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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 open_if_exists(filename, mode='rb'):
 """Returns a file descriptor for the filename if that file exists, otherwise `None`. """

    try:
        return open(filename, mode)
    except IOError, e:
        if e.errno not in (errno.ENOENT, errno.EISDIR):
            raise