Unbreak formlayout for image edits.

anntzer · anntzer · commit 91e1b964155c · 2018-09-17T17:44:45.000+02:00
As of master, the qt editor is broken when editing an axes that contains
an image throws an error when clicking OK.  This is because `self.data`
needs to be modified *in-place* in FormWidget.setup, as FormWidget.get
later checks some explicit types in there.

This means, as a side note, that passing a tuple instead of a list to
generate a QComboBox is broken (one can test that by modifying the
example at the bottom of the module -- the results are different
depending on whether 'list2' is a list or a tuple) but that's something
we can deal with some other day...
diff --git a/lib/matplotlib/backends/qt_editor/formlayout.py b/lib/matplotlib/backends/qt_editor/formlayout.py
@@ -243,8 +243,14 @@ def setup(self):
             elif isinstance(value, str):
                 field = QtWidgets.QLineEdit(value, self)
             elif isinstance(value, (list, tuple)):
+                if isinstance(value, tuple):
+                    value = list(value)
+                # Note: get() below checks the type of value[0] in self.data so
+                # it is essential that value gets modified in-place.
+                # This means that the code is actually broken in the case where
+                # value is a tuple, but fortunately we always pass a list...
+                selindex = value.pop(0)
                 field = QtWidgets.QComboBox(self)
-                selindex, *value = value
                 if isinstance(value[0], (list, tuple)):
                     keys = [key for key, _val in value]
                     value = [val for _key, val in value]
