issue9859: Adds a test.support.detect_api_mismatch function useful to

compare the public APIs of two modules or classes.

Lib/test/support/__init__.py

@@ -88,7 +88,7 @@ __all__ = [
     "skip_unless_symlink", "requires_gzip", "requires_bz2", "requires_lzma",
     "bigmemtest", "bigaddrspacetest", "cpython_only", "get_attribute",
     "requires_IEEE_754", "skip_unless_xattr", "requires_zlib",
-    "anticipate_failure", "load_package_tests",
+    "anticipate_failure", "load_package_tests", "detect_api_mismatch",
     # sys
     "is_jython", "check_impl_detail",
     # network
@@ -2184,6 +2184,21 @@ def fs_is_case_insensitive(directory):
             return False
 
 
+def detect_api_mismatch(ref_api, other_api, *, ignore=None):
+    """Returns the set of items in ref_api not in other_api, except for a
+    defined list of items to be ignored in this check.
+
+    By default this skips private attributes beginning with '_' but
+    includes all magic methods, i.e. those starting and ending in '__'.
+    """
+    missing_items = set(dir(ref_api)) - set(dir(other_api))
+    if ignore:
+        missing_items -= set(ignore)
+    missing_items = set(m for m in missing_items
+                        if not m.startswith('_') or m.endswith('__'))
+    return missing_items
+
+
 class SuppressCrashReport:
     """Try to prevent a crash report from popping up.
 

Lib/test/test_support.py

@@ -280,6 +280,48 @@ class TestSupport(unittest.TestCase):
             self.assertEqual(D["item"], 5)
         self.assertEqual(D["item"], 1)
 
+    def test_detect_api_mismatch(self):
+        class RefClass:
+            attribute1 = None
+            attribute2 = None
+            _hidden_attribute1 = None
+            __magic_1__ = None
+
+        class OtherClass:
+            attribute2 = None
+            attribute3 = None
+            __magic_1__ = None
+            __magic_2__ = None
+
+        missing_items = support.detect_api_mismatch(RefClass, OtherClass)
+        self.assertEqual({'attribute1'}, missing_items)
+
+        missing_items = support.detect_api_mismatch(OtherClass, RefClass)
+        self.assertEqual({'attribute3', '__magic_2__'}, missing_items)
+
+    def test_detect_api_mismatch__ignore(self):
+        class RefClass:
+            attribute1 = None
+            attribute2 = None
+            _hidden_attribute1 = None
+            __magic_1__ = None
+
+        class OtherClass:
+            attribute2 = None
+            attribute3 = None
+            __magic_1__ = None
+            __magic_2__ = None
+
+        ignore = ['attribute1', 'attribute3', '__magic_2__', 'not_in_either']
+
+        missing_items = support.detect_api_mismatch(RefClass, OtherClass,
+                                                    ignore=ignore)
+        self.assertEqual(set(), missing_items)
+
+        missing_items = support.detect_api_mismatch(OtherClass, RefClass,
+                                                    ignore=ignore)
+        self.assertEqual(set(), missing_items)
+
     # XXX -follows a list of untested API
     # make_legacy_pyc
     # is_resource_enabled