I want to use my reference calendar as a scaffold to fill up missing data in my main data. For that purpose I want to join these two dataframes.

```
import pandas as pd
import numpy as np
d1 = { 'Year': [2019,2019,2019,2019,2019,2019],
'Week': [1,2,3,5,5,6],
'Part': ['A','A','A','A','B','B'],
'Static': [20,20,20,20,40,40],
'Value': [np.nan,10,np.nan,50,30,np.nan] }
d2 = { 'Year':[2019,2019,2019,2019,2019,2019,2019,2019,2019,2019],
'Week':[1,2,3,4,5,6,7,8,9,10] }
df1 = pd.DataFrame(d1)
df2 = pd.DataFrame(d2)
```

The expected output is as follows

```
Year Week Part Static Value
0 2019 1 A 20 NaN
1 2019 2 A 20 10.0
2 2019 3 A 20 NaN
3 2019 4 A 20 NaN
4 2019 5 A 20 50.0
5 2019 6 A 20 NaN
6 2019 7 A 20 NaN
7 2019 8 A 20 NaN
8 2019 9 A 20 NaN
9 2019 10 A 20 NaN
10 2019 1 B 40 NaN
11 2019 2 B 40 NaN
12 2019 3 B 40 NaN
13 2019 4 B 40 NaN
14 2019 5 B 40 30.0
15 2019 6 B 40 NaN
16 2019 7 B 40 NaN
17 2019 8 B 40 NaN
18 2019 9 B 40 NaN
19 2019 10 B 40 NaN
```