Kivy and Flask threads blocking each other in Python2

Question

I have the following setup: Kivy 1.11.1, Flask 1.1.1 and Python 2.7.13. I want to fetch a JSON request but I am stuck at Flask app does not fetch requests while the Kivy GUI is running. Even when run in separate threads they block each other.

The main api script eg_api.py:

#!/usr/bin/python2.7 python2.7
# -*- coding: utf-8 -*-

# kivy modules first, if not Kivy may cause problems
import kivy
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.uix.popup import Popup
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.screenmanager import ScreenManager, Screen
kivy.require('1.10.0')

# common modules
import re
import sys
import time
import signal
import threading
from datetime import datetime

# Flask & similar modules
from flask import Flask
from flask import request
from flask_restful import reqparse, abort, Api, Resource
from flask_httpauth import HTTPBasicAuth
import json, ast
import urlparse
import eventlet
from eventlet import wsgi

# imports from other files
from eg_wrapper import Actions

# async server setup
eventlet.monkey_patch()
app = Flask(__name__)
api = Api(app)
auth = HTTPBasicAuth()

# user access setup
USER_DATA = {
    "admin": "SuperSecretPwd"
}

# global variable
data_json = None

@auth.verify_password
def verify(username, password):
    if not (username and password):
        return False
    return USER_DATA.get(username) == password

def start_Flask():
    print("Starting Flask...")
    #app.run(port=5000, debug=False)     #specify separate port to run Flask app
    wsgi.server(eventlet.listen(('', 5000)), app)     # deploy as an eventlet WSGI server

def signal_handler(signal, frame):
    print " CTRL + C detected, exiting ... "
    exit(0)


#######################   flask classes   #########################
class Load(Resource):
    @auth.login_required
    def post(self):
        print "**********  class Load(Resource): *****************"
        data = request.json
        print data
        lw = Actions()
        data_json = lw.Parse_Load_Request(data)
        res = {'data': data_json}
        return res, 201


#######################   resource calls   #######################
api.add_resource(Load, '/load')


########################## kivy screen classes ###############################
class MainScreen(Screen):
    pass


########################## kivy popup classes ###############################

# main screen        
class MainScreen(Screen):
    def __init__(self, **kwargs):
        self.name="MAIN SCREEN"
        super(Screen, self).__init__(**kwargs)

# popup        
class MessageBox10(Popup):
    def __init__(self, obj, **kwargs):
        super(MessageBox10, self).__init__(**kwargs)
        self.obj = obj

# popup        
class MessageBox20(Popup):
    def __init__(self, obj, **kwargs):
        super(MessageBox20, self).__init__(**kwargs)
        self.obj = obj

class Kivy(App):
    w_MessageBox10_1 = "Message_10_1"
    w_MessageBox10_2 = "Message_10_2"
    w_MessageBox10_3 = "Message_10_3"
    w_MessageBox10_4 = "Message_10_4"
    w_MessageBox20_1 = "Message_20_1"
    w_MessageBox20_2 = "Message_20_2"
    w_MessageBox30_1 = "Message_30_1"
    w_MessageBox30_2 = "CONFIRM"
    w_MessageBox30_3 = "CANCEL"

    def do(self):
        print "do something"

    def cancel(self):
        print "load cancelled by user"

    def exit(self):
        print "exiting..."
        exit(1)

    def enter(self):
        # open the init file and write the parameters
        print "********** def enter(self): popup = MessageBox20(self) *************************"
        popup = MessageBox20(self)
        popup.open()

    def build(self):
        sm = Builder.load_string("""

ScreenManager
    MainScreen:
        size_hint: 1, .7
        auto_dismiss: False
        title: app.w_MessageBox10_1       
        title_align: "center"

        BoxLayout:
            orientation: "vertical"
            Label:
                text: app.w_MessageBox10_2
            BoxLayout:
                orientation: "horizontal"
                spacing: 10
                size_hint: 1, .5
                Button:
                    text: app.w_MessageBox30_2  # CONFIRM
                    on_press:
                        app.enter()
                Button:
                    text: app.w_MessageBox30_3  # CANCEL
                    on_press:
                        app.exit()

<MessageBox20>:
    size_hint: 1, .7
    auto_dismiss: False
    title: app.w_MessageBox20_1       
    title_align: "center"

    BoxLayout:
        orientation: "vertical"
        Label:
            text: app.w_MessageBox20_2
        BoxLayout:
            orientation: "horizontal"
            spacing: 10
            size_hint: 1, .5
            Button:
                text: app.w_MessageBox30_2  # "CONFIRM"
                on_press:
                    app.do()
                    root.dismiss()
            Button:
                text: app.w_MessageBox30_3  # "CANCEL"
                on_press:
                    app.cancel()
                    root.dismiss()  

