From 07447a0f562c62f020f00ba1d5248c5de4c264ca Mon Sep 17 00:00:00 2001
From: Jacob Kaplan-Moss <jacob@jacobian.org>
Date: Sat, 15 Sep 2007 19:25:20 +0000
Subject: [PATCH] Fixed #5454: settings.DATABASE_BACKEND may now refer to an
 external package (i.e. one located outside the Django source. Thanks, George
 Vilches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@6316 bcc190cf-cafb-0310-a4f2-bffc1f526a37
---
 django/db/__init__.py | 53 ++++++++++++++++++++++++++++++-------------
 docs/settings.txt     | 11 ++++++---
 2 files changed, 45 insertions(+), 19 deletions(-)

diff --git a/django/db/__init__.py b/django/db/__init__.py
index 33223d200ad..d4ea1403bda 100644
--- a/django/db/__init__.py
+++ b/django/db/__init__.py
@@ -1,6 +1,9 @@
+import os
 from django.conf import settings
 from django.core import signals
+from django.core.exceptions import ImproperlyConfigured
 from django.dispatch import dispatcher
+from django.utils.functional import curry
 
 __all__ = ('backend', 'connection', 'DatabaseError', 'IntegrityError')
 
@@ -8,25 +11,43 @@ if not settings.DATABASE_ENGINE:
     settings.DATABASE_ENGINE = 'dummy'
 
 try:
-    backend = __import__('django.db.backends.%s.base' % settings.DATABASE_ENGINE, {}, {}, [''])
+    # Most of the time, the database backend will be one of the official 
+    # backends that ships with Django, so look there first.
+    _import_path = 'django.db.backends.'
+    backend = __import__('%s%s.base' % (_import_path, settings.DATABASE_ENGINE), {}, {}, [''])
 except ImportError, e:
-    # The database backend wasn't found. Display a helpful error message
-    # listing all possible database backends.
-    from django.core.exceptions import ImproperlyConfigured
-    import os
-    backend_dir = os.path.join(__path__[0], 'backends')
-    available_backends = [f for f in os.listdir(backend_dir) if not f.startswith('_') and not f.startswith('.') and not f.endswith('.py') and not f.endswith('.pyc')]
-    available_backends.sort()
-    if settings.DATABASE_ENGINE not in available_backends:
-        raise ImproperlyConfigured, "%r isn't an available database backend. Available options are: %s" % \
-            (settings.DATABASE_ENGINE, ", ".join(map(repr, available_backends)))
-    else:
-        raise # If there's some other error, this must be an error in Django itself.
+    # If the import failed, we might be looking for a database backend 
+    # distributed external to Django. So we'll try that next.
+    try:
+        _import_path = ''
+        backend = __import__('%s.base' % settings.DATABASE_ENGINE, {}, {}, [''])
+    except ImportError, e_user:
+        # The database backend wasn't found. Display a helpful error message
+        # listing all possible (built-in) database backends.
+        backend_dir = os.path.join(__path__[0], 'backends')
+        available_backends = [f for f in os.listdir(backend_dir) if not f.startswith('_') and not f.startswith('.') and not f.endswith('.py') and not f.endswith('.pyc')]
+        available_backends.sort()
+        if settings.DATABASE_ENGINE not in available_backends:
+            raise ImproperlyConfigured, "%r isn't an available database backend. Available options are: %s" % \
+                (settings.DATABASE_ENGINE, ", ".join(map(repr, available_backends)))
+        else:
+            raise # If there's some other error, this must be an error in Django itself.
 
-get_introspection_module = lambda: __import__('django.db.backends.%s.introspection' % settings.DATABASE_ENGINE, {}, {}, [''])
-get_creation_module = lambda: __import__('django.db.backends.%s.creation' % settings.DATABASE_ENGINE, {}, {}, [''])
-runshell = lambda: __import__('django.db.backends.%s.client' % settings.DATABASE_ENGINE, {}, {}, ['']).runshell()
+def _import_database_module(import_path='', module_name=''):
+    """Lazyily import a database module when requested."""
+    return __import__('%s%s.%s' % (_import_path, settings.DATABASE_ENGINE, module_name), {}, {}, [''])
 
+# We don't want to import the introspect/creation modules unless 
+# someone asks for 'em, so lazily load them on demmand.
+get_introspection_module = curry(_import_database_module, _import_path, 'introspection')
+get_creation_module = curry(_import_database_module, _import_path, 'creation')
+
+# We want runshell() to work the same way, but we have to treat it a
+# little differently (since it just runs instead of returning a module like
+# the above) and wrap the lazily-loaded runshell() method.
+runshell = lambda: _import_database_module(_import_path, "client").runshell()
+
+# Convenient aliases for backend bits.
 connection = backend.DatabaseWrapper(**settings.DATABASE_OPTIONS)
 DatabaseError = backend.DatabaseError
 IntegrityError = backend.IntegrityError
diff --git a/docs/settings.txt b/docs/settings.txt
index 97fc985175a..2e6185f4447 100644
--- a/docs/settings.txt
+++ b/docs/settings.txt
@@ -253,9 +253,14 @@ DATABASE_ENGINE
 
 Default: ``''`` (Empty string)
 
-The database backend to use. Either ``'postgresql_psycopg2'``,
-``'postgresql'``, ``'mysql'``,  ``'mysql_old'``, ``'sqlite3'``,
-``'oracle'``, or ``'ado_mssql'``.
+The database backend to use. The build-in database backends are
+``'postgresql_psycopg2'``, ``'postgresql'``, ``'mysql'``, ``'mysql_old'``,
+``'sqlite3'``, ``'oracle'``, or ``'ado_mssql'``.
+
+You can also use a database backend that doesn't ship with Django by
+setting ``DATABASE_ENGINE`` to a fully-qualified path (i.e.
+``mypackage.backends.whatever``). Writing a whole new database backend from
+scratch is left as an exercise to the reader.
 
 DATABASE_HOST
 -------------