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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _get_fd(fileobj):
 """ Get a descriptor out of a file object. :param fileobj: An integer (existing descriptor) or any object having the `fileno()` method. :raises ValueError: if the descriptor cannot be obtained or if the descriptor is invalid :returns: file descriptor number """

    if isinstance(fileobj, int):
        fd = fileobj
    else:
        try:
            fd = fileobj.fileno()
        except AttributeError:
            fd = None
    if fd is None or fd < 0:
        raise ValueError("invalid fileobj: {!r}".format(fileobj))
    return fd