diff -r 1f57839b7b4b Lib/test/support/__init__.py
--- a/Lib/test/support/__init__.py	Wed Jun 17 10:08:44 2015 -0500
+++ b/Lib/test/support/__init__.py	Fri Jun 19 02:59:13 2015 +0200
@@ -26,6 +26,7 @@
 import sysconfig
 import tempfile
 import time
+import types
 import unittest
 import urllib.error
 import warnings
@@ -2341,3 +2342,49 @@
                                      "memory allocations")
     import _testcapi
     return _testcapi.run_in_subinterp(code)
+
+
+def check__all__(test_instance, module, name_of_module, expected=None,
+                 blacklist=None):
+    """
+    Assert that `module`'s __all__ variable contains all documented names.
+
+    name_of_module must be either a string or iterable of strings. One case for
+    the latter is when module imports names from its C backend (e.g. ``csv``,
+    ``_csv``).
+
+    expected can be a initial set of module names, that wouldn't otherwise be
+    automatically detected as "documented names" (like objects without proper
+    __module__ attribute).
+
+    blacklist can be a set of names that must not be treated as part of
+    documented API even though their names indicate otherwise.
+
+    Usage:
+        import unittest
+        from test import support
+        import foo
+
+        class MiscTestCase(unittest.TestCase):
+            def test__all__(self):
+                support.check__all__(self, foo, 'foo')
+
+    """
+    if isinstance(name_of_module, str):
+        name_of_module = (name_of_module, )
+
+    if expected is None:
+        expected = set()
+
+    if blacklist is None:
+        blacklist = set()
+
+    for name in dir(module):
+        if name.startswith('_') or name in blacklist:
+            continue
+        module_object = getattr(module, name)
+        if isinstance(module_object, types.ModuleType):
+            continue
+        if getattr(module_object, '__module__', None) in name_of_module:
+            expected.add(name)
+    test_instance.assertCountEqual(module.__all__, expected)