        """)

        return sm


if __name__ == '__main__':    

    #CTRL+C signal handler
    signal.signal(signal.SIGINT, signal_handler)
    signal.signal(signal.SIGTERM, signal_handler)

    # start the first thread
    t1 = threading.Thread(target=start_Flask)
    t1.start()

    # start the second thread
    t2 = threading.Thread(target=Kivy().run)
    t2.start()

    # join the two threads
    t1.join()
    t2.join()

The wrapping script eg_wrapper.py:

#!/usr/bin/python2.7 python2.7
#!/bin/sh -i
# -*- coding: utf-8 -*-

"""
This program executes actions called by eg_api.py.
"""

import os
import signal
import re
import json, ast
import sys
import subprocess
import multiprocessing
from subprocess import PIPE, STDOUT
from datetime import datetime

# to solve the send_file issue
import eventlet
eventlet.monkey_patch()


class Actions:

    def Parse_Load_Request(self, request):     # parse the JSON data
        timestamp = datetime.now()
        code = "200"
        id_code = "0000"
        data = request
        task = ""
        res={}
        list_of_tasks = []
        print "********** def Parse_Load_Request(self, r): print data without uni-code chars *****************************"
        data = ast.literal_eval(json.dumps(data)) # Removing uni-code chars
        print "********** def Parse_Load_Request(self, r): print keys for data *****************************"
        print(data.keys())    # print all the keys in the dictionary first
        res["code"] = code
        res["timestamp"] = str(timestamp)
        res["id"] = id_code
        res["data"] = str(list_of_tasks)
        res["task"] = task
        return res


def signal_handler(signal, frame):
    exit(1)


def main():
    # CTRL+C signal handler
    signal.signal(signal.SIGINT, signal_handler)
    signal.signal(signal.SIGTERM, signal_handler)

    #the sys.argv parameter is a list of arguments from the command line
    ex = Actions()
    sys.exit(app.exec_())


if __name__ == "__main__":
    # define the stuff
    global mainProcess, mainProcessPid

    # execute main
    main()

The procedure

I run the eg_api.py and Flask and Kivy do start as threads. They get up and run, eventlet is listening and the Kivy window with the first popup gets up. Then I send the request.

The output

(py2) parovelb@Latitude-E6510:~/Desktop/Python2$ python eg_api.py 
[INFO   ] [Logger      ] Record log in /home/parovelb/.kivy/logs/kivy_19-12-23_34.txt
[INFO   ] [Kivy        ] v1.10.1
[INFO   ] [Python      ] v2.7.15rc1 (default, Nov 12 2018, 14:31:15) 
[GCC 7.3.0]
[INFO   ] [Factory     ] 194 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil, img_gif (img_ffpyplayer ignored)
[INFO   ] [Text        ] Provider: sdl2
Starting Flask...
 (15549) wsgi starting up on http://0.0.0.0:5000
[INFO   ] [Window      ] Provider: sdl2(['window_egl_rpi'] ignored)
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] Backend used <gl>
[INFO   ] [GL          ] OpenGL version <3.3 (Compatibility Profile) Mesa 19.0.8>
[INFO   ] [GL          ] OpenGL vendor <nouveau>
[INFO   ] [GL          ] OpenGL renderer <NVA8>
[INFO   ] [GL          ] OpenGL parsed version: 3, 3
[INFO   ] [GL          ] Shading version <3.30>
[INFO   ] [GL          ] Texture max size <8192>
[INFO   ] [GL          ] Texture max units <16>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
[INFO   ] [Base        ] Start application main loop
[INFO   ] [GL          ] NPOT texture support is available
********** def enter(self): popup = MessageBox20(self) *************************
do something
exiting...
[INFO   ] [Base        ] Leaving application in progress...
 (15549) accepted ('192.168.3.88', 53418)
**********  class Load(Resource): *****************
{u'position': u'b1', u'tasks': [{u'code': u'24258146312', u'actionDescription': u'Preleva 20ml di glucosio dalla sacca', u'barcode': u'000001', u'results': [u'returnCode'], u'actions': [{u'returnCode': u'200', u'type': u'CONFIRM', u'label': u'Conferma'}, {u'returnCode': u'400', u'type': u'ABORT', u'label': u'Annulla'}], u'itemName': u'Glucosio 100ml'}, {u'code': u'24258146313', u'actionDescription': u'Leggi il barcode e il codice RFID', u'itemName': u'Glucosio 100ml', u'barcode': u'000001', u'results': [u'returnCode', u'barcode', u'rfid'], u'actions': [{u'returnCode': u'200', u'type': u'CONFIRM', u'label': u'Conferma'}, {u'returnCode': u'400', u'type': u'ABORT', u'label': u'Annulla'}], u'type': u'BARCODE_CHECK'}, {u'code': u'24258146314', u'itemName': u'Glucosio 100ml', u'actionDescription': u'Posiziona la sacca nello slot 1', u'actions': [{u'returnCode': u'200', u'type': u'CONFIRM', u'label': u'Conferma'}, {u'returnCode': u'400', u'type': u'ABORT', u'label': u'Annulla'}], u'results': [u'returnCode']}], u'weight': u'133.15'}
********** def Parse_Load_Request(self, r): *****************************
********** def Parse_Load_Request(self, r): print data without uni-code chars *****************************
********** def Parse_Load_Request(self, r): print keys for data *****************************
['position', 'tasks', 'weight']
 192.168.3.88 - - [23/Dec/2019 11:08:44] "POST /load HTTP/1.1" 201 247 0.004312

