improve docstring and add test fot to_rgb(<float>)

[Carreau]

Even if the docstring was saying it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Should close #2609

Will post the result of the failing test if one modify to_rgb/to_rgba to accept float for info.

[Carreau]

If one patch like that for example :

diff --git a/lib/matplotlib/colors.py b/lib/matplotlib/colors.py
index a49bc92..d429e62 100644
--- a/lib/matplotlib/colors.py
+++ b/lib/matplotlib/colors.py
@@ -301,7 +301,9 @@ class ColorConverter(object):
                     % (str(arg),))

         try:
-            if cbook.is_string_like(arg):
+            if type(arg) in (int,float):
+                return (arg,)*3
+            elif cbook.is_string_like(arg):
                 argl = arg.lower()
                 color = self.colors.get(argl, None)
                 if color is None:

Test should fail like that :

$ python tests.py matplotlib.tests.test_colors
........F
======================================================================
FAIL: `to_rgb`/`to_rgba` should not accept float; only string representation of float.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.5/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/nose-1.3.0-py2.7.egg/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/Users/matthiasbussonnier/matplotlib/lib/matplotlib/tests/test_colors.py", line 212, in test_colors_no_float
    assert_raises(ValueError, gray_from_float_rgb)
AssertionError: ValueError not raised

----------------------------------------------------------------------
Ran 9 tests in 0.371s

FAILED (failures=1)

[mdboom]

Looks like it just needs some PEP8 fixes.

[Carreau]

Travis should be happy now.

$ python tests.py matplotlib.tests.test_coding_standards
.
----------------------------------------------------------------------
Ran 1 test in 14.457s

OK

[efiring]

to_rgba

[Carreau]

I guess you mean rgb -> rgba :-)

[Carreau]

Fixed, squashed and rebased on master.

[Carreau]

Thanks !