this_range.high = fd (no need to create a copy of this_range and is a bit clear, at least for me ;)

Please avoid creation of set {0..maxfd} as maxfd could be large (it's a smaller problem than closing all fds but still could waste teens miliseconds necessarily)

Please notice also that closerange(a, b) closes [a, b) (thus b is not closed if a != b)
I think that corner cases are broken here. Also max_fd is a number of max opened files and first fd is 0 so max closed fd should be max_fd - 1.

as mentioned about ranges for closerange function, expected result here should be: (0, 3), (4, 5), (6, 9)

(0, 4)

as previously mentioned - need to fix corner cases

Please notice also that closerange(a, b) closes [a, b) (thus b is not closed if a != b) I think that corner cases are broken here. Also max_fd is a number of max opened files and first fd is 0 so max closed fd should be max_fd - 1.

Thank you, that helps a lot. (The parameter names to os.closerange do not make that clear.)

I have re-worked this merge request to incorporate the actual behaviour of os.closerange.