'return (fspath, names) tuple after checking the file exists.'

def _parsearg(self, arg):

    parts = str(arg).split('::')
    if self.config.option.pyargs:
        parts[0] = self._tryconvertpyarg(parts[0])
    relpath = parts[0].replace('/', os.sep)
    path = self.config.invocation_dir.join(relpath, abs=True)
    if (not path.check()):
        if self.config.option.pyargs:
            raise UsageError((('file    or    package    not    found:    ' + arg) + '    (missing    __init__.py?)'))
        else:
            raise UsageError(('file    not    found:    ' + arg))
    parts[0] = path
    return parts
