In [1]: from matplotlib.rcsetup import _validate_linestyle

In [2]: _validate_linestyle(u'solid')
Out[2]: u'solid'

In [3]: _validate_linestyle('solid')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-9192256a512f> in <module>()
----> 1 _validate_linestyle('solid')

/home/adrien/matplotlib-git/lib/matplotlib/rcsetup.pyc in _validate_linestyle(ls)
    928 
    929     raise ValueError("linestyle must be a string or " +
--> 930                      "an even-length sequence of floats.")
    931 
    932 

ValueError: linestyle must be a string or an even-length sequence of floats.

    ...
    if isinstance(ls, six.text_type):
        return _validate_named_linestyle(ls.decode())
    ...

if isinstance(ls, six.text_type):

Python 2.7.11 |Continuum Analytics, Inc.| (default, Dec  6 2015, 18:08:32) 
[...]

In [1]: from matplotlib.rcsetup import _validate_named_linestyle, _validate_linestyle

In [2]: import six

In [3]: _validate_named_linestyle('solid')  # No problem with a valid ASCII string
Out[3]: u'solid'

In [4]: _validate_linestyle('solid')  # Error with a valid ASCII string
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-9192256a512f> in <module>()
----> 1 _validate_linestyle('solid')

/home/adrien/matplotlib-git/matplotlib/lib/matplotlib/rcsetup.pyc in _validate_linestyle(ls)
    928 
    929     raise ValueError("linestyle must be a string or " +
--> 930                      "an even-length sequence of floats.")
    931 
    932 

ValueError: linestyle must be a string or an even-length sequence of floats.

In [5]: isinstance('solid', six.text_type)  # => issue w/ the aforementioned test...
Out[5]: False

In [6]: isinstance(u'solid', six.text_type)  # => expected behavior
Out[6]: True

from __future__ import print_function

import six
from matplotlib.rcsetup import _validate_named_linestyle


def _validate_linestyle(ls):
    """
    A validator for all possible line styles, the named ones *and*
    the on-off ink sequences.
    """
    # Named line style, like u'--' or u'solid'
    if isinstance(ls, six.string_types):  # <= Here is the change!
        return _validate_named_linestyle(ls)

    # On-off ink (in points) sequence *of even length*.
    # Offset is set to None.
    try:
        if len(ls) % 2 != 0:
            # Expecting a sequence of even length
            raise ValueError
        return (None, validate_nseq_float()(ls))
    except (ValueError, TypeError):
        # TypeError can be raised by wrong types passed to float()
        # (called inside the instance of validate_nseq_float).
        pass

    raise ValueError("linestyle must be a string or " +
                     "an even-length sequence of floats.")

# Testing different types of strings for the same valid name of line style.
validated_strings = []

for ls in ['solid', u'solid', bytes('solid')]:
    validated_strings.append(_validate_linestyle(ls))

print(validated_strings)  # => [u'solid', u'solid', u'solid']

try:
    return _validate_named_linestyle(ls)
except KeyError:
    pass