Flask error handling

Question

I'm trying to catch an error generated by flask-sqlalchemy. My code unfortunately can't handle the error because the debugger catches it first. It happens when I try to insert a record into a database with a field that's supposed to be unique but is duplicated.Is there any way around this? This is the traceback.

sqlalchemy.exc.IntegrityError

IntegrityError: (IntegrityError) column username is not unique u'INSERT INTO user (username, password, karma) VALUES (?, ?, ?)' (u'Test', u'test', 0)
Traceback (most recent call last)

    *
      File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 889, in __call__

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return self.wsgi_app(environ, start_response)

    *
      File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 879, in wsgi_app

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] response = self.make_response(self.handle_exception(e))

    *
      File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 876, in wsgi_app

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] rv = self.dispatch_request()

    *
      File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 695, in dispatch_request

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return self.view_functions[rule.endpoint](**req.view_args)

    *
      File "/home/josh/Projects/masterpieces_of_code/app.py", line 70, in create_user

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] db.session.commit()

    *
      File "/usr/share/pyshared/sqlalchemy/orm/scoping.py", line 129, in do

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return getattr(self.registry(), name)(*args, **kwargs)

    *
      File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 653, in commit

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] self.transaction.commit()

    *
      File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 364, in commit

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] self._prepare_impl()

    *
      File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 348, in _prepare_impl

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] self.session.flush()

    *
      File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 1346, in flush

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] self._flush(objects)

    *
      File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 1427, in _flush

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] flush_context.execute()

    *
      File "/usr/share/pyshared/sqlalchemy/orm/unitofwork.py", line 299, in execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] rec.execute(self)

    *
      File "/usr/share/pyshared/sqlalchemy/orm/unitofwork.py", line 443, in execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] uow

    *
      File "/usr/share/pyshared/sqlalchemy/orm/mapper.py", line 1833, in _save_obj

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] execute(statement, params)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1794, in execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] object, *multiparams, **params)

    *
      File "/usr/share/pyshared/sqlalchemy/interfaces.py", line 149, in execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return execute(clauseelement, *multiparams, **params)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1157, in execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] params)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1237, in _execute_clauseelement

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return self.__execute_context(context)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1268, in __execute_context

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] context.parameters[0], context=context)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1806, in _cursor_execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] cursor, statement, parameters, context, False)

    *
      File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flaskext/sqlalchemy.py", line 115, in cursor_execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] return execute(cursor, statement, parameters, context)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1367, in _cursor_execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] context)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1360, in _cursor_execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] context)

    *
      File "/usr/share/pyshared/sqlalchemy/engine/default.py", line 288, in do_execute

       [Display the sourcecode for this frame]  [Open an interactive python shell in this frame] cursor.execute(statement, parameters)

    IntegrityError: (IntegrityError) column username is not unique u'INSERT INTO user (username, password, karma) VALUES (?, ?, ?)' (u'Test', u'test', 0)

This is the Copy/Paste friendly version of the traceback. You can also paste this traceback into the public lodgeit pastebin:

Traceback (most recent call last):
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 889, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 879, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 876, in wsgi_app
    rv = self.dispatch_request()
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 695, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/josh/Projects/masterpieces_of_code/app.py", line 70, in create_user
    db.session.commit()
  File "/usr/share/pyshared/sqlalchemy/orm/scoping.py", line 129, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 653, in commit
    self.transaction.commit()
  File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 364, in commit
    self._prepare_impl()
  File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 348, in _prepare_impl
    self.session.flush()
  File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 1346, in flush
    self._flush(objects)
  File "/usr/share/pyshared/sqlalchemy/orm/session.py", line 1427, in _flush
    flush_context.execute()
  File "/usr/share/pyshared/sqlalchemy/orm/unitofwork.py", line 299, in execute
    rec.execute(self)
  File "/usr/share/pyshared/sqlalchemy/orm/unitofwork.py", line 443, in execute
    uow
  File "/usr/share/pyshared/sqlalchemy/orm/mapper.py", line 1833, in _save_obj
    execute(statement, params)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1794, in execute
    object, *multiparams, **params)
  File "/usr/share/pyshared/sqlalchemy/interfaces.py", line 149, in execute
    return execute(clauseelement, *multiparams, **params)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1157, in execute
    params)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1237, in _execute_clauseelement
    return self.__execute_context(context)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1268, in __execute_context
    context.parameters[0], context=context)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1806, in _cursor_execute
    cursor, statement, parameters, context, False)
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flaskext/sqlalchemy.py", line 115, in cursor_execute
    return execute(cursor, statement, parameters, context)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1367, in _cursor_execute
    context)
  File "/usr/share/pyshared/sqlalchemy/engine/base.py", line 1360, in _cursor_execute
    context)
  File "/usr/share/pyshared/sqlalchemy/engine/default.py", line 288, in do_execute
    cursor.execute(statement, parameters)
IntegrityError: (IntegrityError) column username is not unique u'INSERT INTO user (username, password, karma) VALUES (?, ?, ?)' (u'Test', u'test', 0)

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I forgot to mention how flask reacts when I try to handle the error. I get something related to an undefined global. Here's the traceback for that.

Traceback (most recent call last):
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 889, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 879, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 876, in wsgi_app
    rv = self.dispatch_request()
  File "/home/josh/Projects/masterpieces_of_code/env/lib/python2.6/site-packages/flask/app.py", line 695, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/josh/Projects/masterpieces_of_code/app.py", line 72, in create_user
    except(IntegrityError):
NameError: global name 'IntegrityError' is not defined

Answer 1

You need to put a try/catch in your create_user function (masterpieces_of_code/app.py) in the block of code which does the db.session.commit(), and handle the exception there.

Update: If you are getting a NameError for IntegrityError then it means you are likely to be missing a

from sqlalchemy.exc import IntegrityError

in your app.py. That's not a message from Flask, that's Python telling you about an unknown name (because of the missing import) as it interprets the code in app.py.