Pandas add missing weeks from range to dataframe

I am computing a DataFrame with weekly amounts and now I need to fill it with missing weeks from a provided date range.

This is how I’m generating the dataframe with the weekly amounts:

df['date'] = pd.to_datetime(df['date']) - timedelta(days=6)
weekly_data: pd.DataFrame = (df
                             .groupby([pd.Grouper(key='date', freq='W-SUN')])[data_type]
                             .sum()
                             .reset_index()
                            )

Which outputs:

        date    sum
0 2020-10-11     78
1 2020-10-18    673

If a date range is given as start='2020-08-30' and end='2020-10-30', then I would expect the following dataframe:

        date    sum
0 2020-08-30    0.0
1 2020-09-06    0.0
2 2020-09-13    0.0
3 2020-09-20    0.0
4 2020-09-27    0.0
5 2020-10-04    0.0
6 2020-10-11     78
7 2020-10-18    673
8 2020-10-25    0.0

So far, I have managed to just add the missing weeks and set the sum to 0, but it also replaces the existing values:

weekly_data = weekly_data.reindex(pd.date_range('2020-08-30', '2020-10-30', freq='W-SUN')).fillna(0)

Which outputs:

        date    sum
0 2020-08-30    0.0
1 2020-09-06    0.0
2 2020-09-13    0.0
3 2020-09-20    0.0
4 2020-09-27    0.0
5 2020-10-04    0.0
6 2020-10-11    0.0 # should be 78
7 2020-10-18    0.0 # should be 673
8 2020-10-25    0.0

Answer

Remove reset_index for DatetimeIndex, because reindex working with index and if RangeIndex get 0 values, because no match:

weekly_data = (df.groupby([pd.Grouper(key='date', freq='W-SUN')])[data_type]
                 .sum()
              )

Then is possible use fill_value=0 parameter and last add reset_index:

r = pd.date_range('2020-08-30', '2020-10-30', freq='W-SUN', name='date')
weekly_data = weekly_data.reindex(r, fill_value=0).reset_index()
print (weekly_data)
        date  sum
0 2020-08-30    0
1 2020-09-06    0
2 2020-09-13    0
3 2020-09-20    0
4 2020-09-27    0
5 2020-10-04    0
6 2020-10-11   78
7 2020-10-18  673
8 2020-10-25    0