mypy: Variance error when using middleware with new-style app setup

When using the new-style app setup in Starlette 0.13, where properties of the app are set in the app constructor, mypy can produce an error when middleware is configured. The error is like:

app.py:6: error: Argument "middleware" to "Starlette" has incompatible type "List[Middleware]"; expected "Optional[List[Optional[Middleware]]]"
app.py:6: note: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
app.py:6: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 1 source file)

This can be reproduced with this minimal app:

from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware

middleware = [Middleware(HTTPSRedirectMiddleware)]
app = Starlette(middleware=middleware)

Putting this in a file called app.py and then running mypy app.py (no custom mypy config) produces the above error.

Given that this is the recommended way to configure middleware, mypy should accept the above code.

❯ pip freeze
mypy==0.740
mypy-extensions==0.4.3
starlette==0.13.0
typed-ast==1.4.0
typing-extensions==3.7.4.1

[JayH5]

@taybin for now you can work around this by adding a type declaration to your list of middleware:

import typing

from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.httpsredirect import HTTPSRedirectMiddleware

t_middleware = typing.List[typing.Optional[Middleware]]
middleware: t_middleware = [Middleware(HTTPSRedirectMiddleware)]
app = Starlette(middleware=middleware)

[taybin]

@dmontagu the fix hasn’t been published in a versioned release yet.