exclude_unset doesn't work if query-params are parsed as pydantic-model dependency

[solry]

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Example Code

from pydantic import BaseModel
from fastapi import FastAPI

app = FastAPI()

class TestModel(BaseModel):
    field_true: bool | None = Field(True)

@app.get("/test-unset")
def test_unset(
    model_depends_annotated: Annotated[TestModel, Depends(TestModel)],
    model_depends_non_annotated: TestModel = Depends(),
    model_non_depends: TestModel = TestModel(),
):
    return {
        "model_depends_annotated": model_depends_annotated.dict(
            exclude_unset=True,
        ),
        "model_depends_non_annotated": model_depends_non_annotated.dict(
            exclude_unset=True,
        ),
        "model_non_depends": model_non_depends.dict(
            exclude_unset=True,
        ),
    }

Description

• Run your app, e.g. python -m uvicorn --host 0.0.0.0 --port 8080 app:app
• Open browser and call endpoint http://localhost:8080/test-unset
• Discover the following output:

{
  "model_depends_annotated": {"field_true": true},
  "model_depends_non_annotated": {"field_true": true},
  "model_non_depends": {}
}

• When the expected output is

{
  "model_depends_annotated": {},
  "model_depends_non_annotated": {},
  "model_non_depends": {}
}

• Conclude that the object that is shipped by the dependency injection doesn't have information which fields were set and not set.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.95.2

Python Version

Python 3.10.8

Additional Context

It could be related to #5891, but the same result if I make test_unset an async function.

[starryknight64]

Verified this problem in Python 3.10.11, pydantic 1.10.18, and fastapi 0.95.2 (and uvicorn 0.31.0 in case it matters).

Also verified this problem still exists when updating to Python 3.12.7, pydantic 2.9.2, and fastapi 0.115.0 (and uvicorn 0.31.0 still).

I also tried calling model_dump(exclude_unset=True) directly in Python 3.12 (since the dict function is deprecated) and same results.

I also tried by setting response_model_exclude_unset=True in the decorator for the test_unset function (same results).

I could be wrong, but in my limited experience from what I can tell this looks like a Pydantic issue?

[YuriiMotov]

My attempt to explain why it doesn't work:

Details

When you use the syntax like

model_depends_annotated: Annotated[TestModel, Depends(TestModel)]

or

model_depends_non_annotated: TestModel = Depends()

it's actually the same as:

model_depends_annotated: Annotated[TestModel, Depends(TestModel.__init__)]

where TestModel.__init__ is automatically generated by Pydantic as something like:

def TestModel.__init__(self, field_true: bool | None = Field(True)):
    self.field_true = field_true

FastAPI further analyzes the signature of TestModel.__init__ and adds each parameter as separate dependency.

So, for FastAPI it's equal to the following code:

async def get_test_model(field_true: bool | None = Field(True)):
    return TestModel(field_true=field_true)

@app.get("/test-unset")
def test_unset(model_depends_annotated: Annotated[TestModel, Depends(get_test_model)]):
    pass

As you can see, FastAPI will first validate the field_true parameter and then just pass it to the constructor of TestModel. So, this field will always be set (passed explicitly by FastAPI to model's constructor).

Since it's set explicitly, you can't exclude it with exclude_unset=True..

Current situation

In FastAPI 0.115.0 the support of Pydantic models for Query (and also for Header and Cookie) parameters was introduced officially. So, the code should be changed to:

from typing import Annotated
from pydantic import BaseModel, Field
from fastapi import FastAPI, Query

app = FastAPI()

class TestModel(BaseModel):
    field_true: bool | None = Field(True)

@app.get("/test-unset")
def test_unset(
    model_query: Annotated[TestModel, Query()],
):
    return {
        "model_query": model_query.model_dump(
            exclude_unset=True,
        ),
    }

But it still doesn't work as expected (unset values are not excluded):

curl -X 'GET' \
  'http://localhost:8000/test-unset' \
  -H 'accept: application/json'
{"model_query":{"field_true":true}}

[RichieB2B]

The above example works in FastAPI 0.127.1 when setting None as default:

from typing import Annotated
from pydantic import BaseModel, Field
from fastapi import FastAPI, Query

app = FastAPI()

class TestModel(BaseModel):
    field_true: bool | None = None

@app.get("/test-unset")
def test_unset(
    model_query: Annotated[TestModel, Query()],
):
    return {
        "model_query": model_query.model_dump(
            exclude_unset=True,
        ),
    }

Now it works as expected:

curl -X 'GET' \
  'http://localhost:8000/test-unset' \
  -H 'accept: application/json'
{"model_query":{}}

The fact that it does not work when the default value is not None seems to be FastAPI specific (bug?). In plain pydantic this does work:

from pydantic import BaseModel, Field

class TestModel(BaseModel):
    field_true: bool | None = Field(True)

test_model_not_set = TestModel()
print(f"test_model_not_set = {test_model_not_set.model_dump(exclude_unset=True)}")

test_model_set = TestModel(field_true=True)
print(f"test_model_set = {test_model_set.model_dump(exclude_unset=True)}")

This gives the output you'd expect:

test_model_not_set = {}
test_model_set = {'field_true': True}