UnicodeEncodeError when trying to save a figure

[Nodd]

Run the following code under python2, then try to save the figure but clicking on the floppy button :

import matplotlib.pyplot as plt
fig = plt.figure()
fig.canvas.set_window_title(u"unïcode")
plt.show()

The window for choosing a filename doesn't open and this traceback is printed:

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt5.py", line 707, in save_figure
    start = os.path.join(startpath, self.canvas.get_default_filename())
  File "/usr/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 2254, in get_default_filename
    default_filename = self.get_window_title() or 'image'
  File "/usr/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 2239, in get_window_title
    return self.manager.get_window_title()
  File "/usr/lib/python2.7/site-packages/matplotlib/backends/backend_qt5.py", line 560, in get_window_title
    return str(self.window.windowTitle())
UnicodeEncodeError: 'ascii' codec can't encode character u'\xef' in position 2: ordinal not in range(128)

It works fine on python3.

[mdboom]

I don't know why that str is there -- it appears to go way back to at least the Qt3 days. Removing it seems to work fine for Qt5, Qt4 and PySide.

@mfitzp: You appear to be the last major contributor to the Qt stuff. Do you see any downside to just removing the str() in get_window_title and thereby allowing a unicode string to pass through unaltered down to the Qt level?

[mfitzp]

This exists for compatibility with "version 1" of the PyQt(4) API. In this version the APIs return QVariant types rather than native Python strings and require explicit conversion. The code here is just performing that by default to avoid requiring multiple versions of code for the two APIs.

There is some more information on the Riverbank docs for PyQt4. It notes that v2 of the API is default for Python 3 (I also think it is the default for matplotlib generally in PyQt4). In PyQt5 this distinction doesn't exist.

I guess there are a few options:

• Remove the str() wrapper and return a bare QVariant type on v1 of the API. I'm not totally clued up on QVariant behaviour, e.g. whether it will work with os.path.join. @mdboom Can you check with your PyQt4 install by setting QT_API_PYQT = 'PyQt4' and seeing if the save figure path still works?
• Check for QVariant instance and convert only if it is
• Replace the string str() wrapper with a Python 2 & 3 compatible alternative. Just noticed that matplotlib depends on six which may have a helper for this.

What do you think?

[mfitzp]

The following replacement could be made on a subclassed Qt4 version of FigureManagerQT and then the str() can be removed completely from the Qt5 code (this follows the planned design for using the newer API as the base, and patching to keep old APIs working):

def get_window_title(self):
    s = self.window.windowTitle()

    # For PyQt4 v1 API check if we get a QVariant
    if isinstance(s, QtCore.QVariant):
        return s.toPyObject()

    return s

Let me know if this looks OK and I'll put together a PR.

[mdboom]

Thanks. The fact that this is also used for the save figure path was forgotten, so wasn't included in my testing where I said "everything worked". In that event, actually none of the alternatives work (Qt4 old/new, PySide, Qt5) work on Python 2.7. So it looks like what you propose above work work either.

Just to confirm, when you say set QT_API_PYQT = 'PyQt4', you mean as an environment variable? If that's not right, then I'm probably not testing the v1 API correctly.

This works for me:

def get_window_title(self):
    return six.text_type(self.window.windowTitle())

Having issues getting a Python 3 / Qt environment set up, so I haven't tested there.

[mfitzp]

The above code would be used in addition to removing the str() on the PyQt5 version that is causing the problem. But now I'm looking into it it's possible to also set a v1-like API on PyQt5. I think the six approach is probably the way to go. Does that work for you with PyQt4 and the v1 API?

You're right that setting QT_API_PYQT = 'PyQt4' as an environment variable should set you on the Qt4 v1 API. You can check this by using:

import sip
sip.getapi('QString') 

It should return 1.

[mdboom]

Yes -- using six.text_type works for all Python 2.7 Qt libraries and APIs (PyQt4 API v1/v2, PyQt5, PySide).

[mfitzp]

I've opened a PR with the suggested fix. Testing here it works fine, but also unable to test with Python 3 yet, will look into it.

When applying the fix I noticed there were a few other incidences of str() in the file where its possible that unicode could end up. I've replaced all of them with the six wrapper for consistency.

[Nodd]

Thank you for your reactivity everyone, it is much appreciated !