test_buffer.py

import gi
import sys
gi.require_version('Gst', '1.0')
from gi.repository import Gst

Gst.init(None)

appsrc = Gst.ElementFactory.make("appsrc", "appsrc")
filesink = Gst.ElementFactory.make("filesink", "filesink")
filesink.set_property("location", "test.dat")

pipeline = Gst.Pipeline()
pipeline.add(appsrc)
pipeline.add(filesink)
appsrc.link(filesink)
pipeline.set_state(Gst.State.PLAYING)

data = "1234" * 12
print ("Using data: %s" % data)

buf = Gst.Buffer.new_wrapped(data.encode())

appsrc.emit("push-buffer", buf)

pipeline.send_event(Gst.Event.new_eos())

result = open("test.dat").read()

print ("Result    : %s" % result)

pipeline.set_state(Gst.State.NULL)

Update: Code is finally working. instead of new_allocate i used new_wrapped in order for the data to been wrapped immediately.

data.encode() was used because data is not in range of 0 to 255, so if you tried int(data) there were overflow issues.