Type of open() changes if plugin overrides get_function_hook

[kedder]

When plugin overrides get_function_hook, the type of open function changes from typing.TextIO to typing.IO[Any].

Consider this trivial plugin (type annotations stripped for brevity):

class TrivialPlugin(Plugin):
    def get_function_hook(self, fullname):
        def analyze(function_ctx):
            return function_ctx.default_return_type
        return analyze

With this plugin enabled, reveal_type(open('str')) produces message: note: Revealed type is 'typing.IO[Any]'. Without plugin the type is typing.TextIO.

The issue was reported to mypy-zope pligin: Shoobx/mypy-zope#16.

[ilevkivskyi]

This is part of how plugins work, only one plugin hook is called if there are multiple registered plugins. This is why we have this two-step logic: get_xxx_hook() should only return a non-None value if you are sure you should take care of this function, not other plugins, see https://mypy.readthedocs.io/en/stable/extending_mypy.html#high-level-overview

[kedder]

Hm, I see. The fullname provided to get_xxx_hook is not always enough to make a decision though. It would be nice to have a way to indicate that plugin doesn't want to change the type based on inspection of the context. I was under impression that function_ctx.default_return_type is such a mechanism, but apparently it is not.

[ilevkivskyi]

There is some (very) limited API that you can use to make a bit deeper inspection than just fullname. For example you can use Plugin().lookup_fully_qualified() to retrieve the function node and get something from it to make the decision.