Type of conditional expression is object

The type of the following conditional expression is object:

s: str = "key=value"
a, b = s.split("=", 1) if "=" in s else ("key", "default")
# main.py:2: error: 'builtins.object' object is not iterable

It should be a union type instead, such as Union[List[str], Tuple[str, str]].

(Note that str.split returns List[str] while the else branch has the type Tuple[str, str].)

This is a follow-up to #3487.

Alternatively, inferring Sequence[str] for the conditional expression would help in this case.

Inferring a union type would break backward compatibility. If the fallout is not too bad, it may still make sense.

Note that a good fix for #4975 may automatically lead to inferring Sequence here.

Here is a simpler reproduction:

a, b = [1, 2] if bool() else (1, 2)

This issue is addressed in PR #8335.

Fix joining of Sequence (e.g. variadic tuple) and fixed-length tuple (#…

@intgr

Closing as this was fixed by #8335. Thanks @intgr !

I am also facing a similar problem.

from typing import Protocol


class Shape(Protocol):
    def area(self) -> str:
        ...


class Circle:
    def area(self) -> str:
        return 'π * r²'


class Square:
    def area(self) -> str:
        return 'w * l'


class ShapeFactory:
    def create(self, shape_type: str) -> Shape:
        return Circle() if shape_type == 'circle' else Square()  # Here

Mypy shows this error message:

error: Incompatible return value type (got "object", expected "Shape")

@carlosporta

@carlosporta That's a separate issue. Mypy tries to find the common base class of Circle and Square; since those two don't subclass Shape directly, the common base class happens to be object.

@intgr

@intgr, I understood what is happening, but this only happens using when using a conditional expression. If I change it to a conditional statement mypy identifies the shape protocol correctly as the return type.

This works perfectly, no error messages on mypy:

class ShapeFactory:
    def create(self, shape_type: str) -> Shape:
        if shape_type == 'circle':
            return Circle() 
        else:
            return Square()

Should I create a new issue (Type of conditional expression is object) or am I facing something similar to this issue?

@carlosporta

@carlosporta I suspect an issue already exists for that, not totally sure. If you don't find one, feel free to open a new one.

But there's a historical background to why the ternary condition works as it does, I believe there's no easy fix: #3487 #5095

@intgr

@intgr, I did not find anything related to conditional expression and Protocol, so I created a new issue. If someone goes through a similar problem, here is the issue #11722.

I'm still seeing this issue on mypy==0.910 (which FWICT should include #8335).

repro:

from typing import Tuple, List, cast

x: List[Tuple[str, str]] = [("a", "b"), ("c", "d")]

c1, c2 = zip(*x) if len(x) > 0 else ([], [])

> $ mypy --version                                       
mypy 0.910
                                                                                              
> $ mypy ~/scratch/test_types.py 
/Users/haroon/scratch/test_types.py:5: error: "builtins.object" object is not iterable
Found 1 error in 1 file (checked 1 source file)

@hsghori

@hsghori Please do not report bugs as comments on old bugs, unless you're absolutely certain it's the exact same root cause. And when reporting bugs, ensure that your bug is reproducible in the latest git version, or at least latest release version, of the project.

I cannot reproduce this issue with mypy 0.931

% cat errobject.py
x: list[tuple[str, str]] = [("a", "b"), ("c", "d")]
c1, c2 = zip(*x) if len(x) > 0 else ([], [])
reveal_type(c1)
reveal_type(c2)
% mypy errobject.py
errobject.py:3: note: Revealed type is "typing.Sequence[Any]"
errobject.py:4: note: Revealed type is "typing.Sequence[Any]"

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Type of conditional expression is object #8074

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Type of conditional expression is object #8074

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JukkaL commented on Dec 4, 2019

ilevkivskyi commented on Dec 4, 2019

cjolowicz commented on Dec 5, 2019

intgr commented on Jan 26, 2020

emmatyping commented on Feb 3, 2020

carlosporta commented on Dec 12, 2021

intgr commented on Dec 12, 2021

carlosporta commented on Dec 12, 2021

intgr commented on Dec 12, 2021

carlosporta commented on Dec 12, 2021

hsghori commented on Mar 1, 2022

intgr commented on Mar 2, 2022

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