Testing Traits ClassesΒΆ

A mixin class is provided to facilitate writing tests for HasTraits classes. The methods assertTraitChanges() and assertTraitDoesChange() are available when the UnittestTools is added as a mixin class in the developers TestCase.

The methods behave as a context manager which at entry hooks and traits listener on the class for the desired events and records the arguments pass to the listener at every fired events. This way the developer can easily assert that specific events have been fired. Further analysis and checking can be performed by inspecting the list of recorded events arguments.

Both normal and extended trait names are supported. However, no check is performed regarding the validity of the trait name, thus care is required to safeguard against spelling mistakes in the names of the traits that we need to assert the behaviour.

The following example demonstrates the basic usage of the mixin class in a TestCase:

from traits.testing.unittest_tools import unittest
from traits.api import HasTraits, Float, List, Bool, on_trait_change
from traits.testing.api import UnittestTools


class MyClass(HasTraits):

    number = Float(2.0)
    list_of_numbers = List(Float)
    flag = Bool

    @on_trait_change('number')
    def _add_number_to_list(self, value):
        """ Append the value to the list of numbers. """
        self.list_of_numbers.append(value)

    def add_to_number(self, value):
        """ Add the value to `number`. """
        self.number += value


class MyTestCase(unittest.TestCase, UnittestTools):

    def setUp(self):
        self.my_class = MyClass()

    def test_when_using_with(self):
        """ Check normal use cases as a context manager.
        """
        my_class = self.my_class

        # Checking for change events
        with self.assertTraitChanges(my_class, 'number') as result:
            my_class.number = 5.0

        # Inspecting the last recorded event
        expected = (my_class, 'number', 2.0, 5.0)
        self.assertSequenceEqual(result.events, [expected])

        # Checking for specific number of events
        with self.assertTraitChanges(my_class, 'number', count=3) as result:
            my_class.flag = True
            my_class.add_to_number(10.0)
            my_class.add_to_number(10.0)
            my_class.add_to_number(10.0)

        expected = [(my_class, 'number', 5.0, 15.0),
                    (my_class, 'number', 15.0, 25.0),
                    (my_class, 'number', 25.0, 35.0)]
        self.assertSequenceEqual(result.events, expected)

        # Check using extended names
        with self.assertTraitChanges(my_class, 'list_of_numbers[]'):
            my_class.number = -3.0

        # Check that event is not fired
        my_class.number = 2.0
        with self.assertTraitDoesNotChange(my_class, 'number') as result:
            my_class.flag = True
            my_class.number = 2.0  # The value is the same as the original