Wrong assignement of values in Union[Foo, List[Foo]]

[armingeiser]

Checks

• I added a descriptive title to this issue
• I have searched (google, github) for similar issues and couldn't find anything
• I have read and followed the docs and still think this is a bug

Bug

When validating a model that allows either a single object Foo or a list of Foo objects, the wrong values are assigned. The key of the second parameter is assigned as value of the first parameter (which is a string).

Output of python -c "import pydantic.utils; print(pydantic.utils.version_info())":

             pydantic version: 1.8.2
            pydantic compiled: True
                 install path: /home/anaconda3/envs/py38/lib/python3.8/site-packages/pydantic
               python version: 3.8.5 (default, Sep  4 2020, 07:30:14)  [GCC 7.3.0]
                     platform: Linux-5.11.0-38-generic-x86_64-with-glibc2.10
     optional deps. installed: ['typing-extensions']

The following minimal example repruduces the bug:

from pydantic import BaseModel, Field
from typing import List, Optional, Union


class Foo(BaseModel):
    a: str = Field(
        ...,
    )
    b: Optional[float]


class Bar(BaseModel):
    foo: Union[Foo, List[Foo]]


bar = {
    "foo": [
        {
            "a": "some string",
            "b": 1.0
        },
        {
            "a": "another string",
            "b": 2.0
        }
    ]
}

a = Bar(**bar)

print(a)

• Output: foo=Foo(a='b', b=None)
• Expected Output: foo=[Foo(a='some string', b=1.0), Foo(a='another string', b=2.0)]

I am not sure if this is related somehow to #2092

[vigneshmanick]

can confirm. Have the same issue.

[PrettyWood]

I am not sure if this is related somehow to #2092

@armingeiser It is 😉

With #2092 code and

class Bar(BaseModel, smart_union=True):
    foo: Union[Foo, List[Foo]]

you get

foo=[Foo(a='some string', b=1.0), Foo(a='another string', b=2.0)]

[armingeiser]

@PrettyWood awesome, thanks for the quick reply. Looking forward to that MR being merged.