"Can only assign concrete classes to a variable of type "Type[MyProtocol]"" is hard to debug

[grahamc]

If you consider bad error messages a bug, it is a bug. Otherwise, it is a feature request.

The following code sample is wrong because name is not optional. Defining name's value in MyImplementation or its constructor would fix this code.

from typing import Type, Protocol

class MyProtocol(Protocol):
    name: str

class MyImplementation(MyProtocol):
    pass

foo: Type[MyProtocol] = MyImplementation

However, running mypy from master or 0.770 produces an error which is very challenging to debug:

main.py:9: error: Can only assign concrete classes to a variable of type "Type[MyProtocol]"

Ideally, mypy would tell me the underlying problem. In this case, I took nearly a whole day of experimenting to fix it, ultimately needing to delete code until I got the error message to change. It would be ideal if mypy could say "and it is not concrete because name is not defined statically or in __init__." or something.

• What are the versions of mypy and Python you are using?
  mypy 0.770, python 3.8, yes it persists on master

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

This behavior seems unchanged by flags.

[msullivan]

Yeah that seems painful.

[elbaulp]

I am having the same issue, how did you solve it? defining the attributes inside __init__ did not work

[grahamc]

I deleted code until I had a minimum reproducer, which identified one variable I had forgotten to set.

…

On Mon, Sep 28, 2020, at 6:50 AM, Alejandro Alcalde wrote: I am having the same issue, how did you solve it? defining the attributes inside `__init__` did not work — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#8836 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAASXLBR556OCCGEZGL3ITDSIBS75ANCNFSM4NCVZMCA>.

[Hnasar]

You can use final to narrow down the abstract members:

from typing import Type, Protocol, final

class MyProtocol(Protocol):
    name: str

@final
class MyImplementation(MyProtocol):
    pass
# main.py:7: error: Final class __main__.MyImplementation has abstract attributes "name"  [misc]

foo: Type[MyProtocol] = MyImplementation

Perhaps the error message should allude to that?