__init__.py

#!/usr/bin/env python3

from gi.repository import GObject
from gi.repository import Ide
from gi.repository import Gio

class GofmtPreferences(GObject.Object, Ide.PreferencesAddin):

    def do_load(self, prefs):
        self.completion_id =(
            'code-format','completion',
            'org.gnome.builder.extension-type', 'disabled', '/',
            None,
            _("Gofmt on save"),
            _("Format code go with Gofmt on save, then writes the result to the buffer"),
            None, 30)



    def do_unload(self, prefs):
         prefs.remove_id(self.completion_id)



class GofmtOnSave(GObject.Object, Ide.WorkbenchAddin):


    def do_load(self, workbench):
        context = workbench.get_context()
        bufmgr = context.get_buffer_manager()
        self.handler = bufmgr.connect('save-buffer', self.on_save_buffer)

    def do_unload(self, workbench):
        workbench.remove_id(self.gofmt_enable)
        context = workbench.get_context()
        bufmgr = context.get_buffer_manager()
        bufmgr.disconnect(self.handler)

    def on_save_buffer(self, bufmgr, buffer):
        # If not go code return and exit
        lang = buffer.get_language()
        if lang is None or lang.get_id() not in ('go'):
            return
        launcher = Ide.SubprocessLauncher.new(0)
        launcher.set_flags(Gio.SubprocessFlags.STDIN_PIPE | Gio.SubprocessFlags.STDOUT_PIPE)
         

        #setup our cmdline arguments
        launcher.push_argv('gofmt')
        launcher.set_run_on_host(True)


        #launch the process
        subprocess = launcher.spawn()

        begin, end = buffer.get_bounds()
        text = buffer.get_text(begin, end, True)


        if text == '':
            return
    	#Write the buffer to the gofmt process
        ret, stdout, stderr = subprocess.communicate_utf8(text, None)

        # Write the new contents to the buffer
        buffer.set_text(stdout, len(stdout))
        print("Gofmt complete")