Add a button to Text Editor header to open the file with the default application.

open_text_in_external.py

# ##### BEGIN GPL LICENSE BLOCK #####
#
#  This program is free software; you can redistribute it and/or
#  modify it under the terms of the GNU General Public License
#  as published by the Free Software Foundation; either version 2
#  of the License, or (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
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#  along with this program; if not, write to the Free Software Foundation,
#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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# ##### END GPL LICENSE BLOCK #####

############# Add-on description (use by Blender)

bl_info = {
    "name":
    "Open Text in External Editor",
    "description":
    'Add a button to Text Editor header to open the file with the default application.',
    "author":
    "Hirad Sab",
    "version": (0, 1, 0),
    "blender": (2, 70, 0),
    "location":
    "TextEditor > Header > External",
    "warning":
    "",  # used for warning icon and text in addons panel
    "wiki_url":
    "",
    "tracker_url":
    "",
    "category":
    "Text Editor"
}

##############

import bpy
import os,sys
import platform
import subprocess


def openFile(filepath):
    print('File opened in external editor:', filepath)
    if sys.platform.startswith('darwin'):
        subprocess.call(('open', filepath))
    elif os.name == 'nt': # For Windows
        os.startfile(filepath)
    elif os.name == 'posix': # For Linux, Mac, etc.
        subprocess.call(('xdg-open', filepath))

class OpenTextinExternal(bpy.types.Operator):
    bl_idname = "text.external_editor"
    bl_label = "Open Text in External Editor"

    @classmethod
    def poll(cls, context):
        return True

    def execute(self, context):
        st = context.space_data
        text = st.text
        if text.filepath:
            f = text.filepath
            openFile(f)

        return {'FINISHED'}

def texteditor_view_external_button(self, context):
    layout = self.layout
    layout.operator("text.external_editor", text = "", icon = 'TEXT')


def register():
    bpy.types.TEXT_HT_header.append(texteditor_view_external_button)
    bpy.utils.register_class(OpenTextinExternal)


def unregister():
    bpy.types.TEXT_HT_header.remove(texteditor_view_external_button)
    bpy.utils.unregister_class(OpenTextinExternal)


if __name__ == "__main__":
    register()