ENH: cycler with a tail

Add a generator function to add an infinite 'tail' onto a cycler.

Closes matplotlib#30

cycler.py

@@ -558,3 +558,32 @@ def _cycler(label, itr):
  itr = (v[lab] for v in itr)
 
  return Cycler._from_iter(label, itr)
+
+
+def cycler_with_tail(cyl, tail):
+ '''After the cycle is exhausted continue to yield tail
+
+
+ Parameters
+ ----------
+ cyl : Cycler
+ The cycler to iterate through
+
+ tail : dict
+ The dictionary to yield. Must have the same keys as ``cyl``.
+
+ Yields
+ ------
+ sty : dict
+ '''
+ tk = set(tail.keys())
+ if cyl.keys != tk:
+ raise RuntimeError('The cycler has keys {} and the tail {}. The '
+ 'different keys are {}'.format(cyl.keys,
+ tk, tk ^ cyl.keys))
+
+ for sty in cyl:
+ yield sty
+
+ while True:
+ yield tail

test_cycler.py

@@ -2,7 +2,7 @@
 
 import six
 from six.moves import zip, range
-from cycler import cycler, Cycler, concat
+from cycler import cycler, Cycler, concat, cycler_with_tail
 import pytest
 from itertools import product, cycle, chain
 from operator import add, iadd, mul, imul
@@ -341,3 +341,18 @@ def test_contains():
 
  assert 'a' in ab
  assert 'b' in ab
+
+
+def test_tail():
+ a = cycler('a', range(3))
+ tail = {'a': 4}
+
+ cy_with_tail = cycler_with_tail(a, tail)
+ for j in range(3):
+ next(cy_with_tail)
+
+ for j in range(5):
+ assert {'a': 4} == next(cy_with_tail)
+
+ with pytest.raises(RuntimeError):
+ next(cycler_with_tail(a, {'b': 1}))