Develop Reference

Flask hangs when run as a subprocess alongside pytest

I've spent the last hour and a half trying and failing to debug this test and I am utterly stumped. To simplify the process of testing the Flask server I am building, I have made a relatively simple script which starts the server, then runs pytest, kills the server, writes the outputs to files, and exits with Pytest's exit code. This code was working perfectly until today, and I haven't modified it since (aside from debugging this issue).
Here's the problem: when it gets to a certain point in the tests, it hangs. The weird thing is that this does not happen if I run my tests in any other way.
Debugging my server in VS Code, and running tests in the terminal: works
Running my server using the same code used in the test script and running pytest manually: works
Running pytest using the test script and running the server through the start server script (which uses the same code for running the server as the test script does) in a second terminal: works
Here's the other interesting thing: the tests always hang in the same place, part way through the setup fixture. It sends the clear command, and an echo request to the server (which prints the name of the current test). The database clears successfully, and the server echoes the correct information, but the echo route never exits - my tests never get a response. This echo route behaves perfectly for the 50 or so tests that happen before this point. If I comment out the test that is causing it to fail, it fails on the next test. If I comment out the call to the echo then it hangs on a later test on a completely different request to a different route. When it hangs, the server cannot be killed using a SIGTERM, but instead requires a SIGKILL.
Here is my echo route:
#debug.get('/echo')
def echo() -> IEcho:
"""
Echo an input. This returns the given value, but also prints it to stdout
on the server. Useful for debugging tests.
## Params:
* `value` (`str`): value to echo
"""
try:
value = request.args['value']
except KeyError:
raise http_errors.BadRequest('echo route requires a `value` argument')
to_print = f'{Fore.MAGENTA}[ECHO]\t\t{value}{Fore.RESET}'
# Print it to both stdout and stderr to ensure it is seen across all logs
# Otherwise it could be more difficult to figure out what's up with server
# output
print(to_print)
print(to_print, file=sys.stderr)
return {'value': value}
And here is my code that sends the requests:
def get(token: JWT | None, url: str, params: dict) -> dict:
"""
Returns the response to a GET web request
This also parses the response to help with error checking
### Args:
* `url` (`str`): URL to request to
* `params` (`dict`): parameters to send
### Returns:
* `dict`: response data
"""
return handle_response(requests.get(
url,
params=params,
headers=encode_headers(token),
timeout=3
))
def echo(value: str) -> IEcho:
"""
Echo an input. This returns the given value, but also prints it to stdout
on the server. Useful for debugging tests.
## Params:
* `value` (`str`): value to echo
"""
return cast(IEcho, get(None, f"{URL}/echo", {"value": value}))
#pytest.fixture(autouse=True)
def before_each(request: pytest.FixtureRequest):
"""Clear the database between tests"""
clear()
echo(f"{request.module.__name__}.{request.function.__name__}")
print("After echo") # This never prints
Here is my code for running Pytest in my test script
def pytest():
pytest = subprocess.Popen(
[sys.executable, '-u', '-m', 'pytest', '-v', '-s'],
)
# Wait for tests to finish
print("🔨 Running tests...")
try:
ret = pytest.wait()
except KeyboardInterrupt:
print("❗ Testing cancelled")
pytest.terminate()
# write_outputs(pytest, None)
# write_outputs(pytest, "pytest")
raise
# write_outputs(pytest, "pytest")
if ret == 0:
print("✅ It works!")
else:
print("❌ Tests failed")
return bool(ret)
And here is my code for running my server in my test script:
def backend(debug=False, live_output=False):
env = os.environ.copy()
if debug:
env.update({"ENSEMBLE_DEBUG": "TRUE"})
debug_flag = ["--debug"]
else:
debug_flag = []
if live_output is False:
outputs = subprocess.PIPE
else:
outputs = None
flask = subprocess.Popen(
[sys.executable, '-u', '-m', 'flask'] + debug_flag + ['run'],
env=env,
stderr=outputs,
stdout=outputs,
)
if outputs is not None and (flask.stderr is None or flask.stdout is None):
print("❗ Can't read flask output", file=sys.stderr)
flask.kill()
sys.exit(1)
# Request until we get a success, but crash if we failed to start in 10
# seconds
start_time = time.time()
started = False
while time.time() - start_time < 10:
try:
requests.get(
f'http://localhost:{os.getenv("FLASK_RUN_PORT")}/debug/echo',
params={'value': 'Test script startup...'},
)
except requests.ConnectionError:
continue
started = True
break
if not started:
print("❗ Server failed to start in time")
flask.kill()
if outputs is not None:
write_outputs(flask, None)
sys.exit(1)
else:
if flask.poll() is not None:
print("❗ Server crashed during startup")
if outputs is not None:
write_outputs(flask, None)
sys.exit(1)
print("✅ Server started")
return flask
So in summary, does anyone have any idea what on earth is happening? It freezes on such a simple route that this makes me very concerned. I think I may have found some crazy bug in Flask or in the requests library or something.
Even if you don't know what's happening with this, it'd be really helpful to have any ideas as to how I can debug this further, as I have absolutely no idea what is going on.

