Form Data

Learn essential concepts of form data in FastAPI.

What You'll Learn

This section covers HTML form data, including:

• Receiving form submissions

• Using Form() for form fields

• Combining files and form data

• File uploads with metadata

• Production form handling

Core Concepts

HTML forms send `application/x-www-form-urlencoded` data:

• **Form()**: Import from fastapi for form fields

• **UploadFile**: Handles file uploads in forms

• **Validation**: Form data validated like JSON

• **Streaming**: Efficient file handling

• **Combine Fields**: Mix form fields with files

Simple Form Data Example

from fastapi import FastAPI, Form

app = FastAPI()

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

# HTML Form:
# <form method="post" action="/login/">
#   <input type="text" name="username" />
#   <input type="password" name="password" />
#   <input type="submit" />
# </form>

Form with File Upload Example

from fastapi import FastAPI, Form, UploadFile, File

@app.post("/upload-profile/")
async def upload_profile(
    name: str = Form(...),
    bio: str = Form(...),
    avatar: UploadFile = File(...)
):
    contents = await avatar.read()
    return {
        "name": name,
        "bio": bio,
        "avatar_size": len(contents)
    }

Real-World Usage

Form data is essential for:

• **Web submissions**: Login forms, registration

• **File + metadata**: Upload with description

• **Legacy systems**: API for traditional HTML forms

• **Browser uploads**: Direct form submissions

🔑 Key Takeaways

• ✅ Understand the purpose of form data

• ✅ Know when to apply this pattern

• ✅ Follow best practices consistently

• ✅ Test thoroughly in production scenarios

Next step: Explore the advanced section for production patterns and optimization techniques.