diff --git a/Lib/interfaceChecker.py b/Lib/interfaceChecker.py
new file mode 100644
index 000000000..f2a447648
--- /dev/null
+++ b/Lib/interfaceChecker.py
@@ -0,0 +1,89 @@
+import sys, types
+
+try:
+	import describeFunction
+except ImportError:
+	print "(Can't check interfaces, describeFunction could not be imported)"
+	describeFunction = None
+
+
+def collectMethodNames(theClass, names=None):
+	if names is None:
+		names = {}
+	names.update(theClass.__dict__)
+	for baseClass in theClass.__bases__:
+		collectMethodNames(baseClass, names)
+	names = names.keys()
+	names.sort()
+	return names
+	
+
+def checkClass(moduleName, theClass):
+	className = theClass.__name__
+	if theClass.__module__ <> moduleName:
+		return
+	if not hasattr(theClass, "__implements__"):
+		print "Interfaceless class: %s (%s)" % (className, theClass.__module__)
+		return
+	interfaces = theClass.__implements__
+	if interfaces is None:
+		return
+	if isinstance(interfaces, types.ClassType):
+		interfaces = (interfaces,)
+	gotOne = 0
+	for interface in interfaces:
+		methodNames = collectMethodNames(interface)
+		methodNames.sort()
+		for methodName in methodNames:
+			method = getattr(interface, methodName)
+			if not isinstance(method, types.UnboundMethodType):
+				continue
+			method = method.im_func
+			if not hasattr(theClass, methodName):
+				if not gotOne:
+					print "\nWarnings for module %s\n" % moduleName
+					gotOne = 1
+				print "class %s misses required method:\n  %s" % (className, describeFunction.describe(method))
+				continue
+			method2 = getattr(theClass, methodName).im_func
+			argList = describeFunction._describe(method)
+			argList2 = describeFunction._describe(method2)
+			if len(argList2) < len(argList):
+				if not gotOne:
+					print "\nWarnings for module %s\n" % moduleName
+					gotOne = 1
+				print "Method has not enough arguments:"
+				print "  interface:      %s.%s" % (className, describeFunction.describe(method))
+				print "  implementation: %s.%s" % (className, describeFunction.describe(method2))
+				continue
+			for i in range(len(argList)):
+				if argList[i] <> argList2[i]:
+					if not gotOne:
+						print "\nWarnings for module %s\n" % moduleName
+						gotOne = 1
+					print "Method has different arguments:"
+					print "  interface:      %s.%s" % (className, describeFunction.describe(method))
+					print "  implementation: %s.%s" % (className, describeFunction.describe(method2))
+					break
+			for arg in argList2[len(argList):]:
+				if type(arg) == types.StringType:
+					if not gotOne:
+						print "\nWarnings for module %s\n" % moduleName
+						gotOne = 1
+					print "Additional argument(s) should have default value(s):"
+					print "  interface:      %s.%s" % (className, describeFunction.describe(method))
+					print "  implementation: %s.%s" % (className, describeFunction.describe(method2))
+					break
+
+
+
+def checkInterface(moduleName):
+	if describeFunction is None:
+		return
+	items = sys.modules[moduleName].__dict__.items()
+	items.sort()
+	for name, value in items:
+		if not isinstance(value, types.ClassType):
+			continue
+		checkClass(moduleName, value)
+