sage-shell-mode doesn't handle (unicode) strings containing non-ASCII characters

[EmmanuelCharpentier]

Noticed in this ask.sagemath question.

I'm currently working in a Python 3-based Sage, where strings are systematically Unicode. This will soon become the default.

In a terminal:

sage: S="à"
sage: show(S)
\newcommand{\Bold}[1]{\mathbf{#1}}\verb|à|

In an emacs buffet, inline-typesettig disabled:

sage: S="à"
sage: show(S)
\newcommand{\Bold}[1]{\mathbf{#1}}\verb|à|

In an emacs buffet, inline-typesettig enabled:

sage: S="à"
sage: show(S)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-30-0b3d0aeda691> in <module>()
----> 1 show(S)

/usr/local/sage-P3-2/local/lib/python3.7/site-packages/sage/repl/rich_output/pretty_print.py in show(*args, **kwds)
    256         args[0].show()
    257         return
--> 258     pretty_print(*args, **kwds)

/usr/local/sage-P3-2/local/lib/python3.7/site-packages/sage/repl/rich_output/pretty_print.py in pretty_print(*args, **kwds)
    227             pass
    228         elif len(args) == 1:
--> 229             dm.display_immediately(*args, **kwds)
    230         else:
    231             SequencePrettyPrinter(*args, **kwds).pretty_print()

/usr/local/sage-P3-2/local/lib/python3.7/site-packages/sage/repl/rich_output/display_manager.py in display_immediately(self, obj, **rich_repr_kwds)
    836         """
    837         plain_text, rich_output = self._rich_output_formatter(obj, rich_repr_kwds)
--> 838         self._backend.display_immediately(plain_text, rich_output)
    839 
    840 

/usr/local/sage-P3-2/local/lib/python3.7/site-packages/sage/repl/rich_output/backend_ipython.py in display_immediately(self, plain_text, rich_output)
    302             Example plain text output
    303         """
--> 304         formatdata, metadata = self.displayhook(plain_text, rich_output)
    305         print(formatdata[u'text/plain'])
    306 

/home/charpent/.emacs.d/elpa/sage-shell-mode-20180215.835/emacs_sage_shell_view.py in displayhook(self, plain_text, rich_output)
     55         elif isinstance(rich_output, OutputLatex):
     56             text = "" + plain_text.text.get() + ""
     58             return ({u'text/plain': text}, {})
     59         else:

TypeError: can only concatenate str (not "bytes") to str

This works without error in a Jupyter notebook.

IMHO, this is a bug specific to sage-shell-mode (the last entry in the stack is in its code: .../elpa/sage-shell-mode-20180215.835/emacs_sage_shell_view.py in displayhook(self, plain_text, rich_output))...