django 1.6 where is the User field 'last_login' updated?

Question

The django usermodel django.contrib.auth.models.User has a field 'last_login' record the time when a user is successfully login.

But I donot see a code such as 'last_login=datetime.now()' in function from django.contrib.auth import login or from django.contrib.auth import authenticate. I also checked the django.contrib.auth.signals.user_logged_in.

where is the code for update the field 'last_login'?

Below is all the related source coede,how is the login process invoke the update_last_login? I don't see any code in login views or Authenticate function suoure code.

def update_last_login(sender, user, **kwargs):
    """
    A signal receiver which updates the last_login date for
    the user logging in.
    """
    user.last_login = timezone.now()
    user.save(update_fields=['last_login'])
user_logged_in.connect(update_last_login)

from django.dispatch import Signal
user_logged_in = Signal(providing_args=['request', 'user'])

@sensitive_post_parameters()
@csrf_protect
@never_cache
def login(request, template_name='registration/login.html',
          redirect_field_name=REDIRECT_FIELD_NAME,
          authentication_form=AuthenticationForm,
          current_app=None, extra_context=None):
    """
    Displays the login form and handles the login action.
    """
    redirect_to = request.REQUEST.get(redirect_field_name, '')

    if request.method == "POST":
        form = authentication_form(request, data=request.POST)
        if form.is_valid():

            # Ensure the user-originating redirection url is safe.
            if not is_safe_url(url=redirect_to, host=request.get_host()):
                redirect_to = resolve_url(settings.LOGIN_REDIRECT_URL)

            # Okay, security check complete. Log the user in.
            auth_login(request, form.get_user())

            return HttpResponseRedirect(redirect_to)
    else:
        form = authentication_form(request)

    current_site = get_current_site(request)

    context = {
        'form': form,
        redirect_field_name: redirect_to,
        'site': current_site,
        'site_name': current_site.name,
    }
    if extra_context is not None:
        context.update(extra_context)
    return TemplateResponse(request, template_name, context,
                            current_app=current_app)


class ModelBackend(object):
    """
    Authenticates against settings.AUTH_USER_MODEL.
    """

    def authenticate(self, username=None, password=None, **kwargs):
        UserModel = get_user_model()
        if username is None:
            username = kwargs.get(UserModel.USERNAME_FIELD)
        try:
            user = UserModel._default_manager.get_by_natural_key(username)
            if user.check_password(password):
                return user
        except UserModel.DoesNotExist:
            # Run the default password hasher once to reduce the timing
            # difference between an existing and a non-existing user (#20760).
            UserModel().set_password(password)

Answer 1

user_logged_in signal is connected to django.contrib.auth.models.update_last_login function, it makes:

user.last_login = timezone.now()
user.save(update_fields=['last_login'])