Python 如何计算N天的累计转换率？

我在计算第N天的累计转换率时遇到问题

折算率是指在该期间内有购买行为的用户，因此本例总折算率为3/4=0.75

然而，我确实很难理解如何计算每天的累计转化率，有人知道我该如何解决这个问题吗？我正在使用python进行分析

以下是一些示例数据

data = {"user": ["A","B","C","A","B","D"],
        "date": ["2020-01-01","2020-01-02","2020-01-02","2020-01-04","2020-01-04","2020-01-05"],
        "join_date": ["2020-01-01","2020-01-01","2020-01-01","2020-01-01","2020-01-01","2020-01-03"],
        "purchase": ["NaN","NaN","3","10","5","3"]}
df = pd.DataFrame(data, columns= ["user","date","join_date","purchase"])
print(df)

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你如何从数据上升到75%？