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I'm not sure your solution of locking in the file object is feasible. Every call of PyFile_AsFile() is dangerous, if the  code proceeds to release the GIL before using the FILE* it's in danger. And this code could be in extension modules too (I know there's quite a few in my own img package).

When I did the universal newlines patch I basically ignored the problem (thinking that no-one in their right mind would close a file in one thread while it's still in use in another: how wrong can one be:-).

The only real solution may be to get rid of PyFile_AsFile altogether, and replace it with PyFile_AsFileLocked() and PyFile_Unlock() and make these calls somehow work even when the GIL is held (possibly by temporarily releasing the GIL while acquiring the file lock).