os.sendfile() has a keyword-or-positional parameter named "in". Since it is a keyword in Python, it is not possible to pass it as a keyword argument. You can only pass it as a positional argument or using a var-keyword argument (unlikely anybody uses the latter). The preceding parameter, "out", also can not be passed by keyword because of this.

It is weird, but usually does not cause a problem. You cannot use a keyword argument, period. But it prevents os.sendfile() from converting to Argument Clinic, because Argument Clinic does not allow using Python keywords as parameter names (I already created a patch for conversion, but in needs to solve this issue first).

There are two ways to solve this issue.

1. Rename parameter "in" (and maybe "out" for consistency). "out_fd" and "in_fd" look good names (they are use in Linux manpage).

2. Make "out" and "in" positional-only parameters.

See also bpo-15078.

I’m for renaming both. Since the function is about transmitting or copying (on Linux) files src_fd and dst_fd could also be good candidates.

New changeset 140a7d1f3579e778656a6b6bfad72489e9870a4d by Serhiy Storchaka in branch 'master':
bpo-38378: Rename parameters "out" and "in" of os.sendfile(). (GH-16742)
https://github.com/python/cpython/commit/140a7d1f3579e778656a6b6bfad72489e9870a4d

I would prefer "2. Make "out" and "in" positional-only parameters". It would be a minor incompatible change, but it would make os.sendfile() more consistent with other functions like os.write() which only has positional-only parameters.

Well, since os.sendfile(in=fd) raises a syntax error, I don't think that it's really a backward incompatible change in practice.