Topological sort of a graph, allowing better console (and other) outputs.

Uses algorithm borrowed from networkx graph library to sort a graph in
topological order. The method is only used by output plugins, as
internal plumbery does not really care about the node order.

Also includes a bonobo.util.python.require function that helps importing
thing in a package-less context, or when there are conflict with site
package names.

tests/structs/test_bags.py

@@ -5,7 +5,7 @@ from bonobo import Bag
 from bonobo.constants import INHERIT_INPUT
 from bonobo.structs import Token
 
-args = ('foo', 'bar',)
+args = ('foo', 'bar', )
 kwargs = dict(acme='corp')
 
 
@@ -34,29 +34,29 @@ def test_inherit():
     bag3 = bag.extend('c', c=3)
     bag4 = Bag('d', d=4)
 
-    assert bag.args == ('a',)
+    assert bag.args == ('a', )
     assert bag.kwargs == {'a': 1}
     assert bag.flags is ()
 
-    assert bag2.args == ('a', 'b',)
+    assert bag2.args == ('a', 'b', )
     assert bag2.kwargs == {'a': 1, 'b': 2}
     assert INHERIT_INPUT in bag2.flags
 
-    assert bag3.args == ('a', 'c',)
+    assert bag3.args == ('a', 'c', )
     assert bag3.kwargs == {'a': 1, 'c': 3}
     assert bag3.flags is ()
 
-    assert bag4.args == ('d',)
+    assert bag4.args == ('d', )
     assert bag4.kwargs == {'d': 4}
     assert bag4.flags is ()
 
     bag4.set_parent(bag)
-    assert bag4.args == ('a', 'd',)
+    assert bag4.args == ('a', 'd', )
     assert bag4.kwargs == {'a': 1, 'd': 4}
     assert bag4.flags is ()
 
     bag4.set_parent(bag3)
-    assert bag4.args == ('a', 'c', 'd',)
+    assert bag4.args == ('a', 'c', 'd', )
     assert bag4.kwargs == {'a': 1, 'c': 3, 'd': 4}
     assert bag4.flags is ()
 

tests/structs/test_graphs.py

@@ -1,5 +1,7 @@
 import pytest
 
+from unittest.mock import sentinel
+
 from bonobo.constants import BEGIN
 from bonobo.structs import Graph
 
@@ -41,3 +43,30 @@ def test_graph_add_chain():
     g.add_chain(identity, identity, identity)
     assert len(g.nodes) == 3
     assert len(g.outputs_of(BEGIN)) == 1
+
+
+def test_graph_topological_sort():
+    g = Graph()
+
+    g.add_chain(
+        sentinel.a1,
+        sentinel.a2,
+        sentinel.a3,
+        _input=None,
+        _output=None,
+    )
+
+    assert g.topologically_sorted_indexes == (0, 1, 2)
+    assert g[0] == sentinel.a1
+    assert g[1] == sentinel.a2
+    assert g[2] == sentinel.a3
+
+    g.add_chain(
+        sentinel.b1,
+        sentinel.b2,
+        _output=sentinel.a2,
+    )
+
+    assert g.topologically_sorted_indexes == (0, 3, 4, 1, 2)
+    assert g[3] == sentinel.b1
+    assert g[4] == sentinel.b2