我正在尝试为组填充缺少的值（区号、商店名称、商品名称、日期、销售额）。对于每个组，我需要用52周的数据填写销售金额。我需要用ffill（df.ffill（））和bfill（df.bfill（））创建两个不同的列，然后我需要用ffill&bfill/2对新创建的列求和以获得结果。

area_code   shop_name   item_name   week_date   sales_amount
101 Global Market   Mango Fruits    6/3/2018    5.13
101 Global Market   Mango Fruits    6/10/2018   nan
101 Global Market   Mango Fruits    6/17/2018   7.13
101 Global Market   Chips   6/3/2018    5
101 Global Market   Chips   6/10/2018   nan
102 Global Market   Mango Fruits    6/3/2018    10.34
102 Global Market   Mango Fruits    6/10/2018   nan
102 Global Market   Chips   6/10/2018   nan
102 Global Market   Chips   6/17/2018   nan
102 Global Market   Chips   6/24/2018   nan
102 Global Market   Potato  6/24/2018   nan

After

area_code   shop_name   item_name   week_date   sales_amount
101 Global Market   Mango Fruits    6/3/2018    5.13
101 Global Market   Mango Fruits    6/10/2018   6.13
101 Global Market   Mango Fruits    6/17/2018   7.13
101 Global Market   Chips   6/3/2018    5
101 Global Market   Chips   6/10/2018   5
102 Global Market   Mango Fruits    6/3/2018    10.34
102 Global Market   Mango Fruits    6/10/2018   10.34
102 Global Market   Chips   6/10/2018   Value available before this week for this group
102 Global Market   Chips   6/17/2018   Value available before this week for this group
102 Global Market   Chips   6/24/2018   Value available before this week for this group
102 Global Market   Potato  6/24/2018   Value available before this week for this group

For example - 
Week 1 10
Week 2 nan
week 3 nan

“本周前本组可用值”表示第3周、第2周的值与第1周的值相同。否则，如果第1周和第3周有数据，则根据ffill或bfill填写第2周。如果不喜欢这样，那么只需为每个组填充ffill或bfill值。
如何迭代Dataframe？
如何迭代每个组并填充值？
我试着用，但没有得到任何运气
我需要填写的一周数据从2018年6月3日开始，到2019年6月3日结束
Pandas：用每组的平均值来填充缺失的值