Should wrap to < 80 chars.

This would go before the djblets ones. Alphabetical order.

Blank line between these.

Can you make this a TODO in the comment?

This is more of a developer comment and less of a documentation comment. Doc blocks should always describe what the function does, which may be different than the reason the function exists. Maybe just make these normal comments.

No blank line here.

I'd prefer just doing a standard:

for url_patterns in self.resource_url_patterns_queue:
    self.add_to_url_patterns(url_patterns)

It's a little more standard and readable, whereas using a while loop and pop is less common. It's also potentially faster (in theory) due to fewer method calls and optimizations for iterators.

We can then just reset the list after the loop.

I'm not really sure I love this class modifying the extension's state, especially using public variables. If an extension author tried to use this variable, checking first that Extension itself didn't define it, then we'd end up stomping over it.

Maybe instead we should have a dict owned by this resource class that maps extensions to the pending patterns. That way it's fully contained.

Does this wrap to < 80 chars?

If this is for private use only, I'd prefix the name with a _. Same with any other private functions/variables for the URL building.

For signal handlers, I'd prefer names like "on_extension_initialized"

You can just have this as a parameter to the function:

def notice_initialized_extension(self, sender, ext_class=None, **kwargs):

Can do:

    if extension in self._resource_url_patterns_map:

That'd actually be more forward-compatible with Python 3.

Same here.

Committed to extensions as 6097dc1.