Trying to import TypedDict from typing of tpying_extensions fails

[plannigan]

I have code that runs on Python 3.8 and Python 3.7, but I'd rather not install typing_extensions when it isn't need.

Code:

from typing_extensions import TypedDict

# try:
#     from typing import TypedDict
# except ImportError:
#     from typing_extensions import TypedDict

class MyDict(TypedDict):
    name: str
    id: int

payload: MyDict = {
    "id": 1,
    "name": "test",
}

Environment:

>>python --version
Python 3.7.4
>> pip list
Package           Version
----------------- -------
mypy              0.740  
mypy-extensions   0.4.3  
pip               19.3.1 
setuptools        42.0.1 
typed-ast         1.4.0  
typing-extensions 3.7.4.1

The above code passes with mypy. However, if you reverse the commented out import statements, mypy fails.

>> mypy typeddict.py 
typeddict.py:4: error: Module 'typing' has no attribute 'TypedDict'; maybe "_TypedDict"?
typeddict.py:10: error: Variable "typeddict.MyDict" is not valid as a type
Found 2 errors in 1 file (checked 1 source file)

I was able to use this same import pattern successfully for Protocol, but for some reason it fails for TypedDict.

[Naddiseo]

It looks like TypedDict was added to typing in python3.8: https://docs.python.org/3/whatsnew/3.8.html#typing

[plannigan]

Yes, but my code also has to run on older versions of python.

I have code that runs on Python 3.8 and Python 3.7, but I'd rather not install typing_extensions when it isn't need.

[ilevkivskyi]

There is not much we can do here. Protocol works because it is exposed in typeshed in typing even in versions where it is in fact not available. I am going to transfer this issue to typeshed, so you can try to convince people there.

[JelleZijlstra]

typing.TypedDict doesn't exist at runtime in 3.7, so I'm not sure what the typeshed issue is here. You will need to install typing_extensions to use TypedDict in 3.7.

[ilevkivskyi]

typing.TypedDict doesn't exist at runtime in 3.7, so I'm not sure what the typeshed issue is here.

The issue is that this causes mypy to not understand the try-except import. The first import makes TypedDict a variable with type Any, and this subsequently breaks all the call sites, turning types into variables with Any type.

There is a precedent with Protocol (in typeshed it is exposed as if it is always present), this is why I said "you can try to convince people there".

[vemel]

@plannigan you can check Python version instead

import sys

if sys.version_info >= (3, 8):
    from typing import TypedDict, Literal, overload  # pylint: disable=no-name-in-module
else:
    from typing_extensions import TypedDict, Literal, overload

[Caligatio]

To enforce what @vemel said, checking against the version is the preferred method per the documentation.

EDIT: @'ed the wrong person

[srittau]

@vemel's version is the preferred idiom.

[Sushang-Agnihotri]

Try to Uninstall the related librarie(s) and install it again.
it worked for me.