@app.teardown_request function doesn't called in debug mode [NOT-A-BUG]

[seriyps]

UPD: not a bug, see #661 (comment)

from flask import Flask

app = Flask('teardown')

@app.teardown_request
def shutdown_session(err):
    print repr(err)

@app.route('/')
def index():
    raise ValueError('test exc')

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

So, when I open index page and see 500 error page, console output is ValueError('test exc'). But when I start app in debug mode, eg

if __name__ == '__main__':
    app.run(debug=True)

Output is None, when expected ValueError('test exc').

This is important for me, because I use teardown_request for sqlalchemy transaction management like

@app.teardown_request
def shutdown_session(exception):
    if exception:
        db.session.rollback()
    else:
        db.session.commit()

[mgax]

What Flask version are you using? I just ran your example, and with debug=True, it printed a traceback to the console, and did not enter shutdown_session at all. If you open the page in a browser expect multiple requests to the app (the favicon plus several calls by the werkzeug debugger), maybe that's where you see err==None.

[mgax]

There's a pull request for flask-sqlalchemy that implements commit on app teardown, would you consider testing and helping improve it?

[seriyps]

Thanks for review!
I use Flask 0.9
Sorry, my case code was broken, I didn't check enough before reporting. This is correct code:

from flask import Flask  # , Blueprint

app = Flask('teardown')

# user = Blueprint('user', __name__)
# @user.route('/')
@app.route('/')
def index():
    # do some DB work
    raise ValueError('test exc')

# app.register_blueprint(user, url_prefix='/user')

@app.teardown_request
def shutdown_session(err):
    from flask import request
    print 'shutdown_session:', repr(err), request.base_url, request.method

if __name__ == '__main__':
    app.run(debug=True)

(FYI: if you uncomment commented lines and comment "@app.route('/')" /How I actually do in my app/ issue reproduced too)
This will output

127.0.0.1 - - [06/Jan/2013 16:41:16] "GET / HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1689, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/seriy/workspace/wannatop/pb.py", line 11, in index
    raise ValueError('test exc')
ValueError: test exc
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /?__debugger__=yes&cmd=resource&f=style.css HTTP/1.1" 200 -
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /?__debugger__=yes&cmd=resource&f=jquery.js HTTP/1.1" 200 -
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /?__debugger__=yes&cmd=resource&f=debugger.js HTTP/1.1" 200 -
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /?__debugger__=yes&cmd=resource&f=console.png HTTP/1.1" 200 -
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /?__debugger__=yes&cmd=resource&f=source.png HTTP/1.1" 200 -
shutdown_session: None http://127.0.0.1:5000/ GET  <<--------- problem there!!!!!!!
shutdown_session: None http://127.0.0.1:5000/favicon.ico GET
127.0.0.1 - - [06/Jan/2013 16:41:16] "GET /favicon.ico HTTP/1.1" 404 -

My current workaround is

from flask import got_request_exception

@got_request_exception.connect_via(app)
def _rollback_transaction(sender, exception):
    db.session.rollback()

Your flask-sqlalchemy pull-request looks awesome! I will try it soon and comment on it.

PS: flask also doesn't trigger @app.errorhandler(500) in debug mode, but I think that fact should be documented.

[mgax]

Same as the first example, shutdown_session is not called at all when DEBUG=True. Use curl instead of a browser to make sure you only make one HTTP request. But do you need to explicitly do rollback? I think the session is automatically discarded with a rollback if you don't call commit.

[seriyps]

Just discower that this issue is reproduced if I run python pb.py in first console, then call curl http://127.0.0.1/ in other console MORE then 1 times...

$ python pb.py                                                                                                       [17:42:20]
 * Running on http://127.0.0.1:5000/
 * Restarting with reloader
127.0.0.1 - - [06/Jan/2013 18:22:38] "GET / HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1689, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/seriy/workspace/wannatop/pb.py", line 11, in index
    raise ValueError('test exc')
ValueError: test exc
('shutdown_session:', 'None', 'http://127.0.0.1:5000/', 'GET', datetime.datetime(2013, 1, 6, 18, 22, 46, 368193))
127.0.0.1 - - [06/Jan/2013 18:22:46] "GET / HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
    return self.wsgi_app(environ, start_response)
...................
  File "/home/seriy/workspace/wannatop/pb.py", line 11, in index
    raise ValueError('test exc')
ValueError: test exc
('shutdown_session:', 'None', 'http://127.0.0.1:5000/', 'GET', datetime.datetime(2013, 1, 6, 18, 22, 57, 355056))
127.0.0.1 - - [06/Jan/2013 18:22:57] "GET / HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/home/seriy/workspace/wannatop/env/lib/python2.7/site-packages/flask/app.py", line 1701, in __call__
    return self.wsgi_app(environ, start_response)
...............
  File "/home/seriy/workspace/wannatop/pb.py", line 11, in index
    raise ValueError('test exc')
ValueError: test exc

$ curl http://127.0.0.1:5000/
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
  "http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title>ValueError: test exc // Werkzeug Debugger</title>
......
$ curl http://127.0.0.1:5000/
......
$ curl http://127.0.0.1:5000/
......

I also add datetime.datetime.now() to print statement. Looks like @teardown_request called not after request finishes, but before the next one started!!!

This is very strange behaviour. Anyway, if id doesn't called at all - this is not what I expect.

---

I realised that I doesn't need to call rollback explicitly, because db.session.remove() will call it anyway http://docs.sqlalchemy.org/en/rel_0_8/orm/session.html#sqlalchemy.orm.scoping.scoped_session.remove . My full transaction control code looked like this before @got_request_exception workaround

@app.teardown_request
def _manage_transaction(exception):
    if exception:
        db.session.rollback()
    else:
        db.session.commit()
    db.session.remove()

So it can be replaced by

@app.teardown_request
def _manage_transaction(exception):
    if not exception:
        db.session.commit()
    db.session.remove()

[seriyps]

I discover, that this behaviour is little documented there http://flask.pocoo.org/docs/config/#builtin-configuration-values PRESERVE_CONTEXT_ON_EXCEPTION description.

So, the following piece of code fixes my issue

app.config['PRESERVE_CONTEXT_ON_EXCEPTION'] = False

But I think this should be documented there http://flask.pocoo.org/docs/api/#flask.Flask.teardown_request too!

[james-deee]

FYI, this also affects the teardown_appcontext. This is also the method that is mentioned in the SQLAlchemy setup doc page: http://flask.pocoo.org/docs/0.10/patterns/sqlalchemy/

So might want to add a note there too:
http://flask.pocoo.org/docs/0.10/api/#flask.Flask.teardown_appcontext