ipython/ipython#3922 is related, but I would like to see this tackled in MPL, too.

MEP13 is related to this.

MEP13 is about using properties; I guess you meant MEP14 ("… improving the text handling …"):
https://github.com/matplotlib/matplotlib/wiki/Mep14

Nope -- by using properties as in MEP13, we will be performing error checking of values much earlier than we are now.

I considered that interpretation, too, but properties would just be a syntactic change, no? I mean - the same check could be in the setter today?!

If you want to introduce something like traits, that'd be something different, but MEP13 did not seem to discuss traits, but just the leaving-out-the-parentheses part?

That's true -- I just meant to link this issue as one of many that we have that would benefit from more consistent use of setters/properties/traits. You're right that the underlying technology barely matters -- it's a matter of checking in a single place as early as possible.

What's the exact action for MPL for this issue?

For visitor reference, traitlets discussion/work sitting in #4762.

Guess we can leave this open (...ugh) but need to remember to come back to this once traitlets work is complete

set_ylabel raises an exception now

$ python2 -c 'from pylab import *; gca().set_ylabel("Täst")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/matplotlib/axes/_axes.py", line 254, in set_ylabel
    return self.yaxis.set_label_text(ylabel, fontdict, **kwargs)
  File "/usr/lib/python2.7/site-packages/matplotlib/axis.py", line 1505, in set_label_text
    self.label.set_text(label)
  File "/usr/lib/python2.7/site-packages/matplotlib/text.py", line 1212, in set_text
    self._text = '%s' % (s,)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 1: ordinal not in range(128)

so closing (unless I'm mistaken?). The traitlets work is tracked in a number of other issues.