Body

API reference for the FastAPI Body parameter function

Body

Declare a request body parameter for a path operation. Body parameters are sent as JSON in the request body.

from typing import Annotated
from fastapi import FastAPI, Body
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items/")
async def create_item(
    item: Item,
    importance: Annotated[int, Body()]
):
    return {"item": item, "importance": importance}

Signature

def Body(
    default: Any = Undefined,
    *,
    embed: bool | None = None,
    media_type: str = "application/json",
    alias: str | None = None,
    title: str | None = None,
    description: str | None = None,
    gt: float | None = None,
    ge: float | None = None,
    lt: float | None = None,
    le: float | None = None,
    min_length: int | None = None,
    max_length: int | None = None,
    pattern: str | None = None,
    examples: list[Any] | None = None,
    deprecated: bool | str | None = None,
    include_in_schema: bool = True,
    json_schema_extra: dict[str, Any] | None = None,
) -> Any

Parameters

default · Any · default: "Undefined"

Default value if the parameter field is not set.

embed · bool | None · default: "None"

When embed is True, the parameter will be expected in a JSON body as a key instead of being the JSON body itself. This happens automatically when more than one Body parameter is declared.

media_type · str · default: "application/json"

The media type of this parameter field. Changing it would affect the generated OpenAPI, but currently it doesn’t affect the parsing of the data.

alias · str | None · default: "None"

An alternative name for the parameter field. This will be used to extract the data and for the generated OpenAPI.

title · str | None · default: "None"

Human-readable title for the parameter.

description · str | None · default: "None"

Human-readable description for the parameter.

gt · float | None · default: "None"

Greater than validation. If set, the value must be greater than this. Only applicable to numbers.

ge · float | None · default: "None"

Greater than or equal validation. If set, the value must be greater than or equal to this. Only applicable to numbers.

lt · float | None · default: "None"

Less than validation. If set, the value must be less than this. Only applicable to numbers.

le · float | None · default: "None"

Less than or equal validation. If set, the value must be less than or equal to this. Only applicable to numbers.

min_length · int | None · default: "None"

Minimum length for strings.

max_length · int | None · default: "None"

Maximum length for strings.

pattern · str | None · default: "None"

RegEx pattern for strings.

discriminator · str | None · default: "None"

Parameter field name for discriminating the type in a tagged union.

strict · bool | None · default: "None"

If True, strict validation is applied to the field.

multiple_of · float | None · default: "None"

Value must be a multiple of this. Only applicable to numbers.

allow_inf_nan · bool | None · default: "None"

Allow inf, -inf, nan. Only applicable to numbers.

max_digits · int | None · default: "None"

Maximum number of allowed digits for numbers.

decimal_places · int | None · default: "None"

Maximum number of decimal places allowed for numbers.

examples · list[Any] | None · default: "None"

Example values for this field.

deprecated · bool | str | None · default: "None"

Mark this parameter field as deprecated. It will affect the generated OpenAPI (visible at /docs).

include_in_schema · bool · default: "True"

Whether to include this parameter field in the generated OpenAPI.

json_schema_extra · dict[str, Any] | None · default: "None"

Any additional JSON schema data.

Examples

Single Body Parameter

from typing import Annotated
from fastapi import FastAPI, Body

app = FastAPI()

@app.post("/items/")
async def create_item(name: Annotated[str, Body()]):
    return {"name": name}

Request body:

"iPhone 14"

Embed Single Body Parameter

@app.post("/items/")
async def create_item(name: Annotated[str, Body(embed=True)]):
    return {"name": name}

Request body:

{
  "name": "iPhone 14"
}

Use embed=True when you want a single value to be expected as a key in the JSON body instead of being the body itself.

Multiple Body Parameters

from pydantic import BaseModel

class Item(BaseModel):
    name: str
    price: float

class User(BaseModel):
    username: str

@app.post("/items/")
async def create_item(item: Item, user: User):
    return {"item": item, "user": user}

Request body:

{
  "item": {
    "name": "iPhone 14",
    "price": 999.99
  },
  "user": {
    "username": "johndoe"
  }
}

Mixing Body with Other Parameters

@app.post("/items/{item_id}")
async def update_item(
    item_id: int,
    item: Item,
    user: User,
    importance: Annotated[int, Body(gt=0)]
):
    return {"item_id": item_id, "item": item, "user": user, "importance": importance}

Request body:

{
  "item": {
    "name": "iPhone 14",
    "price": 999.99
  },
  "user": {
    "username": "johndoe"
  },
  "importance": 5
}

Singular Values in Body

@app.put("/items/{item_id}")
async def update_item(
    item_id: int,
    item: Item,
    user: User,
    importance: Annotated[int, Body(gt=0)],
    q: str | None = None
):
    results = {"item_id": item_id, "item": item, "user": user, "importance": importance}
    if q:
        results.update({"q": q})
    return results

Body with Validations

@app.post("/items/")
async def create_item(
    name: Annotated[str, Body(min_length=1, max_length=100)],
    price: Annotated[float, Body(gt=0, le=1000000)],
    quantity: Annotated[int, Body(ge=1, multiple_of=1)]
):
    return {"name": name, "price": price, "quantity": quantity}

When using Body() with singular values (not Pydantic models), remember to use embed=True if you want the value to be wrapped in a JSON object with a key.