Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "typing._SpecialForm")

[hackaugusto]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?

Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from typing import Literal
from typing_extensions import Literal

• What is the actual behavior/output?

No conflicts among the types

• What is the behavior/output you expect?

t.py:3: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "typing._SpecialForm")
Found 1 error in 1 file (checked 1 source file)

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

Python 3.8.1
mypy 0.761
mypy 0.770+dev.a0c5dfc88fa8780db8a3cf7b76c32e4257f218e2
mypy-extensions==0.4.3
typing-extensions==3.7.4.1

• What are the mypy flags you are using? (For example --strict-optional)

None

• If mypy crashed with a traceback, please paste
  the full traceback below.

None

[JelleZijlstra]

That error seems OK to me (it's a bit unfortunate that the _SpecialForm name leaks to a user-visible error message).

Why do you want this code to work? Importing the same name from two different packages outside a conditional or try-except doesn't seem useful.

[hackaugusto]

Why do you want this code to work? Importing the same name from two different packages outside a conditional or try-except doesn't seem useful.

It happened by accident. The project has its own typing module with reexports all symbols from typing with a from typing import *.

I opened the issues mostly because this does work with py3.7 and the error surprised me when I created a fresh venv.

[plannigan]

For anyone trying to write code using Literal has to older versions python, while not requiring typing-extensions be installed for 3.8: mypy v0.782 raised this same error when using the following pattern. However, that is no longer the case in mypy v0.790.

I'm not sure what I was doing before, but I am now getting the error again. Running mypy on

try:
    from typing import Literal
except ImportError:
    from typing_extensions import Literal

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

produces

mypy typing_ex.py
typing_ex.py:4: error: Incompatible import of "Literal" (imported name has type "typing_extensions._SpecialForm", local name has type "typing._SpecialForm")
Found 1 error in 1 file (checked 1 source file)

Note that this pattern does not cause any problems for TypedDict.

The intent of this code is to be used in a library that can run on Python 3.8 and older, but not require typing-extensions to be installed when running on Python 3.8.

Environment:

$ python --version
Python 3.8.1
$ pip freeze
mypy==0.790
mypy-extensions==0.4.3
typed-ast==1.4.1
typing-extensions==3.7.4.3

[wbolster]

@plannigan did you manage to get it to work?

i'm also trying to use typing.Literal, with an optional fall-back to typing_extensions.Literal, in a code base supporting py3.6+. if possible i would like to avoid depending on typing_extensions.

apart from the error about _SpecialForm, which i also see, how would one conditionally use that Literal[...] in annotations? as a string is the only option i guess?

(mypy 0.800)

[hauntsaninja]

https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module

[wbolster]

yes, i'm aware of that, but as i wrote:

if possible i would like to avoid depending on typing_extensions.

on second thought i cannot think of a good reason to avoid typing_extensions; adding such dependency won't hurt (and will likely be pulled in by other dependencies already anyway).

[plannigan]

I guess I was looking for an ask for forgiveness instead of look before you leap. But explicitly checking the python version will work.

[janosh]

For copy-paste convenience, @plannigan is referring to this:

import sys

if sys.version_info >= (3, 8):
    from typing import Literal
else:
    from typing_extensions import Literal

[hauntsaninja]

@janosh your conditions are the wrong way round. Consider copy-pasting from https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module