@user_blueprint.route('/callback/<provider>', methods=['GET','POST'])
def oauth_callback(provider):
    form = TwitterForm(request.form)
    if not current_user.is_anonymous:
        return redirect(url_for('main.home'))
    oauth = OAuthSignIn.get_provider(provider)
    social_id, username, email = oauth.callback()

    if social_id is None:
        flash('Authentication failed!')
        return redirect(url_for('main.home'))
    user = User.query.filter_by(social_id=social_id).first()
    if not user and form.validate_on_submit():
        user = User(
            email=form.email.data,
            password=form.password.data,
            confirmed=False,
            social_id=social_id,
            nickname=username)
        db.session.add(user)
        db.session.commit()
        token = generate_confirmation_token(user.email)
        confirm_url = url_for('user.confirm_email', token=token, _external=True)
        html = render_template('user/activate.html', confirm_url=confirm_url)
        subject = "Please confirm your email"
        send_email(user.email, subject, html)
        login_user(user,True)
        flash('A confirmation email has been sent via email.', 'success')
        return redirect(url_for("user.unconfirmed"))
    return render_template('user/twitterregister.html', form=form)

class TwitterSignIn(OAuthSignIn):
    def __init__(self):
        super(TwitterSignIn, self).__init__('twitter')
        self.service = OAuth1Service(
            name='twitter',
            consumer_key='pNU7LEHOLEAJH3lQRNwOjiJMH',
            consumer_secret='WFe5DVdi5JZ6KtuyRWYP9BkC8905EUp0N56junLtde1vL5w2NR',
            request_token_url='https://api.twitter.com/oauth/request_token',
            authorize_url='https://api.twitter.com/oauth/authorize',
            access_token_url='https://api.twitter.com/oauth/access_token',
            base_url='https://api.twitter.com/1.1/'
        )

    def authorize(self):
        request_token = self.service.get_request_token(
            params={'oauth_callback': self.get_callback_url()}
        )
        session['request_token'] = request_token
        return redirect(self.service.get_authorize_url(request_token[0]))

    def callback(self):
        ***request_token = session.pop('request_token')***
        if 'oauth_verifier' not in request.args:
            return None, None, None
        oauth_session = self.service.get_auth_session(
            request_token[0],
            request_token[1],
            data={'oauth_verifier': request.args['oauth_verifier']}
        )
        me = oauth_session.get('account/verify_credentials.json').json()
        social_id = 'twitter$' + str(me.get('id'))
        username = me.get('screen_name')
        return social_id, username, None   # Twitter does not provide email

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/site-packages/flask_debugtoolbar/__init__.py", line 125, in dispatch_request
    return view_func(**req.view_args)
  File "/Users/Ben/Desktop/flask_practise/project/user/views.py", line 89, in oauth_callback
    social_id, username, email = oauth.callback()
  File "/Users/Ben/Desktop/flask_practise/oauth.py", line 92, in callback
    request_token = session.pop('request_token')
  File "/usr/local/lib/python2.7/site-packages/werkzeug/datastructures.py", line 246, in pop
    rv = super(UpdateDictMixin, self).pop(key)
KeyError: 'request_token'