@pimms.param<EOL><INDENT>def cache_path(cp):<DEDENT>
if cp is None: return None<EOL>if not pimms.is_str(cp): raise ValueError('<STR_LIT>')<EOL>return os.path.expanduser(os.path.expandvars(cp))<EOL>
pseudo_dir.cache_path is the optionally provided cache path; this is the same as the
storage path unless this is None.