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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_fee(speed=FEE_SPEED_MEDIUM):
 """Gets the recommended satoshi per byte fee. :param speed: One of: 'fast', 'medium', 'slow'. :type speed: ``string`` :rtype: ``int`` """

    if speed == FEE_SPEED_FAST:
        return DEFAULT_FEE_FAST
    elif speed == FEE_SPEED_MEDIUM:
        return DEFAULT_FEE_MEDIUM
    elif speed == FEE_SPEED_SLOW:
        return DEFAULT_FEE_SLOW
    else:
        raise ValueError('Invalid speed argument.')