Meta issue: better getattr support

[sobolevn]

Feature

One thing that bothered me for a long time is getattr and its very broad return type even for simplest cases:

from typing import Literal

class Some:
    def __init__(self) -> None:
        self.a = 1

s = Some()

reveal_type(s.a)  # Revealed type is "builtins.int"
reveal_type(getattr(s, 'a'))  # Revealed type is "Any"
reveal_type(getattr(s, 'b'))  # Revealed type is "Any"
reveal_type(getattr(s, 'a', None))  # Revealed type is "Union[Any, None]"

a: Literal['a']
reveal_type(getattr(s, a))  # Revealed type is "Any"

ab: Literal['a', 'b']
reveal_type(getattr(s, ab))  # Revealed type is "Any"

So, I propose to add better support for it to mypy.

The same example, after the support is added:

from typing import Literal

class Some:
    def __init__(self) -> None:
        self.a = 1

s = Some()

reveal_type(s.a)  # Revealed type is "builtins.int"
reveal_type(getattr(s, 'a'))  # Revealed type is "builtins.int"
reveal_type(getattr(s, 'b'))  # error: "Some" has no attribute "b"
reveal_type(getattr(s, 'a', None))  # Revealed type is "builtins.int"

a: Literal['a']
reveal_type(getattr(s, a))  # Revealed type is "builtins.int"

ab: Literal['a', 'b']
reveal_type(getattr(s, ab))  # error: "Some" has no attribute "b"
reveal_type(getattr(s, ab, None))  # Revealed type is "builtins.int | None"

I have even written a custom plugin for it at some point.

Why?

This pattern is quite popular among Python developers. geattr(obj, 'string_key', None) is used for compatibility, optional dependencies, etc. That's something you can find in almost any project.

Technical details

I am going to utilize existing checkmember logic.
I even plan to support attribute plugins, where possible, because a.x and getattr(a, 'x') are semantically identical.
For now only str literals and Literal str types are going to be supported.

Related issues and problems

I am calling this issue "meta", because there are several very related problems I want to mention / solve here:

• Type inference and hasattr #1424
• mypy doesn't understand @abc.abstractmethod on __getattr__ #10409

Next steps

I will start with a PR with a code prototype in a couple of days.

[bbatliner]

Also interested in this, for @property attributes: https://mypy-play.net/?mypy=0.910&python=3.10&gist=b6cb4542f9cd1888be942607a2c8ce1f

from typing import Literal

class Some:
    @property
    def foo(self) -> str: ...


s = Some()

foo_attr = s.foo
reveal_type(foo_attr) # builtins.str

foo_getattr = getattr(s, "foo")
reveal_type(foo_getattr) # Any

foo: Literal["foo"] = "foo"
foo_literal_getattr = getattr(s, foo)
reveal_type(foo_literal_getattr) # Any

[yarikoptic]

I would appreciate anyone who would address this issue and thus avoid uglification of pretty Python codes needed to please mypy.