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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 lookupEnvVar(name, envName, defaultValue):
 """ Use this for looking up environment variables that control Toil and are important enough to log the result of that lookup. :param str name: the human readable name of the variable :param str envName: the name of the environment variable to lookup :param str defaultValue: the fall-back value :return: the value of the environment variable or the default value the variable is not set :rtype: str """

    try:
        value = os.environ[envName]
    except KeyError:
        log.info('Using default %s of %s as %s is not set.', name, defaultValue, envName)
        return defaultValue
    else:
        log.info('Overriding %s of %s with %s from %s.', name, defaultValue, value, envName)
        return value