It turns out that my server output was filling up all the buffer space in the pipe, meaning that it would wait for the buffer to empty. The issue is that my test script was waiting for the tests to exit, and the tests could not progress unless the server was active. As such, the code reached a three-way deadlock. I fixed it by redirecting my output through a file (where limited buffer size wasn't a problem).

I am trying to run an endless worker thread (daemon) from within Django

I have a worker thread which only task is to query a list of active users every 10 minutes from the database, and to send them an SMS message if a certain condition is fulfilled (which is checked every minute); also the worker thread does not hinder the main application at all.
So far I managed to get the thread up and running and sending SMS works also just fine. However, for some reasons the thread stops/gets killed after some random time (hours). I run a try: except Exception as e: within a while True, to catch occurring errors. Additionally, I print out a messages saying what error occurred.
Well, I never see any message and the thread is definitely down. Therefore, I suspect Gunicorn or Django to kill my thread sort of gracefully.
I have put log and print statements all over the code but haven't found anything indicating why my thread is getting killed.
My wsgi.py function where I call the function to start my thread
"""
WSGI config for django_web project.
It exposes the WSGI callable as a module-level variable named ``application``.
For more information on this file, see
https://docs.djangoproject.com/en/2.1/howto/deployment/wsgi/
"""
import os
from django.core.wsgi import get_wsgi_application
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'django_web.settings')
application = get_wsgi_application()
'''
Start background services
Import has to happen after "get_wsgi_application()"; otherwise docker container crashes
'''
try:
from threading import Thread
from timesheet.admin import runWorkmateServices
runWorkmateServices()
except Exception as exp:
print(exp)
The function which is called from within the wsgi.py. I double check if the thread was started to avoid having two up and running.
def runWorkmateServices(request=None):
service_name = 'TimeKeeperWorkMateReminderService'
thread_found = False
for thread in threading.enumerate():
if service_name in thread.name:
thread_found = True
break # Leave loop now
if thread_found:
print(f'Service has already been started: {service_name}')
if request:
messages.add_message(request, messages.ERROR, f'Service has already been started:: {service_name}')
else:
Thread(target=WorkMateReminders, args=(), name=service_name, daemon=True).start()
print(f'Started Service: {service_name}')
if request:
messages.add_message(request, messages.SUCCESS, f'Started Service: {service_name}')
The worker thread itself
def WorkMateReminders():
print('Thread Started: WorkMateReminders')
timer = 0
employees = User.objects.none()
while True:
try:
# Update user list every n * sleep time (10 minutes)
if timer % 10 == 0:
timer = 0
# Get active employees
employees = User.objects.filter(is_active=True, profile__workmate_sms_reminders_activated=True)
print(f'Employees updated at {datetime.now().date()} - {datetime.now().time()}: {employees}')
WorkMateCheckClockOffTimes(employees=employees)
WorkMateClockOnReminder(employees=employees)
WorkMateEndOfBreakReminder(employees=employees)
timer += 1 # increment timer
except Exception as exp:
print(f'Error: {exp}')
time.sleep(60 * 1)
My goal is to have this worker thread running for as long as Django is up.

