How do you handle form data and file uploads in FastAPI?

FastAPI handles HTML form data with Form() and file uploads with File() and UploadFile. Note: you need to pip install python-multipart for form/file support.

from fastapi import FastAPI, Form, File, UploadFile
from typing import Annotated

app = FastAPI()

# Form data (application/x-www-form-urlencoded)
@app.post("/login")
def login(
    username: Annotated[str, Form()],
    password: Annotated[str, Form()],
):
    return {"username": username}

# Single file upload
@app.post("/upload")
async def upload_file(file: UploadFile):
    contents = await file.read()  # bytes
    return {
        "filename":     file.filename,
        "content_type": file.content_type,
        "size":         len(contents),
    }

# Multiple files
@app.post("/upload-multiple")
async def upload_files(files: list[UploadFile]):
    return [{"filename": f.filename} for f in files]

# Mix form fields + file
@app.post("/profile")
async def update_profile(
    username: Annotated[str, Form()],
    avatar:   UploadFile | None = None,
):
    result = {"username": username}
    if avatar:
        content = await avatar.read()
        # Save to disk / cloud storage
        result["avatar_size"] = len(content)
    return result

# Validate file type and size
@app.post("/images")
async def upload_image(file: UploadFile):
    if file.content_type not in ["image/jpeg", "image/png"]:
        from fastapi import HTTPException
        raise HTTPException(400, "Only JPEG/PNG allowed")
    content = await file.read()
    if len(content) > 5 * 1024 * 1024:  # 5 MB limit
        raise HTTPException(400, "File too large")
    return {"filename": file.filename}

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