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class NullBooleanField(BooleanField):
    """
    A field whose valid values are None, True and False. Invalid values are
    cleaned to None.
    """
    # THIS ONE GOES!
    # widget = NullBooleanSelect
    def __init__(self, *args, **kwargs):
        self.widget = Select(choices=[(None, _("Unknown")),
                            (True, _("Yes")),
                            (False, _("No"))])
        super(NullBooleanField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        """
        Explicitly checks for the string 'True' and 'False', which is what a
        hidden field will submit for True and False, and for '1' and '0', which
        is what a RadioField will submit. Unlike the Booleanfield we need to
        explicitly check for True, because we are not using the bool() function
        """
        if value in (True, 'True', '1'):
            return True
        elif value in (False, 'False', '0'):
            return False
        else:
            return None

    def validate(self, value):
        pass