function's annotations on Python 3.7
+  and 3.8
 
 **4.0.0rc3** (2023-04-10)
 

src/typeguard/_transformer.py

@@ -60,7 +60,7 @@
 from collections import defaultdict
 from collections.abc import Generator, Sequence
 from contextlib import contextmanager
-from copy import copy
+from copy import deepcopy
 from dataclasses import dataclass, field
 from typing import Any, ClassVar, cast, overload
 
@@ -448,7 +448,7 @@ def _use_memo(
 
                 # Extract yield, send and return types where possible from a subscripted
                 # annotation like Generator[int, str, bool]
-                return_annotation = copy(node.returns)
+                return_annotation = deepcopy(node.returns)
                 if detector.contains_yields and new_memo.name_matches(
                     return_annotation, *generator_names
                 ):
@@ -622,7 +622,7 @@ def visit_FunctionDef(
                     all_args.extend(node.args.posonlyargs)
 
                 for arg in all_args:
-                    annotation = self._convert_annotation(copy(arg.annotation))
+                    annotation = self._convert_annotation(deepcopy(arg.annotation))
                     if annotation and not self._memo.name_matches(
                         annotation, *anytype_names
                     ):
@@ -898,7 +898,7 @@ def visit_AnnAssign(self, node: AnnAssign) -> Any:
             and node.annotation
             and isinstance(node.target, Name)
         ):
-            annotation = self._convert_annotation(copy(node.annotation))
+            annotation = self._convert_annotation(deepcopy(node.annotation))
             if annotation:
                 self._memo.variable_annotations[node.target.id] = annotation
                 if node.value and not self._memo.name_matches(

tests/dummymodule.py

@@ -344,3 +344,8 @@ def literal(x: Literal["foo"]) -> Literal["foo"]:
 @typechecked
 def typevar_forwardref(x: Type[T]) -> T:
     return x()
+
+
+def never_called(x: List["NonExistentType"]) -> List["NonExistentType"]:  # noqa: F821
+    """Regression test for #337."""
+    return x

tests/test_instrumentation.py

@@ -40,9 +40,9 @@ def dummymodule(method: str):
     config.debug_instrumentation = True
     sys.path.insert(0, str(this_dir))
     try:
+        sys.modules.pop("dummymodule", None)
         if cached_module_path.exists():
             cached_module_path.unlink()
-            sys.modules.pop("dummymodule", None)
 
         if method == "typechecked":
             return import_module("dummymodule")