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diff --git a/Lib/abc.py b/Lib/abc.py
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+# Copyright 2007 Google, Inc. All Rights Reserved.
+# Licensed to PSF under a Contributor Agreement.
+
+"""Abstract Base Classes (ABCs) according to PEP 3119."""
+
+
+def abstractmethod(funcobj):
+    """A decorator indicating abstract methods.
+
+    Requires that the metaclass is ABCMeta or derived from it.  A
+    class that has a metaclass derived from ABCMeta cannot be
+    instantiated unless all of its abstract methods are overridden.
+    The abstract methods can be called using any of the the normal
+    'super' call mechanisms.
+
+    Usage:
+
+        class C(metaclass=ABCMeta):
+            @abstractmethod
+            def my_abstract_method(self, ...):
+                ...
+    """
+    funcobj.__isabstractmethod__ = True
+    return funcobj
+
+
+class _Abstract(object):
+
+    """Helper class inserted into the bases by ABCMeta (using _fix_bases()).
+
+    You should never need to explicitly subclass this class.
+
+    There should never be a base class between _Abstract and object.
+    """
+
+    def __new__(cls, *args, **kwds):
+        am = cls.__dict__.get("__abstractmethods__")
+        if am:
+            raise TypeError("Can't instantiate abstract class %s "
+                            "with abstract methods %s" %
+                            (cls.__name__, ", ".join(sorted(am))))
+        if (args or kwds) and cls.__init__ is object.__init__:
+            raise TypeError("Can't pass arguments to __new__ "
+                            "without overriding __init__")
+        return object.__new__(cls)
+
+    @classmethod
+    def __subclasshook__(cls, subclass):
+        """Abstract classes can override this to customize issubclass().
+
+        This is invoked early on by __subclasscheck__() below.  It
+        should return True, False or NotImplemented.  If it returns
+        NotImplemented, the normal algorithm is used.  Otherwise, it
+        overrides the normal algorithm (and the outcome is cached).
+        """
+        return NotImplemented
+
+
+def _fix_bases(bases):
+    """Helper method that inserts _Abstract in the bases if needed."""
+    for base in bases:
+        if issubclass(base, _Abstract):
+            # _Abstract is already a base (maybe indirectly)
+            return bases
+    if object in bases:
+        # Replace object with _Abstract
+        return tuple([_Abstract if base is object else base
+                      for base in bases])
+    # Append _Abstract to the end
+    return bases + (_Abstract,)
+
+
+class ABCMeta(type):
+
+    """Metaclass for defining Abstract Base Classes (ABCs).
+
+    Use this metaclass to create an ABC.  An ABC can be subclassed
+    directly, and then acts as a mix-in class.  You can also register
+    unrelated concrete classes (even built-in classes) and unrelated
+    ABCs as 'virtual subclasses' -- these and their descendants will
+    be considered subclasses of the registering ABC by the built-in
+    issubclass() function, but the registering ABC won't show up in
+    their MRO (Method Resolution Order) nor will method
+    implementations defined by the registering ABC be callable (not
+    even via super()).
+
+    """
+
+    # A global counter that is incremented each time a class is
+    # registered as a virtual subclass of anything.  It forces the
+    # negative cache to be cleared before its next use.
+    __invalidation_counter = 0
+
+    def __new__(mcls, name, bases, namespace):
+        bases = _fix_bases(bases)
+        cls = super(ABCMeta, mcls).__new__(mcls, name, bases, namespace)
+        # Compute set of abstract method names
+        abstracts = {name
+                     for name, value in namespace.items()
+                     if getattr(value, "__isabstractmethod__", False)}
+        for base in bases:
+            for name in getattr(base, "__abstractmethods__", set()):
+                value = getattr(cls, name, None)
+                if getattr(value, "__isabstractmethod__", False):
+                    abstracts.add(name)
+        cls.__abstractmethods__ = abstracts
+        # Set up inheritance registry
+        cls.__registry = set()
+        cls.__cache = set()
+        cls.__negative_cache = set()
+        cls.__negative_cache_version = ABCMeta.__invalidation_counter
+        return cls
+
+    def register(cls, subclass):
+        """Register a virtual subclass of an ABC."""
+        if not isinstance(cls, type):
+            raise TypeError("Can only register classes")
+        if issubclass(subclass, cls):
+            return  # Already a subclass
+        # Subtle: test for cycles *after* testing for "already a subclass";
+        # this means we allow X.register(X) and interpret it as a no-op.
+        if issubclass(cls, subclass):
+            # This would create a cycle, which is bad for the algorithm below
+            raise RuntimeError("Refusing to create an inheritance cycle")
+        cls.__registry.add(subclass)
+        ABCMeta.__invalidation_counter += 1  # Invalidate negative cache
+
+    def _dump_registry(cls, file=None):
+        """Debug helper to print the ABC registry."""
+        print("Class: %s.%s" % (cls.__module__, cls.__name__), file=file)
+        print("Inv.counter: %s" % ABCMeta.__invalidation_counter, file=file)
+        for name in sorted(cls.__dict__.keys()):
+            if name.startswith("__abc_"):
+                value = getattr(cls, name)
+                print("%s: %r" % (name, value), file=file)
+
+    def __instancecheck__(cls, instance):
+        """Override for isinstance(instance, cls)."""
+        return any(cls.__subclasscheck__(c)
+                   for c in {instance.__class__, type(instance)})
+
+    def __subclasscheck__(cls, subclass):
+        """Override for issubclass(subclass, cls)."""
+        # Check cache
+        if subclass in cls.__cache:
+            return True
+        # Check negative cache; may have to invalidate
+        if cls.__negative_cache_version < ABCMeta.__invalidation_counter:
+            # Invalidate the negative cache
+            cls.__negative_cache_version = ABCMeta.__invalidation_counter
+            cls.__negative_cache = set()
+        elif subclass in cls.__negative_cache:
+            return False
+        # Check the subclass hook
+        ok = cls.__subclasshook__(subclass)
+        if ok is not NotImplemented:
+            assert isinstance(ok, bool)
+            if ok:
+                cls.__cache.add(subclass)
+            else:
+                cls.__negative_cache.add(subclass)
+            return ok
+        # Check if it's a direct subclass
+        if cls in subclass.__mro__:
+            cls.__cache.add(subclass)
+            return True
+        # Check if it's a subclass of a registered class (recursive)
+        for rcls in cls.__registry:
+            if issubclass(subclass, rcls):
+                cls.__registry.add(subclass)
+                return True
+        # Check if it's a subclass of a subclass (recursive)
+        for scls in cls.__subclasses__():
+            if issubclass(subclass, scls):
+                cls.__registry.add(subclass)
+                return True
+        # No dice; update negative cache
+        cls.__negative_cache.add(subclass)
+        return False