Typing updates

[AMDmi3]

What do these changes do?

Bump mypy version to 0.720 and fix/silence new mypy issues.

Are there changes in behavior for the user?

No

Related issue number

#3927

Checklist

• I think the code is well written
• Unit tests for the changes exist (no new code or behavior change, should be covered with existing tests)
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder (may do after more changes are merged)
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[asvetlov]

aiohttp still supports Python 3.5, this syntax doesn't work.

I'm open for discussing the drop of Python 3.5 in aiohttp 4.0 though.
If you want to proceed in this direction please create a new issue for discussion.

Otherwise please use Python 3.5 compatible syntax in the PR

[AMDmi3]

Sorry, I'm not really interested in legacy support or participating in related discussions. If this cannot be merged right away, it should either hang until it can be merged, or have only compatible bits cherry picked, or just be discarded.

[webknjaz]

@AMDmi3 it fails even under Python 3.7: https://ci.appveyor.com/project/aio-libs/aiohttp/builds/26164328/job/0nn5x41jb9ow927u#L1023

[webknjaz]

Curious what Towncrier has to do with this... https://travis-ci.com/aio-libs/aiohttp/jobs/218370235#L527

[asvetlov]

@webknjaz I don't know but towncrier wants to import the served package for some reason.
Maybe for the package version extraction?

[webknjaz]

Ah right.. Package version. Makes sense. AFAIR you can use --version if you have an external mechanism of extracting it.
But the thing which makes me confused is that it barks at the syntax error while running in the Python 3.7 env.

[AMDmi3]

it fails even under Python 3.7

Actually I think there's problem with typing support in multidict module which causes this. Not sure how to fix that properly.

% cat 1.py
from multidict import CIMultiDict
foo: CIMultiDict[str] = CIMultiDict()
% mypy --strict 1.py
% python3 1.py
Traceback (most recent call last):
  File "1.py", line 2, in <module>
    foo: CIMultiDict[str] = CIMultiDict()
TypeError: 'type' object is not subscriptable

% cat 2.py
from multidict import CIMultiDict
foo: CIMultiDict = CIMultiDict()
% mypy --strict 2.py
2.py:2: error: Missing type parameters for generic type
% python3 2.py 

[asvetlov]

Use forwarded type declaration (string) :

foo: 'CIMultiDict[str]' = CIMultiDict()

It is not multidict error, asyncio.Future[int] has the same limitation.
Python 3.7 has from __future__ import annotations (see https://www.python.org/dev/peps/pep-0563/ for details).
Since the feature is the mainstream I don't think that multidict should fix the issue itself by providing __getitem__ in own metaclass.

[antonagestam]

@asvetlov Isn't the issue that the C implementation of CIMultiDict lacks a __class_getitem__ method? If I import it using from multidict._multidict_py import CIMultiDict, using CIMultiDict[str] does not trigger an error, since the Python implementation inherits from the abstract base class.

See MagicStack/immutables#13 and https://gitter.im/python/typing?at=5d25bd59cdb70805c96e239f for a similar issue.

[asvetlov]

__class_getitem__ works for Python 3.7+ only, aiohttp is supposed to support older python versions

[antonagestam]

@asvetlov Yes, but currently using CIMultiDict[str] will break at runtime. The only way around that is to make the C implementation subscriptable. Doing that doesn't break support for older Python versions, but enables the new annotation syntax for users of Python 3.5+.

[asvetlov]

The way is using forward declarations: "CIMultiDict[str]"

[antonagestam]

What do you mean with "the way"? In my opinion, a better way for the multidict project would be to implement __class_getitem__ and letting the user choose whether to use the string syntax or not? But really this discussion ought to happen in the multidict repo.

[asvetlov]

But really this discussion ought to happen in the multidict repo.

Exactly. aiohttp should use strings here.

[AMDmi3]

Switched to string annotations, python 3.6+ build seem to be fixed

[asvetlov]

Outdated, the master supports newer mypy without errors