Add InheritablePropery class for mutable, inheritable descriptors.

* This allows us to define immutable x.getter and x.setter methods in a
   parent class while still inheriting the ability to alter the descriptor in
   a child class without needing to set something like:
       @property
       def x(self):
           super(self.__class__, self)._get_x()
   as a property override for every descriptor.

src/_util.py

@@ -354,3 +354,39 @@ def _write_passphrase(stream, passphrase, encoding):
     passphrase = passphrase.encode(encoding)
     stream.write(passphrase)
     log.debug("Wrote passphrase on stdin.")
+
+
+class InheritableProperty(object):
+  """Based on the emulation of PyProperty_Type() in Objects/descrobject.c"""
+
+  def __init__(self, fget=None, fset=None, fdel=None, doc=None):
+    self.fget = fget
+    self.fset = fset
+    self.fdel = fdel
+    self.__doc__ = doc
+
+  def __get__(self, obj, objtype=None):
+    if obj is None:
+      return self
+    if self.fget is None:
+      raise AttributeError, "unreadable attribute"
+    if self.fget.__name__ == '<lambda>' or not self.fget.__name__:
+      return self.fget(obj)
+    else:
+      return getattr(obj, self.fget.__name__)()
+
+  def __set__(self, obj, value):
+    if self.fset is None:
+      raise AttributeError, "can't set attribute"
+    if self.fset.__name__ == '<lambda>' or not self.fset.__name__:
+      self.fset(obj, value)
+    else:
+      getattr(obj, self.fset.__name__)(value)
+
+  def __delete__(self, obj):
+    if self.fdel is None:
+      raise AttributeError, "can't delete attribute"
+    if self.fdel.__name__ == '<lambda>' or not self.fdel.__name__:
+      self.fdel(obj)
+    else:
+      getattr(obj, self.fdel.__name__)()

src/gnupg.py

@@ -96,6 +96,7 @@ import threading
 
 from _parsers import _fix_unsafe, _sanitise, _is_allowed, _sanitise_list
 from _parsers import _check_preferences
+from _util    import InheritableProperty
 from _util    import _conf, _is_list_or_tuple, _is_stream
 from _util    import _make_binary_stream
 from _util    import log