I can see the point in keeping the behaviour close to the OS-level commands, but the discrepancy between os.mkdir (or os.makedirs) and os.path.dirname is not resolved.

I still think that
>>> os.makedirs(os.path.dirname(filename), exist_ok=True)
should be able to handle the case when "filename" does not contain any directories.
A possible workaround would be to use os.path.abspath somewhere in the mix.

In my humble opinion creating the directory structure required to write a file should not require this level of complexity, since it is such a common operation.