ENH: add concat

Adds a top level function `concat` and a `Cycler` method `concat` which
will concatenate two cyclers.  The method can be chained.

closes matplotlib#1

cycler.py

@@ -389,6 +389,41 @@ def simplify(self):
         trans = self._transpose()
         return reduce(add, (_cycler(k, v) for k, v in six.iteritems(trans)))
 
+    def concat(self, other):
+        return concat(self, other)
+
+
+def concat(left, right):
+    """Concatenate two cyclers.
+
+    The keys must match exactly.
+
+    This returns a single Cycler which is equivalent to
+    `itertools.chain(left, right)`
+
+    Parameters
+    ----------
+    left, right : `Cycler`
+        The two `Cycler` instances to concatenate
+
+    Returns
+    -------
+    ret : `Cycler`
+        The concatenated `Cycler`
+    """
+    if left.keys != right.keys:
+        msg = '\n\t'.join(["Keys do not match:",
+                            "Intersection: {both!r}",
+                            "Disjoint: {just_one!r}"
+                            ]).format(
+                                both=left.keys&right.keys,
+                                just_one=left.keys^right.keys)
+
+        raise ValueError(msg)
+
+    _l = left._transpose()
+    _r = right._transpose()
+    return reduce(add, (_cycler(k, _l[k] + _r[k]) for k in left.keys))
 
 def cycler(*args, **kwargs):
     """

test_cycler.py

@@ -2,10 +2,10 @@
 
 import six
 from six.moves import zip, range
-from cycler import cycler, Cycler
+from cycler import cycler, Cycler, concat
 from nose.tools import (assert_equal, assert_not_equal,
                         assert_raises, assert_true)
-from itertools import product, cycle
+from itertools import product, cycle, chain
 from operator import add, iadd, mul, imul
 
 
@@ -279,3 +279,19 @@ def test_starange_init():
     c2 = cycler('lw', range(3))
     cy = Cycler(list(c), list(c2), zip)
     assert_equal(cy, c + c2)
+
+
+def test_concat():
+    a = cycler('a', range(3))
+    for con, chn in zip(a.concat(a), chain(a, a)):
+        assert_equal(con, chn)
+
+    for con, chn in zip(concat(a, a), chain(a, a)):
+        assert_equal(con, chn)
+
+
+def test_concat_fail():
+    a = cycler('a', range(3))
+    b = cycler('b', range(3))
+    assert_raises(ValueError, concat, a, b)
+    assert_raises(ValueError, a.concat, b)