I'm writing a python debugging library which opens a flask server in a new thread and serves information about the program it's running in. This works fine when the program being debugged isn't a web server itself. However if I try to run it concurrently with another flask server that's running in debug mode, things break. When I try to access the second server, the result alternates between the two servers.
Here's an example:
from flask.app import Flask
from threading import Thread
# app1 represents my debugging library
app1 = Flask('app1')
@app1.route('/')
def foo():
return '1'
Thread(target=lambda: app1.run(port=5001)).start()
# Cannot change code after here as I'm not the one writing it
app2 = Flask('app2')
@app2.route('/')
def bar():
return '2'
app2.run(debug=True, port=5002)
Now when I visit http://localhost:5002/ in my browser, the result may either be 1 or 2 instead of consistently being 2.
Using multiprocessing.Process instead of Thread has the same result.
How does this happen, and how can I avoid it? Is it unavoidable with flask/werkzeug/WSGI? I like flask for its simplicity and ideally would like to continue using it. If that's not possible, what's the simplest library/framework that I can use that won't interfere with any other web servers running at the same time? I'd also like to use threads instead of processes if possible.