Fastapi: Validation error when use alias in pydantic return model

[zihaooo]

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Example

Here's a self-contained, minimal, reproducible, example with my use case:

import fastapi
import starlette.testclient
from pydantic import BaseModel, Field

app = fastapi.FastAPI()


class InModel(BaseModel):
    start_time: str = Field(alias='start_hms')


class OutModel(BaseModel):
    end_time: str = Field(alias='end_hms')


@app.post('/test', response_model=OutModel)
def test(query_body: InModel):
    print(query_body)  # start_time='14:00:00'
    print(query_body.dict())  # {'start_time': '14:00:00'}
    out_model = {'end_time': '12:00:00'}
    return out_model


with starlette.testclient.TestClient(app) as test_client:
    try:
        response = test_client.post('/test', json={'start_hms': '14:00:00'})
        print(response.json())
    except Exception as exc:
        print(exc)

Description

A wonderful framework! Very happy to use it to coding. Here is a little problem that make me really confused. I can easily change the name for request body from public name to field name by add alias in pydantic model (just what i did to InModel in the code), but when I do the same thing to the return model (OutModel in the code), an error occurred.

• Run the code.
• It ends up with an exception:

1 validation error for OutModel
response -> end_hms
  field required (type=value_error.missing).

• But I expected it to return {"end_hms": "14:00:00"}.

Environment

• OS: Ubuntu 20.04
• FastAPI version: 0.58.0
• Python version: 3.8.2
• Pydantic version: 1.5.1
• Starlette version: 0.13.4

[MacMacky]

Hi @zihaooo you could do it like this.

class OutModel(BaseModel):
    end_time: str = Field(alias='end_hms')

    class Config:
        # so we can use either the 'end_time' attribute or the alias attribute in this case 'end_hms'
        allow_population_by_field_name = True

[zihaooo]

@MacMacky Many thanks~

For someone has the same question: https://pydantic-docs.helpmanual.io/usage/model_config/

And I complete the example with @MacMacky 's code:

import fastapi
import starlette.testclient
from pydantic import BaseModel, Field

app = fastapi.FastAPI()


class InModel(BaseModel):
    start_time: str = Field(alias='start_hms')


class OutModel(BaseModel):
    end_time: str = Field(alias='end_hms')
    class Config:
        # so we can use either the 'end_time' attribute or the alias attribute in this case 'end_hms'
        allow_population_by_field_name = True

@app.post('/test', response_model=OutModel)
def test(query_body: InModel):
    print('Enter server side...')
    print(f'Query body: {query_body.dict()}')  # {'start_time': '14:00:00'}
    out_model = {'end_time': '20:00:00'}
    print(f'Return value: {out_model}')
    print('Making response...\n')
    return out_model


with starlette.testclient.TestClient(app) as test_client:
    try:
        print('Enter client side...')
        query_body = {'start_hms': '14:00:00'}
        print(f'Query body: {query_body}')
        print('Making request...\n')
        response = test_client.post('/test', json=query_body)
        print('Back to client side...')
        print(f'Return value: {response.json()}')
        print('Exit client side.\n')
    except Exception as exc:
        print(exc)

Output:

Enter client side...
Query body: {'start_hms': '14:00:00'}
Making request...

Enter server side...
Query body: {'start_time': '14:00:00'}
Return value: {'end_time': '20:00:00'}
Making response...

Back to client side...
Return value: {'end_hms': '20:00:00'}
Exit client side.

[MacMacky]

@zihaooo You're welcome, man. Glad that I helped you.
Will you please close this issue. Thanks.

[tiangolo]

Thanks for the help here everyone! :clap: :bow:

Thanks for reporting back and closing the issue :+1: