please check if this works. The code is a little clearer than using except.

I remember I wrote a test for this 2 years ago. I need to check my private repository... I remember that the bug was pretty annoying and except was my last resort. Is it okay to block this for up to 2 days so that I can find the test?

Yes sure. If you could add a test or two that would be really great.

Thanks for your feedbacks, and thanks again @tobwen for this code snippet. It works perfectly fine as is but we plan to test your suggestion Simon. Will let you know in a few days.

I think this one was it: tobwen@cbc3cca

I hope I can test it tomorrow.

I gave it a try.

if 'image_opts' in layer[0] and layer[0].image_opts.transparent:

Gives me the following error :

TypeError: argument of type 'ImageSource' is not iterable

I fixed it with the following :

        all_transparent = True
        for layer in async_.imap(get_map_from_source, self.sources):
            if layer[0] is not None:
                layers.append(layer)
                try:
                    if not layer[0].image_opts.transparent:
                        all_transparent = False
                        break
                except AttributeError:
                    all_transparent = False
                    break

        if all_transparent:
            self.tile_mgr.image_opts.transparent = True

That works fine, I'll double check that with @damien-faivre and he should add this to this PR.