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As far as I've been able to tell there is no proposed syntax in the docs specifically for package_data. The docs for the resources option seems to suggest separating globs with spaces, which would be fine by me (wouldn't allow paths that contain spaces, but that's a bad idea anyways).
I think that allowing one glob string on each line is more readable, but I'm not too attached either way. Another possibility would be to allow line breaks in the value, for example:
package data = 
 mypackage = templates/*.html
 static/css/*.css
But that's getting a little more complex syntax-wise.
Agreed on getting rid of data_files--it's dangerous. But package_data I find very useful sometimes, and I don't think it's always wrong. At the very least, it's not clear to me how the above use case is intended to be replaced.