While iterrating over the `shop_id`, I am trying to get the `shop_id` values in a multi-index DataFrame:

```
                         total_sales
shop_id date_block_num              
0       0               3.034985e+09
        1               3.547803e+09

```

So I tried:

```
for i, shop_sales in monthly_change.groupby(level=0):
    print(shop_sales['shop_id'])

```

But it returned a key error.

I also tried `shop_sales.index` but I don't understand how I could use the results

```
MultiIndex([(0, 0),
            (0, 1)],
           names=['shop_id', 'date_block_num'])

```