Fix weakly_connected_components() performance on graph view (networkx…

…#7586)

* Fix weakly_connected_components() performance

* Applied the same fix for connected.py

networkx/algorithms/components/connected.py

@@ -62,9 +62,10 @@ def connected_components(G):
 
     """
     seen = set()
+    n = len(G)
     for v in G:
         if v not in seen:
-            c = _plain_bfs(G, v)
+            c = _plain_bfs(G, n, v)
             seen.update(c)
             yield c
 
@@ -148,11 +149,12 @@ def is_connected(G):
     For undirected graphs only.
 
     """
-    if len(G) == 0:
+    n = len(G)
+    if n == 0:
         raise nx.NetworkXPointlessConcept(
             "Connectivity is undefined for the null graph."
         )
-    return sum(1 for node in _plain_bfs(G, arbitrary_element(G))) == len(G)
+    return sum(1 for node in _plain_bfs(G, n, arbitrary_element(G))) == len(G)
 
 
 @not_implemented_for("directed")
@@ -193,13 +195,12 @@ def node_connected_component(G, n):
     For undirected graphs only.
 
     """
-    return _plain_bfs(G, n)
+    return _plain_bfs(G, len(G), n)
 
 
-def _plain_bfs(G, source):
+def _plain_bfs(G, n, source):
     """A fast BFS node generator"""
     adj = G._adj
-    n = len(adj)
     seen = {source}
     nextlevel = [source]
     while nextlevel:

networkx/algorithms/components/weakly_connected.py

@@ -59,9 +59,10 @@ def weakly_connected_components(G):
 
     """
     seen = set()
+    n = len(G)  # must be outside the loop to avoid performance hit with graph views
     for v in G:
         if v not in seen:
-            c = set(_plain_bfs(G, v))
+            c = set(_plain_bfs(G, n, v))
             seen.update(c)
             yield c
 
@@ -164,15 +165,14 @@ def is_weakly_connected(G):
     return len(next(weakly_connected_components(G))) == len(G)
 
 
-def _plain_bfs(G, source):
+def _plain_bfs(G, n, source):
     """A fast BFS node generator
 
     The direction of the edge between nodes is ignored.
 
     For directed graphs only.
 
     """
-    n = len(G)
     Gsucc = G._succ
     Gpred = G._pred
     seen = {source}