best way to copy a big binary file

[Skaperen]

i'm still asking how best to copy a binary file leaving open the possibility of adding in some kind of progress indication which may need to be something custom, or something else that could be done between steps of the copy. so that means doing the copy a little at a time. it also needs to be for open file objects although doing special opens may be possible. being portable is important so it works on other platforms that CPython has been properly ported to.