union_mode='smart' not honored when calling model_dump_json()

[mducar]

Initial Checks

• I confirm that I'm using Pydantic V2

Description

Running the example code below results int the following output:

float then int: 100 <class `'int'>`
{"value":100.0}
int then float: 100 <class 'int'>
{"value":100}

As you can see, even though the object instance stores value=100 as an int in both classes, FloatThenInt promotes it to a float 100.0 when converting to JSON.

Example Code

from pydantic import BaseModel, Field
from typing import Union


class FloatThenInt(BaseModel):
    value: Union[float, int, str] = Field(union_mode='smart')


class IntThenFloat(BaseModel):
    value: Union[int, float, str] = Field(union_mode='smart')


float_then_int = FloatThenInt(value=100)
print(f'float then int: {float_then_int.value} {type(float_then_int.value)}')
print(float_then_int.model_dump_json())

int_then_float = IntThenFloat(value=100)
print(f'int then float: {int_then_float.value} {type(int_then_float.value)}')
print(int_then_float.model_dump_json())

Python, Pydantic & OS Version

$ python -c "import pydantic.version; print(pydantic.version.version_info())"
             pydantic version: 2.7.1
        pydantic-core version: 2.18.2
          pydantic-core build: profile=release pgo=false
                 install path: /Users/mducar/.venv/shared_python3/lib/python3.9/site-packages/pydantic
               python version: 3.9.6 (default, Feb  3 2024, 15:58:27)  [Clang 15.0.0 (clang-1500.3.9.4)]
                     platform: macOS-14.3-arm64-arm-64bit
             related packages: typing_extensions-4.9.0
                       commit: unknown

[henrybetts]

I believe I've come across the same issue, but using strict types;

from pydantic import TypeAdapter, StrictStr, StrictFloat, StrictInt, StrictBool
from typing import Union

AttributeType = Union[StrictStr, StrictFloat, StrictInt, StrictBool, None]

TypeAdapter(AttributeType).dump_json('string')  # '"string"' ✅
TypeAdapter(AttributeType).dump_json(1.0)  # '1.0' ✅
TypeAdapter(AttributeType).dump_json(1)  # '1.0' ❌
TypeAdapter(AttributeType).dump_json(True)  # '1.0' ❌
TypeAdapter(AttributeType).dump_json(None)  # 'null' ✅

Seems to just be the StrictFloat type that isn't actually behaving strictly.

[sydney-runkle]

Hey @mducar @henrybetts,

Thanks for reporting this. Definitely something to be improved. Adding to the union issues milestone!