Pydantic model for array of jsons

Question

I am using FastAPI to write a web service. It is good and fast.

FastAPI is using pydantic models to validate input and output data, everything is good but when I want to declare a nested model for array of jsons like below:

[
   {
      "name": "name1",
      "family": "family1"
   },
   {
      "name": "name2",
      "family": "family2"
   }
]

I get empty response.

I think there is a problem with my model which is:

class Test(BaseModel):
    name: str
    family: str
    class Config:
        orm_mode = True

class Tests(BaseModel):
    List[Test]
    class Config:
        orm_mode = True

So, my question is how should I write a model for array of jsons?

Notice you have "orm_mode = True" , most likely you are experiencing a problem with you ORM and not with your Pydantic models... can't say much more because you have not provided details or your ORM. — user368604
– user368604, Commented Apr 29, 2020 at 12:17
If you are POSTing json data, fastapi will try to convert it automatically to a pydantic model. Otherwise, you may simply declare a field as an array, as you did in Tests. Have you tried removing the "class Config: orm_mode = True" piece of code? — lsabi
– lsabi, Commented Apr 30, 2020 at 16:11

connebs · Accepted Answer · 2020-09-26 13:45:15Z

16

Update (26/09/2020)

In Python 3.9 (not yet released), you can do the same as below but with the built-in list generic type (which is always in scope) rather than needing to import the capitalized List type from typing, e.g.

@app.get("/tests", response_model=list[Test])

The issue here is that you are trying to create a pydantic model where it is not needed. If you want to serialize/deserialize a list of objects, just wrap your singular model in a List[] from python's builtin typing module. There is no need to try to create a plural version of your object with a pydantic BaseModel (and as you can see, it does not work anyway).

With that said, the simplest way to do what you want is to just specify a List[Test] at any point where you need a list of Tests, e.g.

from typing import List

from fastapi import FastAPI
from pydantic import BaseModel


existing_tests = [
    {
        "name": "name1",
        "family": "family1"
    },
    {
        "name": "name2",
        "family": "family2"
    }
]


class Test(BaseModel):
    name: str
    family: str

    class Config:
        orm_mode = True


app = FastAPI()


@app.get("/tests", response_model=List[Test])
async def fetch_tests():
    return existing_tests


@app.post("/tests")
async def submit_tests(new_tests: List[Test]):
    print(new_tests)

But of course if you find yourself repeatedly (or only) specifying Test as a list, you can of course just assign this to a variable and then use that variable where needed, like so:

Tests = List[Test]

@app.get("/tests", response_model=Tests)
async def fetch_tests():
    return existing_tests

@app.post("/tests")
async def submit_tests(new_tests: Tests):
    print(new_tests)

I think the first option is probably slightly clearer in your code though, and unless you are specifying List[Test] many times, using a variable for this purpose is probably not worth the extra layer of indirection.

edited Sep 26, 2020 at 13:45

answered Apr 30, 2020 at 21:49

connebs

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3 Comments

Matt Webster Over a year ago

this is awesome! hadn't seen much good docs on how to return lists with FastAPI which seems like a super common use case.

A. Rabus Over a year ago

This gives me a warning in PyCharm: Expected type 'Optional[type]', got 'GenericAlias' instead, but works.

DrDarkRye Over a year ago

@A.Rabus I get warning too. Should we use the typing List, or Python 3.9 list?

largehadroncollider · Accepted Answer · 2024-06-05 17:16:46Z

1

One thing that I was able to achieve with Pydantic V2 that plays nicely in OpenAPI is importing from RootModel instead of BaseModel:

class Test(BaseModel):
    name: str
    family: str
    class Config:
        orm_mode = True

class Tests(RootModel[List[Test]]):
    pass

but as highlighted above, this is not strictly necessary.

answered Jun 5, 2024 at 17:16

largehadroncollider

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