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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_field_label_css_class(self, bound_field):
 """ Returns the optional label CSS class to use when rendering a field template. By default, returns the Form class property `field_label_css_class`. If the field has errors and the Form class property `field_label_invalid_css_class` is defined, its value is appended to the CSS class. :param bound_field: `BoundField` instance to return CSS class for. :return: A CSS class string or `None` """

        class_name = self.field_label_css_class

        if bound_field.errors and self.field_label_invalid_css_class:
            class_name = join_css_class(
                class_name, self.field_label_invalid_css_class)

        return class_name or None