Comment 3 for bug 2052943

Your four-item summary of the requested functionality is spot-on.

Indeed, these methods could (and should) support other attributes, such as:

====
def add_class(self, these_classes, attname='class'):
    ...

def remove_class(self, these_classes, attname='class'):
    ...
====

For example, if I am working with non-HTML content (such as DITA XML source), then that will have its own conventions for list-of-strings attributes. For example, DITA has profiling condition attributes that are also multi-value:

====
dita_tag.add_class('expert', attname='audience')
====

I am completely fine with not using "class" in the name. (I am just used to XML::Twig's methods.) The terminology used in the BS4 documentation is "multi-valued attributes" so we probably use something consistent with that. Some ideas are:

====
tag.add_value('foo')
tag.remove_value(['foo', 'bar'])

tag.add_multivalue('foo')
tag.remove_multivalue(['foo', 'bar'])
====

If the attribute name argument is required (instead of defaulting to 'class'), it would make the methods' purposes clearer in context:

====
tag.add_value('foo', attname='class')
tag.remove_value(['foo', 'bar'], attname='class')

tag.add_multivalue('foo', attname='class')
tag.remove_multivalue(['foo', 'bar'], attname='class')
====

or

====
tag.add_value('class', 'foo')
tag.remove_value('class', ['foo', 'bar'])

tag.add_multivalue('class', 'foo')
tag.remove_multivalue('class', ['foo', 'bar'])
====

What would the subclassed-list UI look like? How would it handle the addition of a value to an attribute that doesn't exist yet? Could it handle addition/removal of both single values and lists of values?