1 files changed, 33 insertions, 12 deletions
diff --git a/django/db/utils.py b/django/db/utils.py
index e84060f9b3..bd7e10d24c 100644
--- a/django/db/utils.py
+++ b/django/db/utils.py
@@ -6,6 +6,7 @@ import warnings
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
+from django.utils.functional import cached_property
 from django.utils.importlib import import_module
 from django.utils.module_loading import import_by_path
 from django.utils._os import upath
@@ -90,8 +91,7 @@ class DatabaseErrorWrapper(object):
                 except AttributeError:
                     args = (exc_value,)
                 dj_exc_value = dj_exc_type(*args)
-                if six.PY3:
-                    dj_exc_value.__cause__ = exc_value
+                dj_exc_value.__cause__ = exc_value
                 # Only set the 'errors_occurred' flag for errors that may make
                 # the connection unusable.
                 if dj_exc_type not in (DataError, IntegrityError):
@@ -138,16 +138,27 @@ class ConnectionDoesNotExist(Exception):
 
 
 class ConnectionHandler(object):
-    def __init__(self, databases):
-        if not databases:
-            self.databases = {
+    def __init__(self, databases=None):
+        """
+        databases is an optional dictionary of database definitions (structured
+        like settings.DATABASES).
+        """
+        self._databases = databases
+        self._connections = local()
+
+    @cached_property
+    def databases(self):
+        if self._databases is None:
+            self._databases = settings.DATABASES
+        if self._databases == {}:
+            self._databases = {
                 DEFAULT_DB_ALIAS: {
                     'ENGINE': 'django.db.backends.dummy',
                 },
             }
-        else:
-            self.databases = databases
-        self._connections = local()
+        if DEFAULT_DB_ALIAS not in self._databases:
+            raise ImproperlyConfigured("You must define a '%s' database" % DEFAULT_DB_ALIAS)
+        return self._databases
 
     def ensure_defaults(self, alias):
         """
@@ -202,14 +213,24 @@ class ConnectionHandler(object):
 
 
 class ConnectionRouter(object):
-    def __init__(self, routers):
-        self.routers = []
-        for r in routers:
+    def __init__(self, routers=None):
+        """
+        If routers is not specified, will default to settings.DATABASE_ROUTERS.
+        """
+        self._routers = routers
+
+    @cached_property
+    def routers(self):
+        if self._routers is None:
+            self._routers = settings.DATABASE_ROUTERS
+        routers = []
+        for r in self._routers:
             if isinstance(r, six.string_types):
                 router = import_by_path(r)()
             else:
                 router = r
-            self.routers.append(router)
+            routers.append(router)
+        return routers
 
     def _router_func(action):
         def _route_db(self, model, **hints):