Fix PropertyMap ownership problem with GraphView

Now if a GraphView object is deleted, any property map created for it
reverts its ownership to the base Graph object. This eliminates the
problem of property maps created with transient GraphView objects.

src/graph_tool/__init__.py

@@ -337,22 +337,30 @@ class PropertyMap(object):
     def __init__(self, pmap, g, key_type, key_trans=None):
         self.__map = pmap
         self.__g = weakref.ref(g)
+        self.__base_g = lambda: None
+        try:
+            if isinstance(g, GraphView):
+                self.__base_g = weakref.ref(g.base)  # keep reference to the
+                                                     # base graph, in case the
+                                                     # graph view is deleted.
+        except NameError:
+            pass  # ignore if GraphView is yet undefined
         self.__key_type = key_type
         self.__key_trans = key_trans if key_trans is not None else lambda k: k
         self.__register_map()
 
     def __register_map(self):
-        if self.__g() == None:
-            return
-        self.__g()._Graph__known_properties.append((self.key_type(),
-                                                    weakref.ref(self.__map)))
+        for g in [self.__g(), self.__base_g()]:
+            if g is not None:
+                g._Graph__known_properties.append((self.key_type(),
+                                                   weakref.ref(self.__map)))
 
     def __unregister_map(self):
-        if self.__g() == None:
-            return
-        i = self.__g()._Graph__known_properties.index((self.key_type(),
-                                                       weakref.ref(self.__map)))
-        del self.__g()._Graph__known_properties[i]
+        for g in [self.__g(), self.__base_g()]:
+            if g is not None:
+                i = g._Graph__known_properties.index((self.key_type(),
+                                                      weakref.ref(self.__map)))
+                del g._Graph__known_properties[i]
 
     def __del__(self):
         self.__unregister_map()
@@ -382,7 +390,7 @@ class PropertyMap(object):
             k = "Vertex"
         else:
             k = "Graph"
-        g = self.__g()
+        g = self.get_graph()
         if g == None:
             g = "a non-existent graph"
         else:
@@ -392,7 +400,10 @@ class PropertyMap(object):
 
     def get_graph(self):
         """Get the graph class to which the map refers."""
-        return self.__g()
+        g = self.__g()
+        if g is None:
+            g = self.__base_g()
+        return g
 
     def key_type(self):
         """Return the key type of the map. Either 'g', 'v' or 'e'."""
@@ -423,16 +434,17 @@ class PropertyMap(object):
         return PropertyArray(a, prop_map=self)
 
     def _get_data(self):
-        if self.__g() is None:
+        g = self.get_graph()
+        if g is None:
             return None
-        self.__g().stash_filter(edge=True, vertex=True)
+        g.stash_filter(edge=True, vertex=True)
         if self.__key_type == 'v':
-            n = self.__g().num_vertices()
+            n = g.num_vertices()
         elif self.__key_type == 'e':
-            n = self.__g()._Graph__graph.GetMaxEdgeIndex() + 1
+            n = g._Graph__graph.GetMaxEdgeIndex() + 1
         else:
             n = 1
-        self.__g().pop_filter(edge=True, vertex=True)
+        g.pop_filter(edge=True, vertex=True)
         a = self.__map.get_array(n)
         return a
 
@@ -1469,11 +1481,11 @@ class GraphView(Graph):
 
     def __init__(self, g, vfilt=None, efilt=None, directed=None,
                  reversed=False):
+        self.__base = g if not isinstance(g, GraphView) else g.base
         Graph.__init__(self)
         self._Graph__graph = libcore.GraphInterface(g._Graph__graph, True)
         self._Graph__properties = g._Graph__properties
         self._Graph__known_properties = g._Graph__known_properties
-        self.__base = g if type(g) is not GraphView else g.base
 
         if vfilt is not None:
             if type(vfilt) is PropertyMap: