From 7e34a0f37e85a9492a8f4752b0dca190ed25ad67 Mon Sep 17 00:00:00 2001
From: JeremyIV <vonder2@pdx.edu>
Date: Wed, 9 Jun 2021 18:52:05 -0700
Subject: [PATCH 1/2] Improve error message when the graph contains cyclic
 dependencies

---
 graphkit/network.py | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/graphkit/network.py b/graphkit/network.py
index 0df3ddf8..a968bdc7 100644
--- a/graphkit/network.py
+++ b/graphkit/network.py
@@ -27,6 +27,14 @@ class DeleteInstruction(str):
     def __repr__(self):
         return 'DeleteInstruction("%s")' % self
 
+def _format_cycle(cycle):
+    operation_strings = []
+    for second, first in reversed(cycle[1:] + [cycle[0]]):
+        if isinstance(first, DataPlaceholderNode) and operation_strings:
+            operation_strings[-1] += f' needs "{first}" from {second.name}.'
+        if isinstance(first, Operation):
+            operation_strings.append(first.name)
+    return "\n".join(operation_strings)
 
 class Network(object):
     """
@@ -107,8 +115,15 @@ def compile(self):
         self.steps = []
 
         # create an execution order such that each layer's needs are provided.
-        ordered_nodes = list(nx.dag.topological_sort(self.graph))
-
+        try:
+            ordered_nodes = list(nx.dag.topological_sort(self.graph))
+        except nx.exception.NetworkXUnfeasible as ex:
+            try:
+                cycle = nx.find_cycle(self.graph)
+            except nx.exception.NetworkXNoCycle:
+                raise ex
+            
+            raise Exception(f"Cyclic dependency in graph:\n{_format_cycle(cycle)}")
         # add Operations evaluation steps, and instructions to free data.
         for i, node in enumerate(ordered_nodes):
 

From 6e0cdc27b89ee5e013abe5a505b85fd550fea088 Mon Sep 17 00:00:00 2001
From: JeremyIV <vonder2@pdx.edu>
Date: Tue, 15 Jun 2021 15:38:30 -0700
Subject: [PATCH 2/2] Change cyclic dependency error to a NetworkXUnfeasible
 error.

---
 graphkit/network.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/graphkit/network.py b/graphkit/network.py
index a968bdc7..4ba36d53 100644
--- a/graphkit/network.py
+++ b/graphkit/network.py
@@ -122,8 +122,8 @@ def compile(self):
                 cycle = nx.find_cycle(self.graph)
             except nx.exception.NetworkXNoCycle:
                 raise ex
-            
-            raise Exception(f"Cyclic dependency in graph:\n{_format_cycle(cycle)}")
+            message = f"Cyclic dependency in graph:\n{_format_cycle(cycle)}"
+            raise nx.exception.NetworkXUnfeasible(message)
         # add Operations evaluation steps, and instructions to free data.
         for i, node in enumerate(ordered_nodes):