Form Models¶

You can use Pydantic models to declare form fields in FastAPI.

Note

To use forms, first install python-multipart.

Make sure you create a virtual environment, activate it, and then install it, for example:

$ pipinstallpython-multipart

Note

This is supported since FastAPI version 0.113.0. 🤓

Pydantic Models for Forms¶

You just need to declare a Pydantic model with the fields you want to receive as form fields, and then declare the parameter as Form:

Python 3.10+

fromtypingimport Annotated
fromfastapiimport FastAPI, Form
frompydanticimport BaseModel
app = FastAPI()
classFormData(BaseModel):
 username: str
 password: str
@app.post("/login/")
async deflogin(data: Annotated[FormData, Form()]):
 return data

🤓 Other versions and variants

Python 3.10+ - non-Annotated

Tip

Prefer to use the Annotated version if possible.

fromfastapiimport FastAPI, Form
frompydanticimport BaseModel
app = FastAPI()
classFormData(BaseModel):
 username: str
 password: str
@app.post("/login/")
async deflogin(data: FormData = Form()):
 return data

FastAPI will extract the data for each field from the form data in the request and give you the Pydantic model you defined.

Check the Docs¶

You can verify it in the docs UI at /docs:

Forbid Extra Form Fields¶

In some special use cases (probably not very common), you might want to restrict the form fields to only those declared in the Pydantic model. And forbid any extra fields.

Note

This is supported since FastAPI version 0.114.0. 🤓

You can use Pydantic's model configuration to forbid any extra fields:

Python 3.10+

fromtypingimport Annotated
fromfastapiimport FastAPI, Form
frompydanticimport BaseModel
app = FastAPI()
classFormData(BaseModel):
 username: str
 password: str
 model_config = {"extra": "forbid"}
@app.post("/login/")
async deflogin(data: Annotated[FormData, Form()]):
 return data

🤓 Other versions and variants

Python 3.10+ - non-Annotated

Tip

Prefer to use the Annotated version if possible.

fromfastapiimport FastAPI, Form
frompydanticimport BaseModel
app = FastAPI()
classFormData(BaseModel):
 username: str
 password: str
 model_config = {"extra": "forbid"}
@app.post("/login/")
async deflogin(data: FormData = Form()):
 return data

If a client tries to send some extra data, they will receive an error response.

For example, if the client tries to send the form fields:

username: Rick
password: Portal Gun
extra: Mr. Poopybutthole

They will receive an error response telling them that the field extra is not allowed:

{
"detail":[
{
"type":"extra_forbidden",
"loc":["body","extra"],
"msg":"Extra inputs are not permitted",
"input":"Mr. Poopybutthole"
}
]
}

Summary¶

You can use Pydantic models to declare form fields in FastAPI. 😎