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

Body - Fields

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:

=== "Python 3.6 and above"

```Python hl_lines="4"
{!> ../../../docs_src/body_fields/tutorial001.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="2"
{!> ../../../docs_src/body_fields/tutorial001_py310.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:

=== "Python 3.6 and above"

```Python hl_lines="11-14"
{!> ../../../docs_src/body_fields/tutorial001.py!}
```

=== "Python 3.10 and above"

```Python hl_lines="9-12"
{!> ../../../docs_src/body_fields/tutorial001_py310.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`, those are actually functions that return special classes.

!!! 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.

Add extra information

You can declare extra information in Field, Query, Body, etc. And it will be included in the generated JSON Schema.

You will learn more about adding extra information later in the docs, when learning to declare examples.

!!! warning Extra keys passed to Field will also be present in the resulting OpenAPI schema for your application. As these keys may not necessarily be part of the OpenAPI specification, some OpenAPI tools, for example the OpenAPI validator, may not work with your generated schema.

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.