plugin causes open() signature to change

[arnimarj]

Hi,

I ran across an interesting issue. The signature of open() unexpectedly became typing.IO[Any] instead of typing.TextIO when the plugin is enabled.

example.py:

reveal_type(open('str'))

plugin.ini:

[mypy]
plugins = mypy_zope:plugin

Here is small test:

$ more example.py 
reveal_type(open('str'))
$ more plugin.ini 
[mypy]
plugins = mypy_zope:plugin
$ mypy --config plugin.ini example.py 
example.py:1: note: Revealed type is 'typing.IO[Any]'
$ mypy example.py 
example.py:1: note: Revealed type is 'typing.TextIO'

I created a test-case here:

master...arnimarj:bug/open-signature-changes

[kedder]

Interesting. This is triggered by overriding get_function_hook function in a plugin. typing.IO[Any] is what is being passed to it the hook in function_ctx.default_return_type. If hook returns this default type, mypy takes it as granted. If hook is not invoked, mypy infers type as typing.TextIO somehow.

This looks like a mypy bug to me. I'll submit the report upstream.

[kedder]

Apparently mypy doesn't consider this as mypy issue. This bug affects a feature that allows to guess type of a field in a schema, such as

    textline = zope.schema.TextLine(
        title='Title',
        description='Optional text line',
        required=False)

@arnimarj is there any practical issues with the type mismatch? Do you depend on this feature?

[arnimarj]

I ran into the issue when passing down the return value of open down to a function which requires BinaryIO. This is the first time encountering this after having annotated in the order on 10k lines, so maybe not a big deal.

So in practice not a big deal, but if we start using more plugins that may change.

[kedder]

I see. I'm going to try suggestion from python/mypy#8331, maybe we can do something about it.

[arnimarj]

There is a open PR (python/mypy#9275) which removes the open plugin. Just tried the test-case above on that branch, and got this:

$ mypy --config plugin.ini example.py 
example.py:1: note: Revealed type is 'io.TextIOWrapper'

$ mypy example.py 
example.py:1: note: Revealed type is 'typing.TextIO'

[arnimarj]

I just tested this on the latest mypy (0.800), and this is no longer an issue.