Works

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from gi.repository import Gtk
import subprocess

class MainApp:
    def __init__(self):

        # Build our Interface from the XML/Glade file
        gladefile = "MainWindow.glade"
        try:
            self.builder = Gtk.Builder()
            self.builder.add_from_file(gladefile)
        except:
            print("Failed to load Glade file: %s" % gladefile)

        # Connect signals
        self.builder.connect_signals(self)

        # Get the widgets
        self.window = self.builder.get_object("MainWindow")

        # TextViews
        self.TextViewCommandInput = self.builder.get_object("TextViewCommandInput")
        self.TextViewCommandOutput = self.builder.get_object("TextViewCommandOutput")

    def Command(self, cmd, out=1):
        # Print Command
        if out == 1:
            self.TextBufferCommandInput = self.TextViewCommandInput.get_buffer()
            self.TextBufferCommandInput.insert_at_cursor(cmd + "\n")
        # Send Command
        process = subprocess.Popen(cmd, shell=True, stdin=subprocess.PIPE,
            stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
            close_fds=True, universal_newlines=True)
        output = process.stdout.read()

        # Print Command-Output
        if out == 1:
            self.TextBufferCommandOutput = self.TextViewCommandOutput.get_buffer()
            self.TextBufferCommandOutput.insert_at_cursor(output + "\n")

    def Test():
        cmd = "ls -l"
        Command(cmd)

if __name__ == "__main__":
    StartMainApp = MainApp()
    MainApp.main()