Certain order of union members causes Incompatible types error

[Stiivi]

I’m getting this weird error: Incompatible types in assignment (expression has type Dict[str, Union[str, float]], variable has type Dict[str, Union[str, int, float, bool]])

Value = Union[str, int, float, bool]
d: Dict[str, Value]
d = {}

however this works:

Value = Union[str, float, bool, int]

The order of union elements is different.

Other offenses:

Value = Union[int, str, bool, float]
Value = Union[str, int, bool, float]

Causes: Incompatible types in assignment (expression has type Dict[str, Union[str, float]], variable has type Dict[str, Union[int, str, bool, float]])

[gvanrossum]

Might have been fixed by #3025, which got rolled back for other reasons.

[Stiivi]

Another observation that looks related is this incorrect error message for the last statement of the following example. The last statement should cause Incompatible types and it does:

from typing import Union, Dict, Optional

Value = Union[str, float, bool, int]
d: Dict[str, Value]
d = {}

v: Value
v = d.get("athing")

However the error is showing wrong types:

Incompatible types in assignment (expression has type "Union[str, float, bool, None]",
                                    variable has type "Union[str, float, bool, int]")

(indented for better demonstration)

The first union (expression type) in the error should have five elements – it is missing int.

[JelleZijlstra]

That's probably also something deciding that int is compatible with float, so Union[float, int] == float.

[ilevkivskyi]

I think this is essentially a duplicate of #1850

[JukkaL]

This should be fixed by #3025 (I'll need to double check) and yes, this looks like a duplicate of #1850 so I'm closing this now.