fixup! Document CycleError

Author: pablogsal

Lib/functools.py

@@ -219,6 +219,15 @@ def __init__(self, node):
 
 
 class CycleError(ValueError):
+    """Subclass of ValueError raised by TopologicalSorterif cycles exist in the graph
+
+    If multiple cycles exist, only one undefined choice among them will be reported
+    and included in the exception. The detected cycle can be accessed via the second
+    element in the *args* attribute of the exception instance and consists in a list
+    of nodes, such that each node is, in the graph, an immediate predecessor of the
+    next node in the list. In the reported list, the first and the last node will be
+    the same, to make it clear that it is cyclic.
+    """
     pass