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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 apply_widget_css_class(self, field_name):
 """ Applies CSS classes to widgets if available. The method uses the `get_widget_css_class` method to determine if the widget CSS class should be changed. If a CSS class is returned, it is appended to the current value of the class property of the widget instance. :param field_name: A field name of the form. """

        field = self.fields[field_name]
        class_name = self.get_widget_css_class(field_name, field)

        if class_name:
            field.widget.attrs['class'] = join_css_class(
                field.widget.attrs.get('class', None), class_name)