fix PEP420 namespace packages

[ivan-kalev]

See #1074 for details.

[msarahan]

@Ikalev sorry I forgot earlier, this has been proposed once before: #859

There, I suggested one change: throw an exception if copying would overwrite any file(s). I think that this is very important for sanity - otherwise it is ill-defined what the contents of your files are. Any ideas on that point?

[ivan-kalev]

@msarahan It's a bit of a grey zone: If we are merging namespace packages some overwriting might happen and that might be okay in edge cases... But otherwise it would be an error indeed.

I think it's reasonable to ignore said edge cases and assume that this is an error. I'll try to extend the implementation with an explicit check. Should be easy.

[ivan-kalev]

@msarahan should be good to merge now.

[msarahan]

Looks great. Minor PEP8 errors and existing errors on Travis. Will wait for AppVeyor, then merge.

[msarahan]

(I am happy to fix PEP8 - you don't need another push.)

[msarahan]

Thanks again, @Ikalev

[ivan-kalev]

Great, thanks!

[183amir]

Thank you this is awesome. Sorry I was busy and could not find a time to write proper test cases for my pull request.

[ivan-kalev]

Note that this does not solve all cases of #649 . Python 2 namespace packages (declared with setuptools or distutils) contain __init__.py files, which copy_into() might attempt to overwrite. This will obviously not pass our test for merge conflicts. The workaround on Python 2 is to use pkg_resources. declare_namespace() and enable preserve_egg_dir, which hides shared namespace files in private egg folders.

[ivan-kalev]

@msarahan I don't see these changes in the source tree at the moment. I deleted my fork after the merge, but this should be fine, right? I'm confused.

[msarahan]

@Ikalev could you please resubmit this? I don't know how, but I don't see it in the git history. Nor do I see any force pushes that I would have overwritten it with...

[ivan-kalev]

@msarahan Unfortunately I have deleted my fork of the repo, but I can try to extract a patch from this diff and prepare a new PR: https://github.com/conda/conda-build/pull/1090/files

[msarahan]

Sorry for the mess. I can submit this PR for you, if you'd prefer.

[ivan-kalev]

No problem. Here is the new PR: #1105.