[Bug] fix __slots__ in view.py (#230)

python/dgl/view.py

@@ -25,7 +25,7 @@ class NodeView(object):
     --------
     TBD
     """
-    __slot__ = '_graph'
+    __slots__ = ['_graph']
 
     def __init__(self, graph):
         self._graph = graph
@@ -48,7 +48,7 @@ class NodeView(object):
         return F.arange(0, len(self))
 
 class NodeDataView(MutableMapping):
-    __slot__ = ['_graph', '_nodes']
+    __slots__ = ['_graph', '_nodes']
 
     def __init__(self, graph, nodes):
         self._graph = graph
@@ -79,7 +79,7 @@ class NodeDataView(MutableMapping):
 EdgeSpace = namedtuple('EdgeSpace', ['data'])
 
 class EdgeView(object):
-    __slot__ = ['_graph']
+    __slots__ = ['_graph']
 
     def __init__(self, graph):
         self._graph = graph
@@ -102,7 +102,7 @@ class EdgeView(object):
         return self._graph.all_edges(*args, **kwargs)
 
 class EdgeDataView(MutableMapping):
-    __slot__ = ['_graph', '_edges']
+    __slots__ = ['_graph', '_edges']
 
     def __init__(self, graph, edges):
         self._graph = graph