FastAPI: Return response with an array of objects

Question

I'm currently working on a API that returns scraped data. The data is stored as an array but when I go return that array as a response on an endpoint, I get the following error:

pydantic.error_wrappers.ValidationError: 1 validation error for InnerObject
response
  value is not a valid dict (type=type_error.dict)

Here's a simplified version of what I'm trying to achieve:

class InnerObject(BaseModel):
    foo: str

class OuterObject(BaseModel):
    bar: List[InnerObject]

@app.get("/test_single", response_model=InnerObject)
def test():
    return InnerObject(foo="Hello Mars")

@app.get("/test_multiple", response_model=OuterObject)
def test():
    objects = [InnerObject]

    objects.append(InnerObject(foo="Hello Earth"))
    objects.append(InnerObject(foo="Hello Mars"))

    return objects

I have an array of objects that I want to return as a response. It's also possible that I don't need outer/inner models but I have also attempted this and set the response_model to be response_model=List[InnerObject]. Returning a single InnerObject as seen in the "/test_single" endpoint works fine, so I assume it's to do with trying to return a [InnerObject]

Thanks for any responses in advance

Solution

Thanks kosciej16, the problem was that I was adding the object name when declaring the list. So I was going objects = [InnerObject] when I should have been going objects = []

kosciej16 · Accepted Answer · 2021-11-23 10:59:42Z

4

Generally fastapi tries to create OuterObject from thing you return.

In such case you have few options.

Create object explicitly

@app.get("/test_multiple", response_model=OuterObject)
def test():
    objects = []

    objects.append(InnerObject(foo="Hello Earth"))
    objects.append(InnerObject(foo="Hello Mars"))

    return OuterObject(bar=objects)

Change response_model

@app.get("/test_multiple", response_model=List[InnerObject])
def test():
    objects = []

    objects.append(InnerObject(foo="Hello Earth"))
    objects.append(InnerObject(foo="Hello Mars"))

    return objects

Change definition of OuterObject

class OuterObject(List[InnerObject]):
    pass

answered Nov 23, 2021 at 10:59

kosciej16

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