Added monkey patch to handle argparse deepcopy issue introduced in Python 3.15.0b1.

cmd2/argparse_utils.py

@@ -369,6 +369,24 @@ def _validate_completion_callable(self: argparse.Action, value: Any) -> Any:
 register_argparse_argument_parameter("nargs_range")
 register_argparse_argument_parameter("suppress_tab_hint")
 
+############################################################################################################
+# Workaround for Python 3.15.0b1 argparse bug
+# _ColorlessTheme.__getattr__ incorrectly returns "" for dunder methods, which breaks
+# protocols like copy.deepcopy().
+############################################################################################################
+
+if sys.version_info >= (3, 15):
+
+    def _ColorlessTheme_getattr(_self: argparse._ColorlessTheme, name: str) -> Any:  # noqa: N802
+        """Patched __getattr__ that allows dunder lookups to fail correctly."""
+        if name.startswith("__") and name.endswith("__"):
+            raise AttributeError(name)
+        return ""
+
+    # If the bug still exists, then install the patch.
+    if getattr(argparse._ColorlessTheme(), "__deepcopy__", None) == "":
+        argparse._ColorlessTheme.__getattr__ = _ColorlessTheme_getattr
+
 
 ############################################################################################################
 # Patch _ActionsContainer.add_argument to support more arguments

cmd2/utils.py

@@ -480,11 +480,9 @@ def line_buffering(self) -> bool:
         except AttributeError:
             return False
 
-    def __getattr__(self, item: str) -> Any:
-        """When an attribute lookup fails to find the attribute in the usual places, this special method is called."""
-        if item in self.__dict__:
-            return self.__dict__[item]
-        return getattr(self.inner_stream, item)
+    def __getattr__(self, name: str) -> Any:
+        """Forward attribute lookups to the inner stream for attributes not defined on this class."""
+        return getattr(self.inner_stream, name)
 
 
 class ByteBuf:

tests/test_argparse_utils.py

@@ -748,3 +748,19 @@ def test_argparse_output_capture(base_app: cmd2.Cmd) -> None:
     # Prove that the console style settings were used
     assert styled_help_out != unstyled_help_out
     assert su.strip_style("\n".join(styled_help_out)) == "\n".join(unstyled_help_out)
+
+
+@pytest.mark.skipif(
+    sys.version_info < (3, 15),
+    reason="_ColorlessTheme only exists in 3.15+",
+)
+def test_colorless_theme_monkeypatch() -> None:
+    """Test the _ColorlessTheme.__getattr__ monkey patch."""
+
+    # If this assertion fails, then the bug no longer exists and our patch wasn't installed.
+    # We can remove the patch function and this test.
+    assert argparse._ColorlessTheme.__getattr__ == argparse_utils._ColorlessTheme_getattr
+
+    # Our patch raises an Attribute error for dunder attributes.
+    with pytest.raises(AttributeError):
+        getattr(argparse._ColorlessTheme(), "__deepcopy__")  # noqa: B009