Demonstrate the SystemError bug appearing on Python 3 when an exception is cleared for unknown reasons

system_error_bug.enaml

"""Gist demonstarting the SystemError bug on python 3
"""

#-------------------------------------------------------------------------------
#  Imports:
#-------------------------------------------------------------------------------
from atom.api import List, observe

from enaml.widgets.api import RawWidget
from enaml.core.declarative import d_
from enaml.qt.QtGui import QListWidget, QListWidgetItem
from enaml.qt.QtCore import Qt

class QtListStrWidget(RawWidget):
    """ A Qt4 implementation of an Enaml ProxyListStrView.

    """

    __slots__ = '__weakref__'

    #: The list of (str, enabled) tuples being viewed
    items = d_(List() )

    #--------------------------------------------------------------------------
    # Initialization API
    #--------------------------------------------------------------------------
    def create_widget(self, parent):
        """
        Create the QListWidget widget.
        """
        # Create the list model and accompanying controls:
        widget = QListWidget(parent)
        for item, checked in self.items:
            self.add_item(widget, item, checked)

        return widget


    def add_item(self, widget, item, checked=True):
        itemWidget = QListWidgetItem(item)
        widget.addItem(itemWidget)

    #--------------------------------------------------------------------------
    # Observers
    #--------------------------------------------------------------------------
    @observe('items')
    def _update_items(self, change):
        """ An observer which sends state change to the proxy. """

        #this callback may be called before the widget is initialized
        widget = self.get_widget()
        if widget == None:
            return

        raise RuntimeError()


from enaml.widgets.api import Window, Container, PushButton

enamldef Main(Window): main:

    attr items = []

    Container:
        QtListStrWidget:
            items << main.items

        PushButton:
            text= 'Add'
            clicked::
                l = main.items[:]
                l.append('e')
                main.items = l