如何基于python中的where函数获取两列值

问题是：
基于user_id列,我想获取rating和product_id列的值.可以有多个具有相同user_id的条目.我想获取所有用户的记录,并具有rating和product_id列的值,但是对于用户未评分的电影,该电影应显示为Nan,但仍应检索product_id.以下是提供一些数据的表.

| product_id  | user_id         | user_name                                    | rating |
|-------------|-----------------|----------------------------------------------|--------|
|  B0009XRZ92 | A2JFZLAUG3YFQ7  |  Entropy Babe "EB"                           | 5      |
|  B0009XRZ92 | A22HGAAO8KZ2N3  |  R.   Metzelar                               | 5      |
|  B000067A8B |  A2NJO6YE954DBH |  Lawrance M. Bernabo                         | 4      |
|  B0009XRZ92 |  A3HE4MYMWK4AER |  Rebecca M. Eddy "Foster Mom and   Untbunny" | 5      |
|  B003A3R3ZY | A9A2PR663ED1V   |  Roger D. Goff                               | 5      |
|  B0009XRZ92 | A2MRZDJF90JC1U  |  Suzanne K. Armstrong "Suzy Q"               | 5      |
|  B0009XRZ92 |  A2YNBDT3170PCR |  C.   O'Hern                                 | 5      |
|  B0009XRZ92 |  A10VJ7BDVCPKEZ |  Carol S. Bottom                             | 5      |
|  B0009XRZ92 |  AAAQO894MG80B  |  Paul J. Michko                              | 5      |
|  B00067BBQE | A9A2PR663ED1V   |  Roger D. Goff                               | 5      |
|  B0009XRZ92 | A31S5QUMFR8NH2  |  Dana L. Jordan "Mom of Twins"               | 5      |
|  B0009XRZ92 |  A2DS24DHXUH0GM |  Gaz    Rev(iewer)                           | 4      |
|  B00006AUMZ |  A2NJO6YE954DBH |  Lawrance M. Bernabo                         | 4      |
|  B0009XRZ92 |  A16FRHL2ZC7EUR |  M.   Claytor                                | 5      |
|  B0009XRZ92 | A3AV8R0A62PP1N  |  MARCUSHELBLINZ "mmmacman"                   | 5      |
|  B0009XRZ92 |  A3QN84C38DE9FU |  Gillian M. Kratzer                          | 5      |
|  B0009XRZ92 |  A36MLTLVQFEQYL |  Yossarian "alienated socialist"             | 5      |
|  B00006AUMD |  A2NJO6YE954DBH |  Lawrance M. Bernabo                         | 4      |

What I want to do is:

To take one user_id at a time and display the rating and product_id
columns value for that user for all the movies in the table and if the
user didn’t rate some movies then the record should be displayed with
the product_id value and rating as Nan and the whole process should be repeated for all the users.

例如,user_id：A2NJO6YE954DBH的记录将如下所示：

| product_id | rating |
|------------|--------|
| B000067A8B | 4      |
| B00006AUMD | 4      |
| B00006AUMD | 4      |
| B0009XRZ92 | Nan    |
| B003A3R3ZY | Nan    |
| B00067BBQE | Nan    |
|     .      |   .    |
|     .      |   .    |
|     .      |    .   |

我尝试使用Pandas Library为此编写代码,但无济于事.这就是我所做的全部,但未输出我想要的.

import pandas as pd
df =pd.read_csv('out.csv')
unique_users=df.user_id.unique()
for x,y in enumerate(unique_users):
    print(df[['rating','product_id']].where(df.user_id==y))

请帮帮我..谢谢