Slightly modify and clarify the one-way proxy behavior of internal property dictionaries

This fixes ticket #110

doc/graph_tool.rst

@@ -104,7 +104,14 @@
        together with the graph itself. See :ref:`internal property maps <sec_internal_props>`
        for more details.
 
-       All dictionaries below are mutable.
+       .. note::
+
+           All dictionaries below are mutable. However, any dictionary
+           returned below is only an one-way proxy to the internally-kept
+           properties. If you modify this object, the change will be
+           propagated to the internal dictionary, but not
+           vice-versa. Keep this in mind if you intend to keep a copy of
+           the returned object.
 
        .. autoattribute:: properties
        .. autoattribute:: vertex_properties

src/graph_tool/__init__.py

@@ -840,7 +840,15 @@ def edge_difference(g, prop, ediff=None):
 
 class PropertyDict(dict):
     """Wrapper for the dict of vertex, graph or edge properties, which sets the
-    value on the property map when changed in the dict."""
+    value on the property map when changed in the dict.
+
+    .. note::
+
+        The class is only an one-way proxy to the internally-kept properties. If
+        you modify this object, the change will be propagated to the internal
+        dictionary, but not vice-versa. Keep this in mind if you intend to keep
+        a copy of the class instance.
+    """
     def __init__(self, g, old, get_func, set_func, del_func):
         dict.__init__(self)
         dict.update(self, old)
@@ -849,6 +857,13 @@ class PropertyDict(dict):
         self.set_func = set_func
         self.del_func = del_func
 
+    def __getitem__(self, key):
+        if self.get_func != None:
+            val = self.get_func(self.g, key)
+            dict.__setitem__(self, key, val)
+        else:
+            raise KeyError("Property dict cannot be gotten")
+
     def __setitem__(self, key, val):
         if self.set_func != None:
             self.set_func(self.g, key, val)