Simplify the Graphview and SubGraphView system (#3073)

* Simplify the Graphview and SubGraphView system

- add tests to show that extensions of base graph classes (only add new functions)
  should be able to use the Graph.subgraph and Graph.copy methods easily
- Remove ReadOnlyGraph class in favor of existing freeze() function
- Switch all GraphView classes to generic_graph_view function
- Switch all SubGraph classes to subgraph_view function
- Introduce deprecated functions that act like the deprecated classes.

Still need to:
- add docs
- add tests
- make sure backward compatible and marked as deprecated
- remove GraphView and SubGraph construct from rest of codebase
- update release docs

Fixes #2889
Fixes #2793
Fixes #2796
Fixes #2741

* Ease subclassing for to_(un)directed

- add to_directed_class indicator functions to base classes
- start deprecating G.fresh_copy
- update function.to(un)directed

* Remove G.fresh_copy from code replace with __class__

Add deprecation warnings for GraphView classes, ReverseView and
SubGraph. Also for fresh_copy function.

1 files changed, 23 insertions, 36 deletions

diff --git a/networkx/classes/function.py b/networkx/classes/function.py
index 5997642c..06c0f278 100644
--- a/networkx/classes/function.py
+++ b/networkx/classes/function.py
@@ -183,6 +183,7 @@ def freeze(G):
     G.remove_nodes_from = frozen
     G.add_edge = frozen
     G.add_edges_from = frozen
+    G.add_weighted_edges_from = frozen
     G.remove_edge = frozen
     G.remove_edges_from = frozen
     G.clear = frozen
@@ -362,13 +363,7 @@ def induced_subgraph(G, nbunch):
     [(0, 1), (1, 2)]
     """
     induced_nodes = nx.filters.show_nodes(G.nbunch_iter(nbunch))
-    if G.is_multigraph():
-        if G.is_directed():
-            return nx.graphviews.SubMultiDiGraph(G, induced_nodes)
-        return nx.graphviews.SubMultiGraph(G, induced_nodes)
-    if G.is_directed():
-        return nx.graphviews.SubDiGraph(G, induced_nodes)
-    return nx.graphviews.SubGraph(G, induced_nodes)
+    return nx.graphviews.subgraph_view(G, induced_nodes)
 
 
 def edge_subgraph(G, edges):
@@ -413,7 +408,6 @@ def edge_subgraph(G, edges):
     [(0, 1), (3, 4)]
     """
     nxf = nx.filters
-    nxg = nx.graphviews
     edges = set(edges)
     nodes = set()
     for e in edges:
@@ -422,14 +416,14 @@ def edge_subgraph(G, edges):
     if G.is_multigraph():
         if G.is_directed():
             induced_edges = nxf.show_multidiedges(edges)
-            return nxg.SubMultiDiGraph(G, induced_nodes, induced_edges)
-        induced_edges = nxf.show_multiedges(edges)
-        return nxg.SubMultiGraph(G, induced_nodes, induced_edges)
-    if G.is_directed():
-        induced_edges = nxf.show_diedges(edges)
-        return nxg.SubDiGraph(G, induced_nodes, induced_edges)
-    induced_edges = nxf.show_edges(edges)
-    return nxg.SubGraph(G, induced_nodes, induced_edges)
+        else:
+            induced_edges = nxf.show_multiedges(edges)
+    else:
+        if G.is_directed():
+            induced_edges = nxf.show_diedges(edges)
+        else:
+            induced_edges = nxf.show_edges(edges)
+    return nx.graphviews.subgraph_view(G, induced_nodes, induced_edges)
 
 
 def restricted_view(G, nodes, edges):
@@ -475,19 +469,18 @@ def restricted_view(G, nodes, edges):
     [(2, 3)]
     """
     nxf = nx.filters
-    nxg = nx.graphviews
-    h_nodes = nxf.hide_nodes(nodes)
+    hide_nodes = nxf.hide_nodes(nodes)
     if G.is_multigraph():
         if G.is_directed():
-            h_edges = nxf.hide_multidiedges(edges)
-            return nxg.SubMultiDiGraph(G, h_nodes, h_edges)
-        h_edges = nxf.hide_multiedges(edges)
-        return nxg.SubMultiGraph(G, h_nodes, h_edges)
-    if G.is_directed():
-        h_edges = nxf.hide_diedges(edges)
-        return nxg.SubDiGraph(G, h_nodes, h_edges)
-    h_edges = nxf.hide_edges(edges)
-    return nxg.SubGraph(G, h_nodes, h_edges)
+            hide_edges = nxf.hide_multidiedges(edges)
+        else:
+            hide_edges = nxf.hide_multiedges(edges)
+    else:
+        if G.is_directed():
+            hide_edges = nxf.hide_diedges(edges)
+        else:
+            hide_edges = nxf.hide_edges(edges)
+    return nx.graphviews.subgraph_view(G, hide_nodes, hide_edges)
 
 
 @not_implemented_for('undirected')
@@ -496,9 +489,7 @@ def reverse_view(digraph):
 
     Identical to digraph.reverse(copy=False)
     """
-    if digraph.is_multigraph():
-        return nx.graphviews.MultiReverseView(digraph)
-    return nx.graphviews.ReverseView(digraph)
+    return nx.graphviews.reverse_view(digraph)
 
 
 def to_directed(graph):
@@ -508,9 +499,7 @@ def to_directed(graph):
     Note that graph.to_directed defaults to `as_view=False`
     while this function always provides a view.
     """
-    if graph.is_multigraph():
-        return nx.graphviews.MultiDiGraphView(graph)
-    return nx.graphviews.DiGraphView(graph)
+    return graph.to_directed()
 
 
 def to_undirected(graph):
@@ -520,9 +509,7 @@ def to_undirected(graph):
     Note that graph.to_undirected defaults to `as_view=False`
     while this function always provides a view.
     """
-    if graph.is_multigraph():
-        return nx.graphviews.MultiGraphView(graph)
-    return nx.graphviews.GraphView(graph)
+    return graph.to_undirected(as_view=True)
 
 
 def create_empty_copy(G, with_data=True):