Add dcargs.extras.literal_type_from_choices()

dcargs/extras/__init__.py

@@ -5,12 +5,14 @@
 from .._argparse_formatter import set_accent_color
 from .._cli import get_parser
 from ._base_configs import subcommand_type_from_defaults
+from ._choices_type import literal_type_from_choices
 from ._serialization import from_yaml, to_yaml
 
 __all__ = [
     "set_accent_color",
     "subcommand_type_from_defaults",
     "get_parser",
+    "literal_type_from_choices",
     "from_yaml",
     "to_yaml",
 ]

dcargs/extras/_choices_type.py

@@ -0,0 +1,18 @@
+import enum
+from typing import Iterable, Type, TypeVar, Union
+
+from typing_extensions import Literal
+
+T = TypeVar("T", bound=Union[int, str, bool, enum.Enum])
+
+
+def literal_type_from_choices(choices: Iterable[T]) -> Type[T]:
+    """Generate a typing.Literal[] type that constrains values to a set of choices.
+
+    Using Literal[...] directly should generally be preferred, but this helper can be
+    used in the rare case that choices are generated dynamically. (for example, the keys
+    of a dictionary)
+
+    Using the returned type as an annotation currently breaks for mypy, but is
+    understood by Pyright."""
+    return Literal.__getitem__(tuple(choices))  # type: ignore