utils: add a lazy_label helper (#30252)

It allows giving a default value for a label with a getter function to
allow customization.

src/authentic2/utils.py

@@ -3,7 +3,6 @@ import random
 import time
 import logging
 import urllib
-import six
 import urlparse
 import uuid
 import datetime
@@ -24,7 +23,7 @@ from django.contrib.auth import (REDIRECT_FIELD_NAME, login as auth_login, SESSI
 from django import forms
 from django.forms.utils import ErrorList, to_current_timezone
 from django.utils import timezone
-from django.utils import html, http
+from django.utils import html, http, six, encoding
 from django.utils.translation import ugettext as _, ungettext
 from django.shortcuts import resolve_url
 from django.template.loader import render_to_string, TemplateDoesNotExist
@@ -33,7 +32,7 @@ from django.core import signing
 from django.core.urlresolvers import reverse, NoReverseMatch
 from django.utils.formats import localize
 from django.contrib import messages
-from django.utils.functional import empty
+from django.utils.functional import empty, allow_lazy
 from django.utils.http import urlsafe_base64_encode
 from django.utils.encoding import iri_to_uri, force_bytes, uri_to_iri
 from django.shortcuts import render
@@ -1085,3 +1084,12 @@ def user_can_change_password(user, request=None):
         if can is False:
             return can
     return True
+
+
+def lazy_label(default, func):
+    '''Allow using a getter for a label, with late binding.
+
+       ex.: lazy_label(_('Default label'), lambda: app_settings.CUSTOM_LABEL)
+    '''
+    return encoding.force_text(func() or default)
+lazy_label = allow_lazy(lazy_label, six.text_type)