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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_optional_env(key):
 """ Return the value of an optional environment variable, and use the provided default if it's not set. """

    environment_variable_value = os.environ.get(key)
    if environment_variable_value:
        return environment_variable_value
    elif key in CONSTANTS:
        return CONSTANTS[key]
    else:
        raise Exception("The variable {1} is not set".format(key))