improve up-front typechecking

Author: wyattscarpenter

mypy/config_parser.py

@@ -17,15 +17,15 @@
 
 from collections.abc import Mapping, MutableMapping, Sequence
 from typing import Any, Callable, Final, TextIO, Union
-from typing_extensions import TypeAlias as _TypeAlias
+from typing_extensions import TypeAlias, Never
 
 from mypy import defaults
 from mypy.options import PER_MODULE_OPTIONS, Options
 
-_CONFIG_VALUE_TYPES: _TypeAlias = Union[
+_CONFIG_VALUE_TYPES: TypeAlias = Union[
     str, bool, int, float, dict[str, str], list[str], tuple[int, int]
 ]
-_INI_PARSER_CALLABLE: _TypeAlias = Callable[[Any], _CONFIG_VALUE_TYPES]
+_INI_PARSER_CALLABLE: TypeAlias = Callable[[Any], _CONFIG_VALUE_TYPES]
 
 
 class VersionTypeError(argparse.ArgumentTypeError):
@@ -60,21 +60,20 @@ def parse_version(v: str | float) -> tuple[int, int]:
     return major, minor
 
 
-def try_split(v: str | Sequence[str] | Any, split_regex: str = "[,]") -> list[str] | Any:
+def try_split(v: str | Sequence[str] | Any, split_regex: str = "[,]") -> list[str]:
     """Split and trim a str or sequence (eg: list) of str into a list of str.
-    Non-str elements will simply be returned untouched. Feel free to one day
-    fix the typing of the calling code, and remove this caveat and Any."""
+    If an element of the input is not str, a type error will be raised."""
+    def complain(x: object, additional_info: str = "") -> Never:
+        raise argparse.ArgumentTypeError(f"Expected a list or a stringified version thereof, but got: '{x}', of type {type(x)}.{additional_info}")
     if isinstance(v, str):
         items = [p.strip() for p in re.split(split_regex, v)]
         if items and items[-1] == "":
             items.pop(-1)
         return items
     elif isinstance(v, Sequence):
-        return [p.strip() if isinstance(p, str) else p for p in v]
+        return [p.strip() if isinstance(p, str) else complain(p, additional_info=" (As an element of the list.)") for p in v]
     else:
-        raise argparse.ArgumentTypeError(
-            f"Expected a list or a stringified version thereof, but got: '{v}'"
-        )
+        complain(v)
 
 
 def validate_codes(codes: list[str]) -> list[str]: