Why is my file2store variable empty when uploading files with FastAPI?

How to Resolve Empty File2Store Variable When Uploading Files with FastAPI

When attempting to upload files via FastAPI, you may encounter an empty file2store variable despite following the documentation's guidelines. To address this issue, several steps can be taken:

1. Install python-multipart:

Ensure that python-multipart is installed, as it is necessary for parsing uploaded files as form data. Use the following command:

pip install python-multipart

2. Use .file Attribute and def Endpoint:

from fastapi import File, UploadFile

@app.post("/upload")
def upload(file: UploadFile = File(...)):
    # Get actual Python file and read/write contents
    contents = file.file.read()
    with open(file.filename, 'wb') as f:
        f.write(contents)

Note: To prevent blocking the server, define the endpoint with def instead of async def in this case.

3. Asynchronous Reading/Writing with async def Endpoint:

If you need to use async def, use asynchronous methods for reading and writing the contents, as demonstrated in this answer.

4. Handle Chunks for Large Files:

If the uploaded file is larger than the available RAM, load the file into memory in chunks and process the data one chunk at a time, as shown in the provided code example.

Uploading Multiple Files:

To upload multiple files as a list in FastAPI, follow these guidelines:

from fastapi import File, UploadFile
from typing import List

# For a single file, use:
# @app.post("/upload")
def upload(file: List[UploadFile] = File(...)):

# For a list of files, use:
# @app.post("/upload")
async def upload_multiple(files: List[UploadFile] = File(...)):

In summary, by installing python-multipart, using the .file attribute of the UploadFile object, handling chunks for large files, and considering asynchronous operations if necessary, you can effectively resolve the empty file2store variable issue when uploading files with FastAPI.

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