Access settings lazily, not at module import

rest_framework/serializers.py

@@ -786,7 +786,7 @@ class ModelSerializer(Serializer):
     # you'll also need to ensure you update the `create` method on any generic
     # views, to correctly handle the 'Location' response header for
     # "HTTP 201 Created" responses.
-    url_field_name = api_settings.URL_FIELD_NAME
+    url_field_name = None
 
     # Default `create` and `update` behavior...
     def create(self, validated_data):
@@ -869,6 +869,9 @@ class ModelSerializer(Serializer):
         Return the dict of field names -> field instances that should be
         used for `self.fields` when instantiating the serializer.
         """
+        if self.url_field_name is None:
+            self.url_field_name = api_settings.URL_FIELD_NAME
+
         assert hasattr(self, 'Meta'), (
             'Class {serializer_class} missing "Meta" attribute'.format(
                 serializer_class=self.__class__.__name__

rest_framework/settings.py

@@ -26,7 +26,6 @@ from django.utils import six
 from rest_framework import ISO_8601
 from rest_framework.compat import importlib
 
-USER_SETTINGS = getattr(settings, 'REST_FRAMEWORK', None)
 
 DEFAULTS = {
     # Base API policies
@@ -188,10 +187,17 @@ class APISettings(object):
     and return the class, rather than the string literal.
     """
     def __init__(self, user_settings=None, defaults=None, import_strings=None):
-        self.user_settings = user_settings or {}
+        if user_settings:
+            self._user_settings = user_settings
         self.defaults = defaults or DEFAULTS
         self.import_strings = import_strings or IMPORT_STRINGS
 
+    @property
+    def user_settings(self):
+        if not hasattr(self, '_user_settings'):
+            self._user_settings = getattr(settings, 'REST_FRAMEWORK', {})
+        return self._user_settings
+
     def __getattr__(self, attr):
         if attr not in self.defaults.keys():
             raise AttributeError("Invalid API setting: '%s'" % attr)
@@ -212,7 +218,7 @@ class APISettings(object):
         return val
 
 
-api_settings = APISettings(USER_SETTINGS, DEFAULTS, IMPORT_STRINGS)
+api_settings = APISettings(None, DEFAULTS, IMPORT_STRINGS)
 
 
 def reload_api_settings(*args, **kwargs):