Flask destructor

Question

I'm building a web application using Flask. I've sub-classed the Flask object so that I can execute a chunk of code before the app exits (the Flask object get's destroyed). When I run this in my terminal and hit ^C, I'm not seeing the "Can you hear me?" message, so I assume that __del__() isn't getting called.

from flask import Flask

class MyFlask (Flask):

  def __init__(self, import_name, static_path=None, static_url_path=None,
                     static_folder='static', template_folder='templates',
                     instance_path=None, instance_relative_config=False):

    Flask.__init__(self, import_name, static_path, static_url_path,
                         static_folder, template_folder,
                         instance_path, instance_relative_config)

    # Do some stuff ... 

  def __del__(self):
    # Do some stuff ... 
    print 'Can you hear me?'

app = MyFlask(__name__)

@app.route("/")
def hello():
 return "Hello World!"

if __name__ == "__main__":
  app.run()

I'd like this code to execute in the destructor so that it'll run no matter how the application is loaded. i.e, app.run() in testing, gunicorn hello.py in production. Thanks!

You should probably use [signals](http://docs.python.org/2/library/signal.html) in your script to catch the program ending. — korylprince, Jul 22 '13 at 22:48
Good idea. It's easy enough to catch SIGINT, but I need to do some research on what signals will be used when the code is deployed. This is going to be put on Heroku. — tweaksp, Jul 22 '13 at 23:10

score 6 · Answer 1 · answered Jul 23 '13 at 07:36

6

Maybe this is possible:

if __name__ == '__main__':
    init_db()  #or what you need
    try:
        app.run(host="0.0.0.0")
    finally:
        # your "destruction" code
        print 'Can you hear me?'

I have, however, no clue if you can still use app in the finally block ...

answered Jul 23 '13 at 07:36

HolgerSchurig

2,913
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1

This is the best solution, and yes, you can still fully use the app object in the finally block. – Markus Unterwaditzer Jul 24 '13 at 08:32

user2357112 · Accepted Answer · 2013-07-22T22:10:50.530

4

It is not guaranteed that __del__() methods are called for objects that still exist when the interpreter exits. Also, __del__() methods may not have access to global variables, since those may already have been deleted. Relying on destructors in Python is a bad idea.

edited Jul 22 '13 at 22:10

answered Jul 22 '13 at 22:05

user2357112

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The code would only reference the instance itself, no globals. – tweaksp Jul 22 '13 at 22:12
@Chris: It calls `Flask.__del__`. `Flask` is a global. – user2357112 Jul 22 '13 at 22:13
Aw yes, I see. But if this were omitted, how could it be that the destructor doesn't get called? Is something else referencing `app` to the very last second? – tweaksp Jul 22 '13 at 22:16
@Chris: The `app` variable still exists when your script ends. – user2357112 Jul 22 '13 at 22:43
Of course it does, but its reference count should drop to 0 after ^C is typed. My guess is that it's a WSGI thing. – tweaksp Jul 22 '13 at 23:11

Flask destructor

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