It is sometimes useful to create a temporary file as a copy of an existing file (especially e.g. in tests, when you want to mutate the temp file but not the original). I would suggest adding a `copy_from` argument to TemporaryFile and NamedTemporaryFile, which is either the path to an existing file or a file-like object. Then the contents of that source file are copied to the tempfile before it is yielded.