如何使用 python 根据给定日期获取最新日期？

Question

Consider the following two dataframes:

Dataframe1 contains a list of users and stop_dates

Dataframe2 contains a history of user transactions and dates

I want to get the last transaction date before the stop date for all users in Dataframe1 (some users in Dataframe1 have multiple stop dates)

I want the output to look like the following:

Answer 1

Here is one way to accomplish (make sure both date columns are already datetime):

df = pd.merge(df1, df2, on="UserID")

df["Last_Before_Stop"] = df["Stop_Date"].apply(
    lambda x: max(df["Transaction_Date"][df["Transaction_Date"] < x]) if
    len(df["Transaction_Date"][df["Transaction_Date"] < x]) != 0 else
    pd.nan
)

Answer 2

Please always provide data in a form that makes it easy to use as samples (ie as text, not as images - see here ).

You could try:

df1["Stop_Date"] = pd.to_datetime(df1["Stop_Date"], format="%m/%d/%y")
df2["Transaction_Date"] = pd.to_datetime(df2["Transaction_Date"], format="%m/%d/%y")
df = (
    df1.merge(df2, on="UserID", how="left")
    .loc[lambda df: df["Stop_Date"] >= df["Transaction_Date"]]
    .groupby(["UserID", "Stop_Date"])["Transaction_Date"].max()
    .to_frame().reset_index().drop(columns="Stop_Date")
)

• Make datetime s out of the date columns.
• Merge df2 on df1 along UserID .
• Remove the rows which have a Transaction_Date greater than Stop_Date .
• Group the result by UserID and Stop_Date, and fetch the maximum Transaction_Date .
• Bring the result in shape.

Result for

df1 :

   UserID Stop_Date
0       1    2/2/22
1       2    6/9/22
2       3   7/25/22
3       3   9/14/22

df2 :

   UserID Transaction_Date
0       1           1/2/22
1       1           2/1/22
2       1           2/3/22
3       2          1/24/22
4       2          3/22/22
5       3          6/25/22
6       3          7/20/22
7       3          9/13/22
8       3          9/14/22
9       4           2/2/22

is

   UserID Transaction_Date
0       1       2022-02-01
1       2       2022-03-22
2       3       2022-07-20
3       3       2022-09-14

If you don't want to permanently change the dtype to datetime , and also want the result as string, similarly formatted as the input (with padding), then you could try:

df = (
    df1
    .assign(Stop_Date=pd.to_datetime(df1["Stop_Date"], format="%m/%d/%y"))
    .merge(
        df2.assign(Transaction_Date=pd.to_datetime(df2["Transaction_Date"], format="%m/%d/%y")),
        on="UserID", how="left"
    )
    .loc[lambda df: df["Stop_Date"] >= df["Transaction_Date"]]
    .groupby(["UserID", "Stop_Date"])["Transaction_Date"].max()
    .to_frame().reset_index().drop(columns="Stop_Date")
    .assign(Transaction_Date=lambda df: df["Transaction_Date"].dt.strftime("%m/%d/%y"))
)

Result:

   UserID Transaction_Date
0       1         02/01/22
1       2         03/22/22
2       3         07/20/22
3       3         09/14/22