class HtmlElement:
    tag = "html"
    indent = "    "
    def __init__(self, content=None, **kwargs):
        if content is None:
            self.contents = []
        else:
            self.contents = [content]
        self.attributes = kwargs
    def append(self, new_content):
        self.contents.append(new_content)
    def render(self, cur_ind=""):
        print(cur_ind + f"<{self.tag}>")
        for content in self.contents:
            try:
                content.render(cur_ind + self.indent)
            except AttributeError:
                print(cur_ind + self.indent + content)
        print(cur_ind + f"</{self.tag}>")

class Body(HtmlElement):
    tag = "body"

class Paragraph(HtmlElement):
    tag = "p"



para = Paragraph("hello")
body = Body(para)
body.append("world")
doc = HtmlElement(body)
doc.render()