PyYAML: YAMLObject subclasses are ignored

I suspect the solution to my problem is something really trivial.

I wrote a module called pitz that contains a class Issue:

    >>> pitz.Issue.yaml_tag
    u'ditz.rubyforge.org,2008-03-06/issue'

Then I try to load a document with that same tag, but I get a
ConstructorError:

    ConstructorError: could not determine a constructor for the tag
    '!ditz.rubyforge.org,2008-03-06/issue'

I think there's some namespace/scoping problem, because when I define
the same class in my interpreter and then run yaml.load(issue_file),
everything works OK.

So, how do I tell yaml to build pitz.Issue instances when it runs across
that tag?

Do I need to use add_constructor?  If so, I'd *really* appreciate some
example usage.

Thanks!

Matt


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