Make typing_extensions a drop in for typing

[Gobot1234]

Currently I find it annoying trying to remember which types from typing_extensions are and aren't in the versions of typing that I have access to. I think this could be solved by making typing_extensions a fully drop in replacement. I think this would also help with some older issues like subclassing Generic with a ParamSpec as a parameter used to be.

#50 (comment)

[srittau]

I would be fine with this, but I am mostly indifferent to it.

[vwxyzjn]

Yeah, this is very inconvenient. I tried using it as a drop-in replacement because we have dozens of files, but it doesn't work:

from typing_extensions import Literal, Mapping, Optional, Tuple, Union

yields

ImportError: cannot import name 'Mapping' from 'typing_extensions'

[vwxyzjn]

Effectively we needed to do things like

from typing import Literal, Mapping, Optional, Tuple, Union
try:
    import Literal
except ImportError:
    from typing extensions import Literal

which is a labor-intensive process and not that maintainable.

[JelleZijlstra]

I'm sympathetic to this idea; it's hard to remember what you need to import from typing-extensions depending on the Python version. At runtime we can simply add from typing import * to typing_extensions.py. The stubs for typing-extensions can work similarly. Some type checkers may also need minor changes.

For reference, these are the names in typing.__all__ but not typing_extensions.__all__.

{'AbstractSet',
 'Any',
 'AnyStr',
 'BinaryIO',
 'ByteString',
 'Callable',
 'Collection',
 'Container',
 'Dict',
 'ForwardRef',
 'FrozenSet',
 'Generator',
 'Generic',
 'Hashable',
 'IO',
 'ItemsView',
 'Iterable',
 'Iterator',
 'KeysView',
 'List',
 'Mapping',
 'MappingView',
 'Match',
 'MutableMapping',
 'MutableSequence',
 'MutableSet',
 'NamedTuple',
 'Optional',
 'Pattern',
 'Reversible',
 'Sequence',
 'Set',
 'Sized',
 'SupportsAbs',
 'SupportsBytes',
 'SupportsComplex',
 'SupportsFloat',
 'SupportsInt',
 'SupportsRound',
 'TextIO',
 'Tuple',
 'TypeVar',
 'Union',
 'ValuesView',
 'cast',
 'no_type_check',
 'no_type_check_decorator'}

[AlexWaygood]

I also quite like this idea (though I don't feel particularly strongly about it, like @srittau). @JelleZijlstra's suggested implementation sounds good to me.

[vwxyzjn]

Thanks @JelleZijlstra! It would really help with our use case.

As a side note, we tried to patch it somehow:

if sys.version_info < (3, 8):
    from typing_extensions import get_origin as get_origin_typing_extensions
    def get_origin_wrapped(t):
        if sys.version_info < (3, 8):
            return get_origin_typing_extensions(t)
        return typing.get_origin(t)
    setattr(typing, "Literal", typing_extensions.Literal)
    setattr(typing, "get_origin", get_origin_wrapped)

[julien-blanchon]

Yes, please it's a huge pain to use both typing and typing_extensions depending on the method

[JelleZijlstra]

PR in #196. I didn't do the import * approach I suggested above; it's better to be explicit about this.