HTMLParser derives from _markupbase.ParserBase, which has the following method:

class HTMLParser:

    ...

    def error(self, message):
        raise NotImplementedError(
            "subclasses of ParserBase must override error()")

HTMLParser does not implement this method and the documentation for HTMLParser (https://docs.python.org/3.6/library/html.parser.html) does not mention that its sub-classes need to override it.

I am not sure whether this is a documentation omission, whether HTMLParser should provide an (empty?) implementation, or whether ParserBase should not raise a NotImplementedError (to make linters happy).