Convert Windows relative path to POSIX relative path so I can join POSIX absolute path and Windows r

Instead of using os which depends on the platform, or posixpath which will only work with POSIX paths, Python's pathlib module seems to be a good choice here:

This module offers classes representing filesystem paths with semantics appropriate for different operating systems

Here is a solution for the example from the question above:

def example(abspath, relpath):
    import pathlib
    abspath = pathlib.PurePosixPath(abspath)
    relpath = pathlib.PurePath(relpath)
    return str(abspath.joinpath(relpath))

pathlib.PurePosixPath is being used for abspath because the final path is supposed to be a POSIX path and pathlib.PurePath works on both Windows and Linux:

pathlib.PurePath - A generic class that represents the system’s path flavour (instantiating it creates either a PurePosixPath or a PureWindowsPath)

An example call on Linux might look like:

>>> example("/x/y/z", "a/b/c")
'/x/y/z/a/b/c'

An example call on Windows might look like:

>>> example("/x/y/z", "a\\b\\c")
'/x/y/z/a/b/c'

So it produces the correct POSIX path on both platforms.