Nested tagged unions stopped working

[levrik]

Bug Report

Starting with mypy 0.900 tagged unions don't work anymore if they are nested.

To Reproduce

from typing import Literal, Union
from enum import Enum

class ModelType(str, Enum):
    Simple1 = "simple1"
    Simple2 = "simple2"
    Complex1 = "complex1"
    Complex2 = "complex2"

class BaseModel:
    type: ModelType
    
class BaseSimpleModel(BaseModel):
    type: Literal[ModelType.Simple1, ModelType.Simple2]
    simple: str
    
class Simple1Model(BaseSimpleModel):
    type: Literal[ModelType.Simple1]
    
class Simple2Model(BaseSimpleModel):
    type: Literal[ModelType.Simple2]
    
SimpleModel = Union[Simple1Model, Simple2Model]

class BaseComplexModel(BaseModel):
    type: Literal[ModelType.Complex1, ModelType.Complex2]
    complex: str

class Complex1Model(BaseComplexModel):
    type: Literal[ModelType.Complex1]
    
class Complex2Model(BaseComplexModel):
    type: Literal[ModelType.Complex2]
    
ComplexModel = Union[Complex1Model, Complex2Model]
    
Model = Union[SimpleModel, ComplexModel]


model: Model = Simple1Model()

if model.type == ModelType.Simple1 or model.type == ModelType.Simple2:
    reveal_type(model)
    reveal_type(model.simple)
else:
    reveal_type(model)
    reveal_type(model.complex)

Type-check with mypy >= 0.900 and it won't pass, try with mypy 0.812 for example, and it will pass.

I've also prepared it on mypy-play.

Working (mypy 0.812):
https://mypy-play.net/?mypy=0.812&python=3.9&gist=b15073e2bdc681689f2e6f9bc8556ead
Broken (mypy 0.910):
https://mypy-play.net/?mypy=0.910&python=3.9&gist=b15073e2bdc681689f2e6f9bc8556ead

Expected Behavior

main.py:43: note: Revealed type is 'Union[main.Simple1Model, main.Simple2Model]'
main.py:44: note: Revealed type is 'builtins.str'
main.py:46: note: Revealed type is 'Union[main.Complex1Model, main.Complex2Model]'
main.py:47: note: Revealed type is 'builtins.str'

Actual Behavior

main.py:43: note: Revealed type is "Union[main.Simple1Model, main.Simple2Model, main.Complex1Model, main.Complex2Model]"
main.py:44: error: Item "Complex1Model" of "Union[Union[Simple1Model, Simple2Model], Union[Complex1Model, Complex2Model]]" has no attribute "simple"
main.py:44: error: Item "Complex2Model" of "Union[Union[Simple1Model, Simple2Model], Union[Complex1Model, Complex2Model]]" has no attribute "simple"
main.py:44: note: Revealed type is "Union[builtins.str, Any]"
main.py:46: note: Revealed type is "Union[main.Simple1Model, main.Simple2Model, main.Complex1Model, main.Complex2Model]"
main.py:47: error: Item "Simple1Model" of "Union[Union[Simple1Model, Simple2Model], Union[Complex1Model, Complex2Model]]" has no attribute "complex"
main.py:47: error: Item "Simple2Model" of "Union[Union[Simple1Model, Simple2Model], Union[Complex1Model, Complex2Model]]" has no attribute "complex"
main.py:47: note: Revealed type is "Union[Any, builtins.str]"

As you can see mypy isn't able to correctly nail down the union members anymore when unions are nested.

Your Environment

• Mypy version used: 0.912
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.9
• Operating system and version: macOS

[hauntsaninja]

Thanks for following up!
Using mypy_primer -p test.py --bisect-output 'has no attribute' --old v0.812 --debug I bisected this to #9097 (cc @ethan-leba in case you have time to take a look)

[hauntsaninja]

Not obvious and not fully equivalent code, but if you change your check to: if model.type is ModelType.Simple1 or model.type is ModelType.Simple2: you'll pass type checking.

(which is similar behaviour to #6366 (comment) , see other comments on that issue as well)

[levrik]

@hauntsaninja Thanks a lot for pointing me to this workaround.

But I've noticed another issue demonstrated here:

from typing import Literal, Union
from enum import Enum

class ModelType(str, Enum):
    Simple1 = "simple1"
    Simple2 = "simple2"
    Complex1 = "complex1"
    Complex2 = "complex2"

class BaseModel:
    type: ModelType
    
class BaseSimpleModel(BaseModel):
    type: Literal[ModelType.Simple1, ModelType.Simple2]
    simple: str
    
class Simple1Model(BaseSimpleModel):
    type: Literal[ModelType.Simple1]
    
class Simple2Model(BaseSimpleModel):
    type: Literal[ModelType.Simple2]
    
SimpleModel = Union[Simple1Model, Simple2Model]

class BaseComplexModel(BaseModel):
    type: Literal[ModelType.Complex1, ModelType.Complex2]
    complex: str

class Complex1Model(BaseComplexModel):
    type: Literal[ModelType.Complex1]
    
class Complex2Model(BaseComplexModel):
    type: Literal[ModelType.Complex2]
    
ComplexModel = Union[Complex1Model, Complex2Model]
    
Model = Union[SimpleModel, ComplexModel]


model: Model = Simple1Model()

if model.type is ModelType.Simple1:
    reveal_type(model)

Mypy play link: https://mypy-play.net/?mypy=0.910&python=3.9&gist=3fea5f8e3bfa94c7b9835fb53133af4d

Expected output:

main.py:43: note: Revealed type is 'main.Simple1Model'

Actual output:

main.py:43: note: Revealed type is 'Union[main.Simple1Model, main.Simple2Model]'

It seems to nail down the type just one level to the right nested union but not further down.

I'm not fully sure if this is connected to the particular error I was reporting here as this also happens in mypy 0.812.
Should I open a separate issue?

[hauntsaninja]

I think I fixed the original issue in #11521 (not yet released) and the issue in your second comment got fixed in #11204