tiangolo.fastapi/docs/tutorial/body-fields.md

The same way you can declare additional validation and metadata in path operation function parameters with Query, Path and Body, you can declare validation and metadata inside of Pydantic models using Pydantic's Field.

Import Field

First, you have to import it:

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

!!! warning Notice that Field is imported directly from pydantic, not from fastapi as are all the rest (Query, Path, Body, etc).

Declare model attributes

You can then use Field with model attributes:

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

Field works the same way as Query, Path and Body, it has all the same parameters, etc.

!!! note "Technical Details" Actually, Query, Path and others you'll see next create objects of subclasses of a common Param class, which is itself a subclass of Pydantic's FieldInfo class.

And Pydantic's `Field` returns an instance of `FieldInfo` as well.

`Body` also returns objects of a subclass of `FieldInfo` directly. And there are others you will see later that are subclasses of the `Body` class.

Remember that when you import `Query`, `Path`, and others from `fastapi`, <a href="https://fastapi.tiangolo.com/tutorial/path-params-numeric-validations/#recap" target="_blank">those are actually functions that return classes of the same name</a>.

!!! tip Notice how each model's attribute with a type, default value and Field has the same structure as a path operation function's parameter, with Field instead of Path, Query and Body.

JSON Schema extras

In Field, Path, Query, Body and others you'll see later, you can declare extra parameters apart from those described before.

Those parameters will be added as-is to the output JSON Schema.

If you know JSON Schema and want to add extra information apart from what we have discussed here, you can pass that as extra keyword arguments.

!!! warning Have in mind that extra parameters passed won't add any validation, only annotation, for documentation purposes.

For example, you can use that functionality to pass a JSON Schema example field to a body request JSON Schema:

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

And it would look in the /docs like this:

Recap

You can use Pydantic's Field to declare extra validations and metadata for model attributes.

You can also use the extra keyword arguments to pass additional JSON Schema metadata.