What File Descriptor object does Python AsyncIO's loop.add_reader() expect?

These functions expect a file descriptor, that is, the underlying integers the operating system uses, not Python's file objects. File objects that are based on file descriptors return that descriptor on the fileno() method, so for example:

>>> sys.stderr.fileno()
2

In Unix, file descriptors can be attached to files or a lot of other things, including other processes.

Edit for the OP's edit:

As Max in the comments says, you can not use epoll on local files (and asyncio uses epoll). Yes, that's kind of weird. You can use it on pipes, though, for example:

import asyncio
import urllib.parse
import sys
import pdb
import os

def fileCallback(*args):
    print("Received: " + sys.stdin.readline())

loop = asyncio.get_event_loop()
task = loop.add_reader(sys.stdin.fileno(), fileCallback)
loop.run_forever()

This will echo stuff you write on stdin.