def positive_percent(df):
    """
    compute the percentage of positive Tweets: 2 is positive
    :param df: a tweet dataframe
    :return: percentage of positive tweets
    """
    assert 'sentiment_vader_percent' in df, "The pandas dataframe should contain the sentiment of each tweet"
    positive = 0
    for sentiment in list(df['sentiment_vader_percent']):
        if int(float(sentiment)) == 2:
            positive += 1
        else:
            pass
    return positive / df.shape[0]