Most WSGI servers spawn workers that are killed/recycled fairly regularly, spawning threads from these workers is not the best solution to your problem. There are several ways to go about this
Cron
Create a management command that does what you want and configure cron to run it every 10 minutes
Celery/Celerybeat
Set up a celery worker, this is a process that runs asynchronously to your Django application and using celerybeat you can have tasks run at intervals

Telnet connection to TS3 ServerQuery keeps getting slower and slower

I wrote a bot for TeamSpeak 3 that runs over ServerQuery (a telnet interface).
But the bot keeps responding later and later, in the beginning it takes like 0.1 sec, after like 1 minute the bot takes about 10 seconds to respond, and using commands makes it even faster.
Any idea why?
So basically the telnet interface sends data from the TS3 Server to my python script, the ts3 module recieves and processes the data, then the script will make a decision of what the action will be.
As modules I am using MySQLdb and ts3(https://github.com/benediktschmitt/py-ts3)
My sourcecode is here: https://pastebin.com/cJuyB9ZH
Another script, which just takes all clients and pushes them into a database every 5 min, runs multiple days without any issues.
I checked the code multiple times now and even deleted variables right after they have been used, but it still has the same issue.
My guess would be that is sortof clogges the RAM, so I looked through the code multiple times, but couldn't find out why or where.
Sidenote: I know I sometimes call a commit() when its totally not necessary, but I don't know if that might cause problems, but I dont see how.
Short(er) version of my code:
import ts3
import MySQLdb
# Some other imports like time and threading and such
## Connect to TS3
tsConn = ts3.query.TS3Connection(tsAddr, tsPort)
try:
tsConn.login(client_login_name=tsUser, client_login_password=tsPass)
tsConn.use(sid=tsSID, virtual=True)
print(" ==>> CONNECTED TO TS3 SERVER: " + tsAddr)
except ts3.query.TS3QueryError as e:
print("Login to TS Server failed! Aborting...")
exit(1)
## Connect to mySQL
try:
qConn = MySQLdb.connect(host=qHost, user=qUser, passwd=qPass, db=qDB)
qServer = qConn.cursor()
print(" ==>> CONNECTED TO mySQL SERVER: " + qHost)
except OperationalError:
print("Cannot connect to mySQL Database! Aborting...")
exit(1)
running = True
while running:
tsConn.send_keepalive()
qServer.execute("SELECT 1") # keepalive
try:
e = tsConn.wait_for_event(timeout=1)
except TS3TimeoutError:
pass
else:
try:
# <some command processing here>
except KeyError:
try:
if event[0]["reasonid"] == "0":
tsConn.sendtextmessage(targetmode=1, target=event[0]["clid"], msg=greetingmsg.format(event[0]["client_nickname"]))
except:
pass

How to capture ctrl-c for killing a Flask python script

I have the following script which just boots up a web server serving a dynamically created website. In order to get dynamic data the script opens a file to read the data.
My concern is how can I catch CTRL-C command for killing the python script so I can close the file before script thread is killed.
I tried the following couple things but neither work:
from flask import Flask, render_template
import time
# Initialize the Flask application
app = Flask(__name__)
fileNames = {}
fileDesc = {}
for idx in range(1,4):
fileNames["name{}".format(idx)] = "./name" + str(idx) + ".txt"
fileDesc["name{}".format(idx)] = open(fileNames["name{}".format(idx)],'r')
try:
#app.route('/')
def index():
# code for reading data from files
return render_template('index.html', var1 = var1)
#app.errorhandler(Exception)
def all_exception_handler(error):
print("Closing")
for key, value in fileDesc:
val.close()
print("Files closed")
if __name__ == '__main__':
app.run(
host="192.168.0.166",
port=int("8080"),
debug=True
)
except KeyboardInterrupt:
print("Closing")
for key, value in fileDesc:
val.close()
print("Files closed")
Thanks in advance.

