Form Data

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When you need to receive form fields instead of JSON, you can use Form.

Info

To use forms, first install python-multipart.

E.g. pip install python-multipart.

Import Form

Import Form from fastapi:

from fastapi import FastAPI, Form
app = FastAPI()
@app.post("/login/")
async def login(username: str = Form(...), password: str = Form(...)):
    return {"username": username}

Define Form parameters

Create form parameters the same way you would for Body or Query:

from fastapi import FastAPI, Form
app = FastAPI()
@app.post("/login/")
async def login(username: str = Form(...), password: str = Form(...)):
    return {"username": username}

For example, in one of the ways the OAuth2 specification can be used (called “password flow”) it is required to send a username and password as form fields.

The spec requires the fields to be exactly named username and password, and to be sent as form fields, not JSON.

With Form you can declare the same metadata and validation as with Body (and Query, Path, Cookie).

Info

Form is a class that inherits directly from Body.

Tip

To declare form bodies, you need to use Form explicitly, because without it the parameters would be interpreted as query parameters or body (JSON) parameters.

About “Form Fields”

The way HTML forms (<form></form>) sends the data to the server normally uses a “special” encoding for that data, it’s different from JSON.

FastAPI will make sure to read that data from the right place instead of JSON.

Technical Details

Data from forms is normally encoded using the “media type” application/x-www-form-urlencoded.

But when the form includes files, it is encoded as multipart/form-data. You’ll read about handling files in the next chapter.

If you want to read more about these encodings and form fields, head to the MDN web docs for POST.

Warning

You can declare multiple Form parameters in a path operation, but you can’t also declare Body fields that you expect to receive as JSON, as the request will have the body encoded using application/x-www-form-urlencoded instead of application/json.

This is not a limitation of FastAPI, it’s part of the HTTP protocol.

Recap

Use Form to declare form data input parameters.