configure_traits freezes console in Spyder/Anaconda

Question

Trying the very first example in traitsui doc:

from traits.api import HasTraits, Str, Int
from traitsui.api import View, Item
import traitsui

class SimpleEmployee(HasTraits):
    first_name = Str
    last_name = Str
    department = Str
    employee_number = Str
    salary = Int

view1 = View(Item(name = 'first_name'),
             Item(name = 'last_name'),
             Item(name = 'department'))

sam = SimpleEmployee()
sam.configure_traits(view=view1)

makes the Spyder IPython console hang, even after the UI window has been closed.

MacOSX 10.14.6, Spyder 4.0.0, Python 3.7.0, IPython 7.10.2, traitsui 6.1.3

Probably something to configure about the UI event loop, but what and how ?

Answer 1

The difference between the Canopy IPython console and the Spyder IPython console is that

app = QtGui.QApplication.instance()
print(app)

returns

<PyQt5.QtWidgets.QApplication at 0xXXXXXXXX>

for both, but

is_event_loop_running_qt4(app)

returns True in Canopy IPython terminal and False in Spyder IPython terminal.

It is necessary to patch view_application() in traits.qt4.view_application by commenting out the lines starting a new ViewApplication instance depending on that test. Instead of changing the original file, it is possible to run the following code:

# -*- coding: utf-8 -*-

"""Avoid Spyder console to hang when using configure_traits()
"""

from IPython import get_ipython

from pyface.qt import QtCore, QtGui, qt_api
from pyface.util.guisupport import is_event_loop_running_qt4

from traitsui.api import toolkit
from traitsui.qt4.view_application import ViewApplication

KEEP_ALIVE_UIS = set()

def on_ui_destroyed(object, name, old, destroyed):
    """ Remove the UI object from KEEP_ALIVE_UIS.
    """
    assert name == 'destroyed'
    if destroyed:
        assert object in KEEP_ALIVE_UIS
        KEEP_ALIVE_UIS.remove(object)
        object.on_trait_change(on_ui_destroyed, 'destroyed', remove=True)

def _view_application(context, view, kind, handler, id, scrollable, args):
    """ Creates a stand-alone PyQt application to display a specified traits UI
        View.

    Parameters
    ----------
    context : object or dictionary
        A single object or a dictionary of string/object pairs, whose trait
        attributes are to be edited. If not specified, the current object is
        used.
    view : view object
        A View object that defines a user interface for editing trait attribute
        values.
    kind : string
        The type of user interface window to create. See the
        **traitsui.view.kind_trait** trait for values and
        their meanings. If *kind* is unspecified or None, the **kind**
        attribute of the View object is used.
    handler : Handler object
        A handler object used for event handling in the dialog box. If
        None, the default handler for Traits UI is used.
    scrollable : Boolean
        Indicates whether the dialog box should be scrollable. When set to
        True, scroll bars appear on the dialog box if it is not large enough
        to display all of the items in the view at one time.


    """
    if (kind == 'panel') or ((kind is None) and (view.kind == 'panel')):
        kind = 'modal'

    # !!!!!!! The following 4 lines commented out !!!!!!!!!!!
    # app = QtGui.QApplication.instance()
    # if app is None or not is_event_loop_running_qt4(app):
    #     return ViewApplication(context, view, kind, handler, id,
    #                            scrollable, args).ui.result
    # !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    ui = view.ui(context,
                 kind=kind,
                 handler=handler,
                 id=id,
                 scrollable=scrollable,
                 args=args)

    # If the UI has not been closed yet, we need to keep a reference to
    # it until it does close.
    if not ui.destroyed:
        KEEP_ALIVE_UIS.add(ui)
        ui.on_trait_change(on_ui_destroyed, 'destroyed')
    return ui.result

def view_application(self, context, view, kind=None, handler=None,
                     id='', scrollable=None, args=None):
    """ Creates a PyQt modal dialog user interface that
        runs as a complete application, using information from the
        specified View object.

    Parameters
    ----------
    context : object or dictionary
        A single object or a dictionary of string/object pairs, whose trait
        attributes are to be edited. If not specified, the current object is
        used.
    view : view or string
        A View object that defines a user interface for editing trait
        attribute values.
    kind : string
        The type of user interface window to create. See the
        **traitsui.view.kind_trait** trait for values and
        their meanings. If *kind* is unspecified or None, the **kind**
        attribute of the View object is used.
    handler : Handler object
        A handler object used for event handling in the dialog box. If
        None, the default handler for Traits UI is used.
    id : string
        A unique ID for persisting preferences about this user interface,
        such as size and position. If not specified, no user preferences
        are saved.
    scrollable : Boolean
        Indicates whether the dialog box should be scrollable. When set to
        True, scroll bars appear on the dialog box if it is not large enough
        to display all of the items in the view at one time.

    """
    return _view_application(context, view, kind, handler,
                                             id, scrollable, args)

from traitsui.qt4.toolkit import GUIToolkit

GUIToolkit.view_application = view_application

Thus, the behavior is OK in both Spyder and Canopy IPython consoles. Note that configure_traits(kind='modal') is also OK in both.