How can I call to subprocess with input and output as file-like variables?

Hello I am triying to callo to ffmpeg from ubuntu, the idea is set the input a file-like object get from s3 apply a filter and upload again without write locally, the code that The first try was the following code

import subprocess
import boto3
from io import BytesIO

s3_client = boto3.client('s3')

f = BytesIO()
s3_client.download_fileobj('video', '15_19_12.mkv', f)

result = subprocess.run(['ffmpeg', "-codec copy", "aaa.mp4"], input=f.getbuffer())

But I this doesn't recognice args like "-codec copu"

Then I update to

proc = subprocess.Popen(['ffmpeg', "-codec", "copy"], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
out, err = proc.communicate(input=f.getbuffer())

in this case, doesnt recognice the input

So how can I use Popen or another in order to set f.getbuffer and get the output file in another Bytes?

Thanks

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