<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def service_login(self):
 """ Perform login against a service :return: * The rendering of the ``settings.CAS_WARN_TEMPLATE`` if the user asked to be warned before ticket emission and has not yep been warned. * The redirection to the service URL with a ticket GET parameter * The redirection to the service URL without a ticket if ticket generation failed and the :attr:`gateway` attribute is set * The rendering of the ``settings.CAS_LOGGED_TEMPLATE`` template with some error messages if the ticket generation failed (e.g: user not allowed). :rtype: django.http.HttpResponse """

        try:
            # is the service allowed
            service_pattern = ServicePattern.validate(self.service)
            # is the current user allowed on this service
            service_pattern.check_user(self.user)
            # if the user has asked to be warned before any login to a service
            if self.request.session.get("warn", True) and not self.warned:
                messages.add_message(
                    self.request,
                    messages.WARNING,
                    _(u"Authentication has been required by service %(name)s (%(url)s)") %
                    {'name': service_pattern.name, 'url': self.service}
                )
                if self.ajax:
                    data = {"status": "error", "detail": "confirmation needed"}
                    return json_response(self.request, data)
                else:
                    warn_form = forms.WarnForm(initial={
                        'service': self.service,
                        'renew': self.renew,
                        'gateway': self.gateway,
                        'method': self.method,
                        'warned': True,
                        'lt': self.request.session['lt'][-1]
                    })
                    return render(
                        self.request,
                        settings.CAS_WARN_TEMPLATE,
                        utils.context({'form': warn_form})
                    )
            else:
                # redirect, using method ?
                list(messages.get_messages(self.request))  # clean messages before leaving django
                redirect_url = self.user.get_service_url(
                    self.service,
                    service_pattern,
                    renew=self.renewed
                )
                if not self.ajax:
                    return HttpResponseRedirect(redirect_url)
                else:
                    data = {"status": "success", "detail": "auth", "url": redirect_url}
                    return json_response(self.request, data)
        except ServicePattern.DoesNotExist:
            error = 1
            messages.add_message(
                self.request,
                messages.ERROR,
                _(u'Service %(url)s not allowed.') % {'url': self.service}
            )
        except models.BadUsername:
            error = 2
            messages.add_message(
                self.request,
                messages.ERROR,
                _(u"Username not allowed")
            )
        except models.BadFilter:
            error = 3
            messages.add_message(
                self.request,
                messages.ERROR,
                _(u"User characteristics not allowed")
            )
        except models.UserFieldNotDefined:
            error = 4
            messages.add_message(
                self.request,
                messages.ERROR,
                _(u"The attribute %(field)s is needed to use"
                  u" that service") % {'field': service_pattern.user_field}
            )

        # if gateway is set and auth failed redirect to the service without authentication
        if self.gateway and not self.ajax:
            list(messages.get_messages(self.request))  # clean messages before leaving django
            return HttpResponseRedirect(self.service)

        if not self.ajax:
            return render(
                self.request,
                settings.CAS_LOGGED_TEMPLATE,
                utils.context({'session': self.request.session})
            )
        else:
            data = {"status": "error", "detail": "auth", "code": error}
            return json_response(self.request, data)