Debugging and testing

Running widgets as scripts

To run a widget without canvas - for debugging and for nitpicking about the GUI - the widget module must be executable as a script. This is handled by WidgetPreview. It is typically used as follows

if __name__ == "__main__":

where OWMyWidgetName is the widget’s class.

We can also pass the data to the widget. For instance,

if __name__ == "__main__":

passes the Iris data set to the widget. Passing data in this way requires that there is a single or default signal for the argument’s data type. Multiple signals can be passed as keyword arguments in which the names correspond to signal handlers:

if __name__ == "__main__":
    data ="iris")

If the signal handler accepts multiple inputs, they can be passed as a list, like in the following method in the Table widget.

if __name__ == "__main__":
        [(Table("iris"), "iris"),
        (Table("brown-selected"), "brown-selected"),
        (Table("housing"), "housing")

Preview ends by tearing down the widget and calling sys.exit with the widget’s exit code. This can be prevented by adding a no_exit=True argument. We can also prevent showing the widget and starting the event loop by using no_exec=True. This, together with some previewers method described below, can be used for debugging the widget. For example, OWRank’s preview,

if __name__ == "__main__":
    from Orange.classification import RandomForestLearner
        set_learner=(RandomForestLearner(), (3, 'Learner', None)),

can be temporarily modified to

if __name__ == "__main__":
    from Orange.classification import RandomForestLearner
    previewer = WidgetPreview(OWRank)""), no_exit=True)
        set_learner=(RandomForestLearner(), (3, 'Learner', None)))

which shows the widget twice, allows us a finer control of signal passing, and offers adding some breakpoints.

class Orange.widgets.utils.widgetpreview.WidgetPreview(widget_cls)[source]

A helper class for widget previews.


an instance of the widget or None




the widget class



run(input_data=None, *, no_exec=False, no_exit=False, **kwargs)[source]

Run a preview of the widget;

It first creates a widget, unless it exists from the previous call. This can only happen if no_exit was set to True.

Next, it passes the data signals to the widget. Data given as positional argument must be of a type for which there exist a single or a default handler. Signals can also be given by keyword arguments, where the name of the argument is the name of the handler method. If the data is a list of tuples, the sequence of tuples is sent to the same handler.

Next, the method shows the widget and starts the event loop, unless no_exec argument is set to True.

Finally, unless the argument no_exit is set to True, the method tears down the widget, deletes the reference to the widget and calls Python’s garbage collector, as an effort to catch any crashes due to widget members (typically QGraphicsScene elements) outliving the widget. It then calls sys.exit with the exit code from the application’s main loop.

If no_exit is set to True, the run keeps the widget alive. In this case, subsequent calls to run or other methods (send_signals, exec_widget) will use the same widget.

  • input_data – data used for the default input signal of matching type

  • no_exec (bool) – if set to True, the widget is not shown and the event loop is not started

  • no_exit (bool) – if set to True, the widget is not torn down

  • **kwargs – data for input signals


Initialize QApplication and construct the widget.

send_signals(input_data=None, **kwargs)[source]

Send signals to the widget


Show the widget and start the QApplication’s main loop.


Save settings and delete the widget.

Unit-testing Widgets

Orange provides a base class WidgetTest with helper methods for unit testing.

class Orange.widgets.tests.base.WidgetTest(methodName='runTest')[source]
assert_domain_equal(domain1, domain2)[source]

Test domains for equality.

Unlike in domain1 == domain2 uses Variable.__eq__, which in case of DiscreteVariable ignores values, this method also checks that both domain have equal values.