熊猫从另一个数据帧填充一个数据帧上的空值

Question

This is something that I would be able to replicate easily in Excel with an XLOOKUP function, but I'm trying to do it with pandas.

I have 2 dataframes, say something like this:

df1

|first_name | last_name | dob      | value |
| Goku      | Saiyan    | 1/1/2021 |       |
| Vegetta   |  Super    | 8/7/1990 |       |
| Gohan     |  Son      | 4/20/1969|       |

df2

|first_name | last_name | dob      | value |
| Goku      | Saiyan    | 1/1/2021 |   50  |
| Vegetta   |  Super    | 8/7/1990 |   92  |
| Gohan     |  Son      | 4/20/1969|   31  |
| Trunks    |  Donald   | 7/1/1921 |   49  |
| New Name  |  Another  | 1/31/1992|   67  |

I would like to fill the value column in df1 from the value column in df2.

I cannot use combine_first because the dataframes have different index and different sizes.

If I use pd.merge then I get the value_x and value_y where value_y has the data that I want, but I need to do more process to have it where I want on df1['value']

I basically want to match the first name, last name and dob on both dataframes and receive the value from df2.

It's probably a simple issue, but I have been struggling with the different methods that I've tried and I think there must be something that I'm missing because it shouldn't be that complicated.

Any help will be really appreciated.

Answer 1

If your value column from df1 does not contain existing value, you can drop it and use merge :

>>> pd.merge(df1.drop(columns='value'), df2, how='left',
             on=['first_name', 'last_name', 'dob'])

  first_name last_name        dob  value
0       Goku    Saiyan   1/1/2021     50
1    Vegetta     Super   8/7/1990     92
2      Gohan       Son  4/20/1969     31

Answer 2

Use map.

Create a dict of firts_name; value from df2 and map to df1's first_name.

df1 =df1.assign(value=df1['first_name'].map(dict(zip(df2['first_name'],df2['value']))))



   first_name last_name        dob  value
0       Goku    Saiyan   1/1/2021     50
1    Vegetta     Super   8/7/1990     92
2      Gohan       Son  4/20/1969     31