How to Efficiently Upload Files to a FastAPI Server Using `form-data`?

Uploading Files with FastAPI Using form-data and SpooledTemporaryFile

To upload files using FastAPI with multipart/form-data, it's recommended to install python-multipart as multipart files are sent via form-data.

pip install python-multipart

Here's an improved example of uploading a file using FastAPI:

from fastapi import File, UploadFile
from typing import List

@app.post("/upload")
def upload(file: UploadFile = File(...)):
    try:
        # Using file.file for synchronous operations (e.g., opening a file on disk)
        contents = file.file.read()
        with open(file.filename, 'wb') as f:
            f.write(contents)
    except Exception:
        return {"message": "An error occurred while uploading the file."}
    finally:
        file.file.close()

    return {"message": f"Successfully uploaded {file.filename}"}

If you need to process larger files in chunks, consider reading the file in smaller increments. You can either use a manual loop:

@app.post("/upload")
def upload(file: UploadFile = File(...)):
    try:
        with open(file.filename, 'wb') as f:
            while contents := file.file.read(1024 * 1024):
                f.write(contents)
    except Exception:
        return {"message": "An error occurred while uploading the file."}
    finally:
        file.file.close()

    return {"message": f"Successfully uploaded {file.filename}"}

Or, use the shutil.copyfileobj() method, which reads and writes data in chunks:

from shutil import copyfileobj

@app.post("/upload")
def upload(file: UploadFile = File(...)):
    try:
        with open(file.filename, 'wb') as f:
            copyfileobj(file.file, f)
    except Exception:
        return {"message": "An error occurred while uploading the file."}
    finally:
        file.file.close()

    return {"message": f"Successfully uploaded {file.filename}"}

Additional Notes:

• FastAPI uses SpooledTemporaryFile for file upload, which stores data in memory. For files larger than 1MB, data is written to a temporary file on disk.
• If defining an endpoint with async def, use asynchronous file processing as mentioned in this answer: [https://stackoverflow.com/a/69868184/6616846](https://stackoverflow.com/a/69868184/6616846)
• For uploading multiple files, you can use a list of UploadFile objects in your endpoint function.
• For HTML form examples, refer to the links provided in the original answer.

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