How to fill a new Dataframe based on another Dataframe [duplicate]

Question

Below is a dataframes for example :

Employee= pd.DataFrame(
    {"EmployeeKey": ["1", "2", "3"], 
     "End_Contract": [None, "2022-02-02", None],
     "Contract": ["35", "35", "35"]}
)

I use a calendar like the following :

Calendar = pd.DataFrame({'Year':[2022,2022,2022], 
                         'Month ':[1,2,3], 
                         'Working day':[21,20,21]})

I try to fill a new dataframe from the last two dataframes.

the final result I'm trying to find

| EmployeeKey | EndContract | Contract| Year   | Month    |  Working day     | 
| --------    | ----------- |-------  |------- | -------  |  --------------    
| 1           | None        |    35   | 2022   |  1       |      21          |
| 1           | None        |    35   | 2022   |  2       |      20          |
| 1           | None        |    35   | 2022   |  3       |      21          |
| 2           | 2022-02-02  |    35   | 2022   |  1       |      21          |
| 2           | 2022-02-02  |    35   | 2022   |  2       |      20          |
| 2           | 2022-02-02  |    35   | 2022   |  3       |      21          |
| 3           | None        |    35   | 2022   |  1       |      21          |
| 3           | None        |    35   | 2022   |  2       |      20          |
| 3           | None        |    35   | 2022   |  3       |      21          |

I can't find a way to get this result !

Answer 1

Use:

cal = pd.DataFrame({'year':[2022,2021], 'month':[2,3], 'working days':[21,21]})
Employee['End_Contract']=pd.to_datetime(Employee['End_Contract'])
Employee['y']=Employee['End_Contract'].dt.year
Employee['m']=Employee['End_Contract'].dt.month
Employee.merge(cal, left_on=['y', 'm'], right_on=['year', 'month'])

Based on your comment, if I understand you well, you can use the how parameter with outer value. From pandas documentation:

outer: use union of keys from both frames, similar to a SQL full outer join; sort keys lexicographically.

So:

Employee.merge(cal, left_on=['y', 'm'], right_on=['year', 'month'], how = 'outer')

The result: