Cherrypy sqlalchemy exception try catch block

Question

I've searched everywhere for information about how CherryPy handles exceptions but I can't seem to figure out why my try/catch block is not being invoked properly. I think the problem is the exception is being thrown/handled on another thread, or at the very least being intercepted/handled before my catch block.

In the following code the catch block at the very bottom never gets called. Instead it returns a response to the browser with the full exception message before I've been able to handle the exception.

How can I catch this exception before it gets processed for response by cherrypy?

import cherrypy
from cherrypy.process import wspbus, plugins
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, String
import os


# BELOW CLASSES COPIED FROM 
# https://bitbucket.org/Lawouach/cherrypy-recipes/src/d140e6da973aa271e6b68a8bc187e53615674c5e/web/database/?at=default
class SATool(cherrypy.Tool):
    def __init__(self):
        """
        The SA tool is responsible for associating a SA session
        to the SA engine and attaching it to the current request.
        Since we are running in a multithreaded application,
        we use the scoped_session that will create a session
        on a per thread basis so that you don't worry about
        concurrency on the session object itself.

        This tools binds a session to the engine each time
        a requests starts and commits/rollbacks whenever
        the request terminates.
        """
        cherrypy.Tool.__init__(self, 'on_start_resource',
                               self.bind_session,
                               priority=20)

    def _setup(self):
        cherrypy.Tool._setup(self)
        cherrypy.request.hooks.attach('on_end_resource',
                                      self.commit_transaction,
                                      priority=80)

    def bind_session(self):
        """
        Attaches a session to the request's scope by requesting
        the SA plugin to bind a session to the SA engine.
        """
        session = cherrypy.engine.publish('bind-session').pop()
        cherrypy.request.db = session

    def commit_transaction(self):
        """
        Commits the current transaction or rolls back
        if an error occurs. Removes the session handle
        from the request's scope.
        """
        if not hasattr(cherrypy.request, 'db'):
            return
        cherrypy.request.db = None
        cherrypy.engine.publish('commit-session')


class SAEnginePlugin(plugins.SimplePlugin):
    def __init__(self, bus):
        """
        The plugin is registered to the CherryPy engine and therefore
        is part of the bus (the engine *is* a bus) registery.

        We use this plugin to create the SA engine. At the same time,
        when the plugin starts we create the tables into the database
        using the mapped class of the global metadata.
        """
        plugins.SimplePlugin.__init__(self, bus)
        self.sa_engine = None
        self.session = scoped_session(sessionmaker(autoflush=True,
                                                   autocommit=False))

    def start(self):
        self.bus.log('Starting up DB access')

        self.sa_engine = create_engine('oracle+cx_oracle://%s:%s@%s' % 
            (os.getenv('DB_USERNAME'), os.getenv('DB_PASSWORD'), os.getenv('DB_SERVER')), 
            echo=True)

        self.bus.subscribe("bind-session", self.bind)
        self.bus.subscribe("commit-session", self.commit)

    def stop(self):
        self.bus.log('Stopping down DB access')
        self.bus.unsubscribe("bind-session", self.bind)
        self.bus.unsubscribe("commit-session", self.commit)
        if self.sa_engine:
            self.sa_engine.dispose()
            self.sa_engine = None

    def bind(self):
        """
        Whenever this plugin receives the 'bind-session' command, it applies
        this method and to bind the current session to the engine.

        It then returns the session to the caller.
        """
        self.session.configure(bind=self.sa_engine)
        return self.session

    def commit(self):
        """
        Commits the current transaction or rollbacks if an error occurs.

        In all cases, the current session is unbound and therefore
        not usable any longer.
        """
        try:
            self.session.commit()
        except:
            self.session.rollback()
            raise
        finally:
            self.session.remove()

# SQL Alchemy model
Base = declarative_base()
class Registration(Base):
    __tablename__ = 'beta_registration'
    email = Column('email', String, primary_key=True)


class Root():
    @cherrypy.expose
    def index(self):
        registration = Registration()
        registration.email = "test@test.com"

        db = cherrypy.request.db
        try:
            db.add(registration)
        except Exception as e:
            # **** never gets here *****
            # should be IntegrityError on second call from sqlalchemy.exc
            raise cherrypy.HTTPError("409 Conflict", "The email address has already been registered")

SAEnginePlugin(cherrypy.engine).subscribe()
cherrypy.tools.db = SATool()

cherrypy.config.update({
'tools.sessions.on' : True,
'tools.sessions.storage_type' : 'File',
'tools.sessions.storage_path' : 'adf'
})


cherrypy.quickstart(Root(), '/')

Log:

