class Page:
    def __init__(self, num, content):
        self.num = num
        self.content = content
    def output(self):
        return f'{self.content}'

class HTMLPageMixin:
    def to_html(self):
        return f'<html><body>{self.output()}</body></html>'

class WebPage(Page, HTMLPageMixin):
    pass

page = WebPage(0, 'web content')
print(page.to_html())