The problem

Flask does not fetch the request when I send it. When I press the button of the first popup, Flask still does not fetch the request. When I press the button on the second popup, Flask still does not fetch the request. Kivy blocks Flask no matter the order how I start the threads. I checked this thread but I does not help. What am I doing wrong?

Can you create a minimal example that someone could copy/paste to see the same issue? — Erik, Dec 22 '19 at 23:16
@Erik, I followed your suggestion and shrink it as possible. The issue is that between the lines `Starting Flask... (15549) wsgi starting up on http://0.0.0.0:5000` and `[INFO ] [Base ] Leaving application in progress...` there are no `******** def Parse_Load_Request ******` lines — parovelb, Dec 23 '19 at 10:45

score 0 · Answer 1 · answered Dec 31 '19 at 12:20

It is no exactly the answer to the question but it is an alternative solution to the Flask blocking Kivy and vice versa. I gave it a try with multiprocessing and it seems it does the work. Hope it helps somebody and if some expert coder decide to upgrade the answer is welcome to do so.

CODE

#!/usr/bin/python2.7 python2.7
# -*- coding: utf-8 -*-

# kivy modules first, if not Kivy may cause problems
import kivy
from kivy.app import App
from kivy.lang import Builder
from kivy.uix.label import Label
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.screenmanager import ScreenManager, Screen
kivy.require('1.10.0')


# common modules
import sys
import signal
from multiprocessing import Process


# Flask & similar modules
from flask import Flask
from flask_restful import reqparse, abort, Api, Resource
import eventlet
from eventlet import wsgi


# async server setup
eventlet.monkey_patch()
app = Flask(__name__)
api = Api(app)


def start_Flask():
    print("Starting Flask...")
    # deploy as an eventlet WSGI server on port 5000
    wsgi.server(eventlet.listen(('', 5000)), app)     


def signal_handler(signal, frame):
    # for fetching CTRL+C and relatives
    print " CTRL + C detected, exiting ... "
    exit(1)


# Kivy screen class
class MainScreen(Screen):
    def __init__(self, **kwargs):
        self.name="MAIN SCREEN"
        super(Screen, self).__init__(**kwargs)


# Kivy app class
class Kivy(App):
    w_MessageBox10_1 = "MAIN SCREEN"
    w_MessageBox10_2 = "One golden glance of what should be"
    w_MessageBox30_2 = "CHORUS"
    w_MessageBox30_3 = "EXIT"


    # exit button action   
    def exit(self):
        print "exiting... one shaft of light will show the way..."
        p1.terminate()  # terminate Flask by pressing on cancel
        exit(1)


    # do magic button action
    def do_magic(self):
        # your code goes here or maybe not
        print "***** it's a kind of magic *************************"


    # Kivy UI builder file
    def build(self):
        sm = Builder.load_string("""

ScreenManager
    MainScreen:
        size_hint: 1, .7
        auto_dismiss: False
        title: app.w_MessageBox10_1       
        title_align: "center"

        BoxLayout:
            orientation: "vertical"
            Label:
                text: app.w_MessageBox10_2
            BoxLayout:
                orientation: "horizontal"
                spacing: 10
                size_hint: 1, .5
                Button:
                    text: app.w_MessageBox30_2  # DO MAGIC
                    on_press:
                        app.do_magic()
                Button:
                    text: app.w_MessageBox30_3  # EXIT
                    on_press:
                        app.exit()


        """)

        return sm


if __name__ == '__main__':    

    # #CTRL+C signal handler
    signal.signal(signal.SIGINT, signal_handler)
    signal.signal(signal.SIGTERM, signal_handler)

    global p1
    p1 = Process(target=start_Flask)    # assign Flask to a process
    p1.start()                          # run Flask as process
    Kivy().run()                        # run Kivy UI

Kivy and Flask threads blocking each other in Python2

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