tiangolo.fastapi/docs/tutorial/extra-models.md

Continuing with the previous example, it will be common to have more than one related model.

This is especially the case for user models, because:

• The input model needs to be able to have a password.
• The output model should not have a password.
• The database model would probably need to have a hashed password.

!!! danger Never store user's plaintext passwords. Always store a "secure hash" that you can then verify.

If you don't know, you will learn what a "password hash" is in the [security chapters](security/simple-oauth2.md#password-hashing){.internal-link target=_blank}.

Multiple models

Here's a general idea of how the models could look like with their password fields and the places where they are used:

{!./src/extra_models/tutorial001.py!}

About **user_in.dict()

Pydantic's .dict()

user_in is a Pydantic model of class UserIn.

Pydantic models have a .dict() method that returns a dict with the model's data.

So, if we create a Pydantic object user_in like:

user_in = UserIn(username="john", password="secret", email="[email protected]")

and then we call:

user_dict = user_in.dict()

we now have a dict with the data in the variable user_dict (it's a dict instead of a Pydantic model object).

And if we call:

print(user_dict)

we would get a Python dict with:

{
    'username': 'john',
    'password': 'secret',
    'email': '[email protected]',
    'full_name': None,
}

Unwrapping a dict

If we take a dict like user_dict and pass it to a function (or class) with **user_dict, Python will "unwrap" it. It will pass the keys and values of the user_dict directly as key-value arguments.

So, continuing with the user_dict from above, writing:

UserInDB(**user_dict)

Would result in something equivalent to:

UserInDB(
    username="john",
    password="secret",
    email="[email protected]",
    full_name=None,
)

Or more exactly, using user_dict directly, with whatever contents it might have in the future:

UserInDB(
    username = user_dict["username"],
    password = user_dict["password"],
    email = user_dict["email"],
    full_name = user_dict["full_name"],
)

A Pydantic model from the contents of another

As in the example above we got user_dict from user_in.dict(), this code:

user_dict = user_in.dict()
UserInDB(**user_dict)

would be equivalent to:

UserInDB(**user_in.dict())

...because user_in.dict() is a dict, and then we make Python "unwrap" it by passing it to UserInDB prepended with **.

So, we get a Pydantic model from the data in another Pydantic model.

Unwrapping a dict and extra keywords

And then adding the extra keyword argument hashed_password=hashed_password, like in:

UserInDB(**user_in.dict(), hashed_password=hashed_password)

...ends up being like:

UserInDB(
    username = user_dict["username"],
    password = user_dict["password"],
    email = user_dict["email"],
    full_name = user_dict["full_name"],
    hashed_password = hashed_password,
)

!!! warning The supporting additional functions are just to demo a possible flow of the data, but they of course are not providing any real security.

Reduce duplication

Reducing code duplication is one of the core ideas in FastAPI.

As code duplication increments the chances of bugs, security issues, code desynchronization issues (when you update in one place but not in the others), etc.

And these models are all sharing a lot of the data and duplicating attribute names and types.

We could do better.

We can declare a UserBase model that serves as a base for our other models. And then we can make subclasses of that model that inherit its attributes (type declarations, validation, etc).

All the data conversion, validation, documentation, etc. will still work as normally.

That way, we can declare just the differences between the models (with plaintext password, with hashed_password and without password):

{!./src/extra_models/tutorial002.py!}

Union or anyOf

You can declare a response to be the Union of two types, that means, that the response would be any of the two.

It will be defined in OpenAPI with anyOf.

To do that, use the standard Python type hint typing.Union:

{!./src/extra_models/tutorial003.py!}

List of models

The same way, you can declare responses of lists of objects.

For that, use the standard Python typing.List:

{!./src/extra_models/tutorial004.py!}

Response with arbitrary dict

You can also declare a response using a plain arbitrary dict, declaring just the type of the keys and values, without using a Pydantic model.

This is useful if you don't know the valid field/attribute names (that would be needed for a Pydantic model) beforehand.

In this case, you can use typing.Dict:

{!./src/extra_models/tutorial005.py!}

Recap

Use multiple Pydantic models and inherit freely for each case.

You don't need to have a single data model per entity if that entity must be able to have different "states". As the case with the user "entity" with a state including password, password_hash and no password.