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I have an api in which the server sends a stream of data to the client. On the server-side, the stream object is populated. I can verify this by writing to a file on the server before sending. But when the client receives the stream and tries to write it to memory, it always results in an empty file. Here is my code

TL;DR: Stream is not empty when sent from api, but is empty when client recieves and tries to write

Server-side - sending io.BytesIO object

from io import BytesIO
from fastapi import FastAPI
app = FastAPI()

# capture image and store in io.BytesIO stream
def pic_to_stream(self):
        io = BytesIO()
        self.c.capture(io, 'jpeg')  # populate stream with image
        return io

# api endpoint to send stream
@app.get('/dropcam/stream')
def dc_stream():
    stream, ext = cam.pic_to_stream()
    # with open('example.ext', 'wb') as f:
    #     f.write(stream.getbuffer())  # this results in non-empty file on server
    return StreamingResponse(stream, media_type=ext)

Client-side - receiving io.BytesIO object and trying to write

import requests

def recieve_stream():
    url = 'http://abc.x.y.z/dropcam/stream'
    local_filename = 'client/foo_stream.jpeg'
    with requests.get(url, stream=True) as r:
        with open(local_filename, 'wb') as f:
            f.write(r.content) # this results in an empty file on client

Does anyone have any ideas? I have looked at these links and more with no luck. Thank you!

alex_noname
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SkippyElvis
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  • You might find [this answer](https://stackoverflow.com/a/71643439/17865804) and [this answer](https://stackoverflow.com/a/73815575/17865804) helpful as well – Chris Apr 25 '23 at 05:18

1 Answers1

2

Perhaps you did not consider that BytesIO is a seekable stream and supports the current byte position. Therefore after writing to it you need to do stream.seek(0) before reading from beginning or passing to StreamingResponse.

alex_noname
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