add @export syntax for handling __all__

This adds a decorator that checks for __all__ and then exports decorated
functions and classes to it.

r2/r2/lib/export.py

@@ -0,0 +1,54 @@
+# The contents of this file are subject to the Common Public Attribution
+# License Version 1.0. (the "License"); you may not use this file except in
+# compliance with the License. You may obtain a copy of the License at
+# http://code.reddit.com/LICENSE. The License is based on the Mozilla Public
+# License Version 1.1, but Sections 14 and 15 have been added to cover use of
+# software over a computer network and provide for limited attribution for the
+# Original Developer. In addition, Exhibit A has been modified to be consistent
+# with Exhibit B.
+#
+# Software distributed under the License is distributed on an "AS IS" basis,
+# WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for
+# the specific language governing rights and limitations under the License.
+#
+# The Original Code is reddit.
+#
+# The Original Developer is the Initial Developer.  The Initial Developer of
+# the Original Code is reddit Inc.
+#
+# All portions of the code written by reddit are Copyright (c) 2006-2012 reddit
+# Inc. All Rights Reserved.
+###############################################################################
+
+import sys
+
+__all__ = ["export", "ExportError"]
+
+
+class ExportError(Exception):
+    def __init__(self, module):
+        msg = "Missing __all__ declaration in module %s.  " \
+              "@export cannot be used without declaring __all__ " \
+              "in that module." % (module)
+        Exception.__init__(self, msg)
+
+
+def export(exported_entity):
+    """Use a decorator to avoid retyping function/class names.
+  
+    * Based on an idea by Duncan Booth:
+    http://groups.google.com/group/comp.lang.python/msg/11cbb03e09611b8a
+    * Improved via a suggestion by Dave Angel:
+    http://groups.google.com/group/comp.lang.python/msg/3d400fb22d8a42e1
+    * Copied from Stack Overflow
+    http://stackoverflow.com/questions/6206089/is-it-a-good-practice-to-add-names-to-all-using-a-decorator
+    """
+    all_var = sys.modules[exported_entity.__module__].__dict__.get('__all__')
+    if all_var is None:
+        raise ExportError(exported_entity.__module__)
+    if exported_entity.__name__ not in all_var:  # Prevent duplicates if run from an IDE.
+        all_var.append(exported_entity.__name__)
+    return exported_entity
+
+export(export)  # Emulate decorating ourself
+