HTML Representations — MarkupSafe documentation

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HTML Representations

In many frameworks, if a class implements an __html__ method it will be used to get the object’s representation in HTML. MarkupSafe’s escape() function and Markup class understand and implement this method. If an object has an __html__ method it will be called rather than converting the object to a string, and the result will be assumed safe and not escaped.

For example, an Image class might automatically generate an <img> tag:

class Image:
    def __init__(self, url):
        self.url = url

    def __html__(self):
        return '<img src="%s">' % self.url
>>> img = Image('/static/logo.png')
>>> Markup(img)
Markup('<img src="/static/logo.png">')

Since this bypasses escaping, you need to be careful about using user-provided data in the output. For example, a user’s display name should still be escaped:

class User:
    def __init__(self, id, name): = id = name

    def __html__(self):
        return '<a href="/user/{}">{}</a>'.format(
  , escape(
>>> user = User(3, '<script>')
>>> escape(user)
Markup('<a href="/users/3">&lt;script&gt;</a>')