Apparently CoreFX adds the \\?\ prefix automatically:

https://blogs.msdn.microsoft.com/jeremykuhne/2016/06/21/more-on-new-net-path-handling

It's great that Windows 10 Anniversary Edition will be getting long path support without requiring the extended path prefix, at least for NTFS volumes. I assume that includes slash-to-backslash normalization, relative paths, and drive-relative paths. I wonder about long drive-relative paths since they depend on hidden environment variables such as "=D:". The default environment block isn't that big.

Python 3.5 supports back to Vista, so I still think automatically handling long Unicode paths, like how CoreFX reportedly works, makes for a more friendly cross-platform development experience. There are too many pitfalls with \\?\ paths -- they have to be Unicode (except that limitation is removed in Windows 10), fully qualified, use backslash only, and UNC paths have to be translated to use the \\?\UNC DOS device.