I am struggling with the same thing in my project. Something that did work for me was using signal to capture CTRL-C.
import sys
import signal
def handler(signal, frame):
print('CTRL-C pressed!')
sys.exit(0)
signal.signal(signal.SIGINT, handler)
signal.pause()
When this piece of code is put in the script that is running the Flask app, the CTRL-C can be captured. As of now, you have to use CTRL-C twice and then the handler is executed though. I'll investigate further and edit the answer if I find something new.
Edit 1
Okay I've done some more research and came up with some other methods, as the above is quite hack 'n slash.
In production, clean-up code such as closing databases or files is done via the #app.teardown_appcontext decorator. See this part of the tutorial.
When using the simple server, you can shut it down via exposing the werkzeug shutdown function. See this post.
Edit 2
I've tested the Werkzeug shutdown function, and it also works together with the teardown_appcontext functions. So I suggest to write your teardown functions using the decorator and writing a simple function that just does the shutdown of the werkzeug server. That way production and development code are the same.

Use atexit to handle this, from: https://stackoverflow.com/a/30739397/5782985
import atexit
#defining function to run on shutdown
def close_running_threads():
for thread in the_threads:
thread.join()
print "Threads complete, ready to finish"
#Register the function to be called on exit
atexit.register(close_running_threads)
#start your process
app.run()

SMTP - Fast and reliable connection probing without auth?

Briefing
I am currently building a python SMTP Mail sender program.
I added a feature so that the user would not be able to log in if there was no active internet connection, I tried many solutions/variations to make the real time connection checking as swift as possible, there were many problems such as:
The thread where the connection handler was running suddenly lagged when I pulled out the ethernet cable ( to test how it would handle the sudden disconnect )
The whole program crashed
It took several seconds for the program to detect the change
My current solution
I set up a data handling class which would contain all the necessary info ( the modules needed to share info effectively )
import smtplib
from socket import gaierror, timeout
class DataHandler:
is_logged_in = None
is_connected = None
server_conn = None
user_address = ''
user_passwd = ''
#staticmethod
def try_connect():
try:
DataHandler.server_conn = smtplib.SMTP('smtp.gmail.com', 587, timeout=1) # The place where the connection is checked
DataHandler.is_connected = True
except (smtplib.SMTPException, gaierror, timeout):
DataHandler.is_connected = False # Connection status changed upon a connection error
I put a connection handler class on a second thread, the server connection process slowed down the gui when it was all on one thread.
from root_gui import Root
import threading
from time import sleep
from data_handler import DataHandler
def handle_conn():
DataHandler.try_connect()
smtp_client.refresh() # Refreshes the gui according to the current status
def conn_manager(): # Working pretty well
while 'smtp_client' in globals():
sleep(0.6)
try:
handle_conn() # Calls the connection
except NameError: # If the user quits the tkinter gui
break
smtp_client = Root()
handle_conn()
MyConnManager = threading.Thread(target=conn_manager)
MyConnManager.start()
smtp_client.mainloop()
del smtp_client # The connection manager will detect this and stop running
My question is:
Is this a good practice or a terrible waste of resources? Is there a better way to do this because no matter what I tried, this was the only solution that worked.
From what I know the try_connect() function creates a completely new smtp object each time it is run ( which is once in 0.6 seconds! )
Resources/observations
The project on git: https://github.com/cernyd/smtp_client
Observation: the timeout parameter when creating the smtp object improved response times drastically, why is that so?