[09/Apr/2014:16:43:54] ENGINE Error in 'commit-session' listener <bound method SAEnginePlugin.commit of <plugins.saplugin.SAEnginePlugin object at 0x105a0cd10>>
Traceback (most recent call last):
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/process/wspbus.py", line 197, in publish
    output.append(listener(*args, **kwargs))
  File "/Users/xatter/Sites/connectfor/plugins/saplugin.py", line 57, in commit
    self.session.commit()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 149, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 765, in commit
    self.transaction.commit()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 370, in commit
    self._prepare_impl()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 350, in _prepare_impl
    self.session.flush()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1879, in flush
    self._flush(objects)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1997, in _flush
    transaction.rollback(_capture_exception=True)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 57, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1961, in _flush
    flush_context.execute()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 370, in execute
    rec.execute(self)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/unitofwork.py", line 523, in execute
    uow
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 64, in save_obj
    mapper, table, insert)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/orm/persistence.py", line 562, in _emit_insert_statements
    execute(statement, multiparams)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 717, in execute
    return meth(self, multiparams, params)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 317, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 814, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 927, in _execute_context
    context)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1076, in _handle_dbapi_exception
    exc_info
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 185, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 920, in _execute_context
    context)
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 425, in do_execute
    cursor.execute(statement, parameters)
IntegrityError: (IntegrityError) duplicate key value violates unique constraint "beta_registration_pk"
DETAIL:  Key (email)=(test@test.com) already exists.
 'INSERT INTO beta_registration (email) VALUES (%(email)s)' {'email': u'test@test.com'}

[09/Apr/2014:16:43:54]  Traceback (most recent call last):
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/_cprequest.py", line 102, in run
    hook()
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/_cprequest.py", line 62, in __call__
    return self.callback(**self.kwargs)
  File "/Users/xatter/Sites/connectfor/plugins/satool.py", line 47, in commit_transaction
    cherrypy.engine.publish('commit-session')
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/process/wspbus.py", line 215, in publish
    raise exc
ChannelFailures: IntegrityError('(IntegrityError) duplicate key value violates unique constraint "beta_registration_pk"\nDETAIL:  Key (email)=(test@test.com) already exists.\n',)

[09/Apr/2014:16:43:54] HTTP 
Request Headers:
  Content-Length: 25
  COOKIE: remember_token=_tm4c-HNpJWTHB1PXj8Wbg; session_id=25757ddac3a84730ce3b87f32b80a4288f5421b4
  HOST: localhost:5000
  ORIGIN: http://localhost:5000
  CONNECTION: keep-alive
  Remote-Addr: 127.0.0.1
  ACCEPT: application/json, text/plain, */*
  USER-AGENT: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/7.0.3 Safari/537.75.14
  REFERER: http://localhost:5000/
  ACCEPT-LANGUAGE: en-us
  Content-Type: application/json;charset=UTF-8
  ACCEPT-ENCODING: gzip, deflate
[09/Apr/2014:16:43:54] HTTP Traceback (most recent call last):
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/_cprequest.py", line 670, in respond
    self.hooks.run('on_end_resource')
  File "/Users/xatter/Sites/connectfor/venv/lib/python2.7/site-packages/cherrypy/_cprequest.py", line 112, in run
    raise exc
ChannelFailures: IntegrityError('(IntegrityError) duplicate key value violates unique constraint "beta_registration_pk"\nDETAIL:  Key (email)=(test@test.com) already exists.\n',)

127.0.0.1 - - [09/Apr/2014:16:43:54] "POST /beta_registration HTTP/1.1" 500 1303 "http://localhost:5000/" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/7.0.3 Safari/537.75.14"

Answer 1

The problem is that SQLAlchemy does not touch the db until you try to commit the changes and the SATool that your are using commits when the request has ended. Which means it commit when your function has already return and the try/except has already been executed.

To catch those errors you will have to make the commit yourself that means removing this method of the SATool:

def _setup(self):
    cherrypy.Tool._setup(self)
    cherrypy.request.hooks.attach('on_end_resource',
                                  self.commit_transaction,
                                  priority=80)

And then publish to the engine that you want to commit with:

cherrypy.engine.publish('commit-session')

So your Root object will be something list this:

class Root(object):

    @cherrypy.expose
    def index(self):
        registration = Registration()
        registration.email = "test@test.com"

        db = cherrypy.request.db
        db.add(registration)
        try:
            cherrypy.engine.publish('commit-session')               
        except Exception as e:
            raise cherrypy.HTTPError("409 Conflict", "The email address has already been   registered")

This means that you will need to make that:

cherrypy.engine.publish('commit-session')

explicitly on all the request that modify the db.

You can also split the tool to enable/disable the autocommit or just parametrize the tool. This may require a little more detail for that. Just comment if you wanna know more about that.

Answer 2

Try this... except Exception as e

import cherrypy

cherrypy.config.update({
'tools.sessions.on' : True,
'tools.sessions.storage_type' : 'File',
'tools.sessions.storage_path' : 'adf'
})

class Root(object):
    @cherrypy.expose
    def index(self):
        try:
            asdf = cherrypy.session['asdf']
        except Exception as e:
            raise cherrypy.HTTPError("409 Conflict", "The email address has already been registered")
            #return str(e)
        return 1

    @cherrypy.expose
    @cherrypy.tools.json_out()
    def main(self):
        return 'hi'

cherrypy.quickstart(Root())

With this code I see two errors. Cherrypy's automatic error handling and my customer error. This is using python 3.3.3 and cherrypy 3.2.4

Hope this helps!