Slightly refactor process_error_codes (#20618)

process_error_codes was confusing to read due to the lack of distinction
it made between error codes (ErrorCode) and error code names (str). This
refactor fixes this, and also makes the documentation comment at the
topic of the function a real comment.

The current CI ensures this does not introduce errors.

Author: wyattscarpenter

mypy/options.py

@@ -437,18 +437,20 @@ def __repr__(self) -> str:
         return f"Options({pprint.pformat(self.snapshot())})"
 
     def process_error_codes(self, *, error_callback: Callable[[str], Any]) -> None:
-        # Process `--enable-error-code` and `--disable-error-code` flags
-        disabled_codes = set(self.disable_error_code)
-        enabled_codes = set(self.enable_error_code)
+        """Process `--enable-error-code` and `--disable-error-code` flags."""
+        disabled_code_names = set(self.disable_error_code)
+        enabled_code_names = set(self.enable_error_code)
 
-        valid_error_codes = set(error_codes.keys())
+        valid_error_code_names = set(error_codes.keys())
 
-        invalid_codes = (enabled_codes | disabled_codes) - valid_error_codes
-        if invalid_codes:
-            error_callback(f"Invalid error code(s): {', '.join(sorted(invalid_codes))}")
+        invalid_code_names_here = (
+            enabled_code_names | disabled_code_names
+        ) - valid_error_code_names
+        if invalid_code_names_here:
+            error_callback(f"Invalid error code(s): {', '.join(sorted(invalid_code_names_here))}")
 
-        self.disabled_error_codes |= {error_codes[code] for code in disabled_codes}
-        self.enabled_error_codes |= {error_codes[code] for code in enabled_codes}
+        self.disabled_error_codes |= {error_codes[code] for code in disabled_code_names}
+        self.enabled_error_codes |= {error_codes[code] for code in enabled_code_names}
 
         # Enabling an error code always overrides disabling
         self.disabled_error_codes -= self.enabled_error_codes