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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_css_classes(cls, instance):
 """ """

        css_classes = []
        if hasattr(cls, 'default_css_class'):
            css_classes.append(cls.default_css_class)
        for attr in getattr(cls, 'default_css_attributes', []):
            css_class = instance.glossary.get(attr)
            if isinstance(css_class, six.string_types):
                css_classes.append(css_class)
            elif isinstance(css_class, list):
                css_classes.extend(css_class)
        return css_classes