Cannot inference common base class when root of hierarchy is Any

[asottile]

I believe this to be a bug

code

(this has been boiled down from a more complete example)

from typing import Type

from external import ExtBase


class Base(ExtBase):
    pass


class Concrete1(Base):
    pass


class Concrete2(Concrete1):
    pass


def f(x: Type[Base]) -> None:
    pass


def g(x: bool) -> None:
    #reveal_type(Base)
    #reveal_type(Concrete1)
    #reveal_type(Concrete2)
    #reveal_type(Concrete1 if x else Concrete2)
    f(Concrete1 if x else Concrete2)

output

$  mypy t2.py --ignore-missing-imports
t2.py:27: error: Argument 1 to "f" has incompatible type "Type[object]"; expected "Type[Base]"

output with types revealed

$ mypy t2.py --ignore-missing-imports
t2.py:23: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t2.Base'
t2.py:24: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t2.Concrete1'
t2.py:25: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t2.Concrete2'
t2.py:26: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> builtins.object'
t2.py:27: error: Argument 1 to "f" has incompatible type "Type[object]"; expected "Type[Base]"

version information

$ mypy --version
mypy 0.600

against master

same error

peculiarly working

diff --git a/t2.py b/t2.py
index 91a8309..6f35532 100644
--- a/t2.py
+++ b/t2.py
@@ -24,4 +24,8 @@ def g(x: bool) -> None:
     #reveal_type(Concrete1)
     #reveal_type(Concrete2)
     #reveal_type(Concrete1 if x else Concrete2)
-    f(Concrete1 if x else Concrete2)
+    if x:
+        t = Concrete1
+    else:
+        t = Concrete2
+    f(t)

[asottile]

Note that this also passes (why I believe to be ~related to Any here):

diff --git a/t2.py b/t2.py
index 91a8309..1e9896b 100644
--- a/t2.py
+++ b/t2.py
@@ -3,7 +3,7 @@ from typing import Type
 from external import ExtBase
 
 
-class Base(ExtBase):
+class Base:
     pass
 
 

[gvanrossum]

This looks like another case where the conditional expression should take the context into account. I wonder if it works as expected when #5095 is reverted?

[asottile]

Seems to work as expected when reverting that, yes:

$ git -C mypy/ show | head -6
commit 3e529fc7655266bec48044aaa71acf4df203c72d
Author: Anthony Sottile <[email protected]>
Date:   Fri Jun 1 15:42:13 2018 -0700

    Revert "Don't always infer unions for conditional expressions (#5095)"
    
$ mypy --ignore-missing-imports t.py 
$

$ # with reveal
$ mypy --ignore-missing-imports t.py 
t.py:23: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t.Base'
t.py:24: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t.Concrete1'
t.py:25: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t.Concrete2'
t.py:26: error: Revealed type is 'def (*_args: Any, **_kwds: Any) -> t.Concrete1'

[gvanrossum]

OK, then the fix would entail improving on that diff (it partially reverted #5041, which would have the desired effect here but not in general). Perhaps we should always try the make_simplified_union() call and accept the return value if it's not a Union -- that seems to be the case here.

[ilevkivskyi]

TBH, I would also allow Optional[...] as a result of make_simplified_union, so that the rules would be:

• in union context, always use union
• otherwise, use make_simplified_union if the result is a non-union, or a union of two types one of which is None
• otherwise fallback to join

The point is that Any can "swallow" None, so that unimported if x else None results in just Any, not Optional[Any] (this however reveals few dozens of real errors in our internal codebases).

[hauntsaninja]

I fixed this in #14404