Function ReadOnly

Lib/typing.py:3455–3471

A special typing construct to mark an item of a TypedDict as read-only. For example:: class Movie(TypedDict): title: ReadOnly[str] year: int def mutate_movie(m: Movie) -> None: m["year"] = 1992 # allowed m["title"] = "The Matrix

(self, parameters)

@_SpecialForm
def ReadOnly(self, parameters):
    """A special typing construct to mark an item of a TypedDict as read-only.

    For example::

        class Movie(TypedDict):
            title: ReadOnly[str]
            year: int

        def mutate_movie(m: Movie) -> None:
            m["year"] = 1992  # allowed
            m["title"] = "The Matrix"  # typechecker error

    There is no runtime checking for this property.
    """
    item = _type_check(parameters, f'{self._name} accepts only a single type.')
    return _GenericAlias(self, (item,))


class NewType: