Is there a way to check the size of the incoming POST in Pyramid, without saving the file to disk and using the os module?
You should be able to check the request.content_length. WSGI does not support streaming the request body so content length must be specified. If you ever access request.body, request.params or request.POST it will read the content and save it to disk.
The best way to handle this, however, is as close to the client as possible. Meaning if you are running behind a proxy of any sort, have that proxy reject requests that are too large. Once it gets to Python, something else may have already stored the request to disk.
Check size of HTTP POST without saving to disk