Using GTK/GIO async reads in Python

Question

I'm trying to do asynchronous reads with GTK/GIO in Python, but while the read itself works okay (in that the callback is called only after the wanted number of bytes has been input), I haven't been able to find any way to actually access the result.

Given this code:

from gi.repository import Gtk, GLib, Gio
import sys

def callback(stream, task):
        result = stream.read_all_finish(task)
        print(stream, task, task.get_user_data(), task.get_task_data(), result, buf)
        Gtk.main_quit()

buf = bytearray(4)

stream = Gio.UnixInputStream.new(sys.stdin.fileno(), False)
stream.read_all_async(buf, GLib.PRIORITY_DEFAULT, None, callback)

Gtk.main()

When run, it'll wait until at least 4 bytes have been input, so a single a<return> will not cause it to exit, but a second one will. The print will result in something like:

<Gio.UnixInputStream object at 0x7f1045dc3708 (GUnixInputStream at 0x1b8c160)> <Gio.Task object at 0x7f1045dc3bd0 (GTask at 0x1b8e860)> 28890912 28207104 (True, bytes_read=4) bytearray(b'\x00\x00\x00\x00')

In other words...

get_user_data() and get_task_data() seem to be of no help, since they return integers (whose meaning I have no clue about)
The return value from read_all_finish() seems to be of no help, since it's just a success/fail bool plus a bytes_read integer field
The buffer that we gave to read_all_async() seems to be of no help, since it remains unchanged, even though one might expect it to hold the result of the read...

I've also tried...

Inspecting the same things before the read_all_finish() call, just in case that call did something like clear the buffer for some reason - no dice, that gives the same result
Using array('b', [0]*4) instead of the bytearray - that gives the same effect as well
Using an integer count in place of buf (because it seems that an older version of the API worked that way) - that just results in TypeError: Must be sequence, not int
Using a file instead of stdin - no change
Using read_async()/read_finish() instead - no change
...and probably a few more things I can't even remember anymore

At this point, I'm really at my wit's end here - Am I just doing something horribly wrong, or is GIO's python binding horribly broken with respect to the async methods?

score 0 · Answer 1 · answered Jun 11 '18 at 04:45

Despite not complaining about a standard python bytearray/array, it won't actually work with those; You need to use a GLib.ByteArray for it to work.

In other words, this'll work:

from gi.repository import Gtk, GLib, Gio
import sys

def callback(stream, task):
        result = stream.read_all_finish(task)
        print(buf)
        Gtk.main_quit()

buf = GLib.ByteArray.new_take(bytes(4))

stream = Gio.UnixInputStream.new(sys.stdin.fileno(), False)
stream.read_all_async(buf, GLib.PRIORITY_DEFAULT, None, callback)

Gtk.main()

Using GTK/GIO async reads in Python

1 Answers1