ENH: add persistent style

Function to generate a default dict with a cycler bound to it.  This
functionality is currently in the documentation as an example.

Closes matplotlib#27

cycler.py

@@ -47,6 +47,7 @@
 from itertools import product, cycle
 from six.moves import zip, reduce
 from operator import mul, add
+from collections import defaultdict
 import copy
 
 __version__ = '0.10.0'
@@ -558,3 +559,29 @@ def _cycler(label, itr):
         itr = (v[lab] for v in itr)
 
     return Cycler._from_iter(label, itr)
+
+
+class OutOfStyles(StopIteration):
+    pass
+
+
+def persistent_style(cyl, repeat=False):
+    '''Create a defaultdict mapping keys -> styles
+
+    Parameters
+    ----------
+    cyl : Cycler
+        The c
+    '''
+    def next_style():
+        try:
+            next(cy_iter)
+        except StopIteration:
+            raise OutOfStyles()
+
+    if repeat:
+        cy_iter = cyl()
+        return defaultdict(lambda: next(cy_iter))
+    else:
+        cy_iter = iter(cyl)
+        return defaultdict(next_style)

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, persistent_style, OutOfStyles
 import pytest
 from itertools import product, cycle, chain
 from operator import add, iadd, mul, imul
@@ -341,3 +341,24 @@ def test_contains():
 
     assert 'a' in ab
     assert 'b' in ab
+
+
+@pytest.mark.parametrize('repeat', [True, False])
+def test_persistent(repeat):
+    a = cycler('a', range(3)) + cycler('b', range(3))
+    dd = persistent_style(a, repeat=repeat)
+    one = dd['one']
+    two = dd['two']
+    three = dd['three']
+
+    assert one == dd['one']
+    assert two == dd['two']
+    assert three == dd['three']
+    if not repeat:
+        with pytest.raises(OutOfStyles):
+            dd['four']
+    else:
+        assert one == dd['four']
+        assert one == dd['four']
+        assert two == dd['five']
+        assert three == dd['six']