无法从输入中导入协议

Question

I have fresh installed Python 3.7-32 on Windows 10.

I want to try Protocols python approach and I do next:

file test_protocols.py with only one row:

from typing import Protocol

then:

>python test_protocols.py

And I have next error message that is needed to be explained:

Traceback (most recent call last):
  File "test_protocols.py", line 1, in <module> 
    from typing import Protocol
ImportError: cannot import name 'Protocol' from 'typing' (C:\Programing\Python\Python37-32\lib\typing.py)

What do I do wrong?

Maybe I have read PEP-0544 wrong but from my point of view I do the same as it is documented.

Answer 1

As of January 20, 2019, PEP 544 's status is Draft . As far as I understand, it's not implemented in CPython yet.

UPD : it should work since Python 3.8, try updating.

Answer 2

In the implementation section of PEP 544 , it says

The mypy type checker fully supports protocols (modulo a few known bugs). This includes treating all the builtin protocols, such as Iterable structurally. The runtime implementation of protocols is available in typing_extensions module on PyPI.

Thus, in your code, add from typing_extensions import Protocol .

Answer 3

If it does not exist in typing do

pip install typing_extensions

from typing_extensions import Protocol

Depending upon OS and Python version Protocol class might be within typing module or in typing_extensions.