[1.5.x] Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

Backport of b0ce6fe656 from master

1 files changed, 29 insertions, 0 deletions

diff --git a/django/utils/module_loading.py b/django/utils/module_loading.py
index f8aadb34df..ede585e562 100644
--- a/django/utils/module_loading.py
+++ b/django/utils/module_loading.py
@@ -2,6 +2,35 @@ import imp
 import os
 import sys
 
+from django.core.exceptions import ImproperlyConfigured
+from django.utils import six
+from django.utils.importlib import import_module
+
+
+def import_by_path(dotted_path, error_prefix=''):
+    """
+    Import a dotted module path and return the attribute/class designated by the
+    last name in the path. Raise ImproperlyConfigured if something goes wrong.
+    """
+    try:
+        module_path, class_name = dotted_path.rsplit('.', 1)
+    except ValueError:
+        raise ImproperlyConfigured("%s%s doesn't look like a module path" % (
+            error_prefix, dotted_path))
+    try:
+        module = import_module(module_path)
+    except ImportError as e:
+        msg = '%sError importing module %s: "%s"' % (
+            error_prefix, module_path, e)
+        six.reraise(ImproperlyConfigured, ImproperlyConfigured(msg),
+                    sys.exc_info()[2])
+    try:
+        attr = getattr(module, class_name)
+    except AttributeError:
+        raise ImproperlyConfigured('%sModule "%s" does not define a "%s" attribute/class' % (
+            error_prefix, module_path, class_name))
+    return attr
+
 
 def module_has_submodule(package, module_name):
     """See if 'module' is in 'package'."""