type checking for attributes

[glyph]

This example doesn't complain:

from zope.interface import Interface, Attribute, implementer

class IWhatever(Interface):
    stuff: str = Attribute("name")


@implementer(IWhatever)
class WhateverBroken(object):
    stuff: int

even though the type of stuff doesn't match.

This closed PR, from 2017, suggests that this syntax was intended to work one day:

zopefoundation/zope.interface#98

I'm guessing this is what the README is already talking about, when it says "Interface compatibility checker will not type-check non-method attributes"?

[kedder]

@glyph, yes, that README sentence refers to this issue. This feature was supported in the earlier version, but since interface checking was reimplemented, I had to drop it. The reason is documentde in _report_implementation_problems:

mypy-zope/src/mypy_zope/plugin.py

Lines 553 to 559 in ae7ed4b

	# We could check the field type for compatibility with the code
	# below, however this yields too many false-positives in
	# real-life projects. The problem is that we can only check
	# types defined on a class, instead of instance types, so all
	# "descriptor" properties will be falsly reported as type
	# mismatches. Instead we opt-out from property type checking
	# until we figure out the workaround.

[kedder]

If I remember correctly, the issue is boils down to the type difference between class and instance attributes, e.g.

class Descriptor:
    __get__(self, obj) -> int:
        return 0

class IWhatever(Interface):
    stuff: int = Attribute("name")

@implementer(IWhatever)
class Whatever:
    stuff = Descriptor()

The type of Whatever.stuff will be Descriptor, but type of Whatever().stuff will be int. IWhatever.stuff is int, but Whatever.stuff is Descriptor, which would be reported as mismatch.

[glyph]

@kedder Thanks for the explanation. I went looking for how this was handled by Protocol to see how it could work, but it looks like Protocols have this same issue! I added __set__ and __delete__ methods to make sure that it wasn't an issue with the property declared as mutable being incomplete, but this still results in an error:

from typing import Protocol

class Descriptor:
    def __get__(self, obj: object) -> int:
        return 0

    def __set__(self, obj: object, value: int) -> None:
        return None

    def __delete__(self, obj: object) -> None:
        return None

class Whatever:
    stuff = Descriptor()


class WhateverProto(Protocol):
    stuff: int


whatever: WhateverProto = Whatever()

[glyph]

Looks like it's a known issue: python/mypy#5481

[glyph]

Would it be possible to make this a configuration option? There are some hacks which would make this possible (like what attrs's stubs do with Factory)

[kedder]

We can definiely make it an option.

@glyph, can you try uncommenting these lines and check if that actually works for your project?

mypy-zope/src/mypy_zope/plugin.py

Lines 561 to 567 in ae7ed4b

	# api.check_subtype(
	# impl_mtype,
	# iface_mtype,
	# context=ctx,
	# subtype_label=f'"{impl_name}" has type',
	# supertype_label=f'interface "{iface_name}" defines "{member}" as',
	# )

[glyph]

Gotcha. There may be … significant latency on this experiment, since we're burning down LOTS of unrelated mypy errors on Twisted right now, so seeing how this works in practice might take a while. But @adiroiban @rodrigc might be interested to know that this is possible.