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diff --git a/netmodel/util/toposort.py b/netmodel/util/toposort.py
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+#######################################################################
+# Implements a topological sort algorithm.
+#
+# Copyright 2014 True Blade Systems, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# Notes:
+#  Based on http://code.activestate.com/recipes/578272-topological-sort
+#   with these major changes:
+#    Added unittests.
+#    Deleted doctests (maybe not the best idea in the world, but it cleans
+#     up the docstring).
+#    Moved functools import to the top of the file.
+#    Changed assert to a ValueError.
+#    Changed iter[items|keys] to [items|keys], for python 3
+#     compatibility. I don't think it matters for python 2 these are
+#     now lists instead of iterables.
+#    Copy the input so as to leave it unmodified.
+#    Renamed function from toposort2 to toposort.
+#    Handle empty input.
+#    Switch tests to use set literals.
+#
+########################################################################
+
+from functools import reduce as _reduce
+
+__all__ = ['toposort', 'toposort_flatten']
+
+def toposort(data):
+    """Dependencies are expressed as a dictionary whose keys are items
+and whose values are a set of dependent items. Output is a list of
+sets in topological order. The first set consists of items with no
+dependences, each subsequent set consists of items that depend upon
+items in the preceeding sets.
+"""
+
+    # Special case empty input.
+    if len(data) == 0:
+        return
+
+    # Copy the input so as to leave it unmodified.
+    data = data.copy()
+
+    # Ignore self dependencies.
+    for k, v in data.items():
+        v.discard(k)
+    # Find all items that don't depend on anything.
+    extra_items_in_deps = _reduce(set.union, data.values()) - set(data.keys())
+    # Add empty dependences where needed.
+    data.update({item:set() for item in extra_items_in_deps})
+    while True:
+        ordered = set(item for item, dep in data.items() if len(dep) == 0)
+        if not ordered:
+            break
+        yield ordered
+        data = {item: (dep - ordered)
+                for item, dep in data.items()
+                    if item not in ordered}
+    if len(data) != 0:
+        raise ValueError('Cyclic dependencies exist among these items: {}'.format(', '.join(repr(x) for x in data.items())))
+
+
+def toposort_flatten(data, sort=True):
+    """Returns a single list of dependencies. For any set returned by
+toposort(), those items are sorted and appended to the result (just to
+make the results deterministic)."""
+
+    result = []
+    for d in toposort(data):
+        result.extend((sorted if sort else list)(d))
+    return result