Pandas- adding missing dates to DataFrame while keeping column/index values?

Question

I have a pandas dataframe that incorporates dates, customers, items, and then dollar value for purchases.

   date     customer   product   amt  
 1/1/2017   tim         


        

Answer 1

Let's use product from itertools, pd.date_range, and merge:

from itertools import product

daterange = pd.date_range(df['date'].min(), df['date'].max(), freq='D')
d1 = pd.DataFrame(list(product(daterange, 
                               df['customer'].unique(),
                               df['product'].unique())), 
                  columns=['date', 'customer', 'product'])
d1.merge(df, on=['date', 'customer', 'product'], how='left').fillna(0)

Output:

         date customer product  amt
0  2017-01-01      tim   apple  3.0
1  2017-01-01      tim   melon  0.0
2  2017-01-01      tim  orange  0.0
3  2017-01-01      jim   apple  0.0
4  2017-01-01      jim   melon  2.0
5  2017-01-01      jim  orange  0.0
6  2017-01-01      tom   apple  5.0
7  2017-01-01      tom   melon  4.0
8  2017-01-01      tom  orange  0.0
9  2017-01-02      tim   apple  0.0
10 2017-01-02      tim   melon  0.0
11 2017-01-02      tim  orange  0.0
12 2017-01-02      jim   apple  0.0
13 2017-01-02      jim   melon  0.0
14 2017-01-02      jim  orange  0.0
15 2017-01-02      tom   apple  0.0
16 2017-01-02      tom   melon  0.0
17 2017-01-02      tom  orange  0.0
18 2017-01-03      tim   apple  0.0
19 2017-01-03      tim   melon  0.0
20 2017-01-03      tim  orange  0.0
21 2017-01-03      jim   apple  0.0
22 2017-01-03      jim   melon  0.0
23 2017-01-03      jim  orange  0.0
24 2017-01-03      tom   apple  0.0
25 2017-01-03      tom   melon  0.0
26 2017-01-03      tom  orange  0.0
27 2017-01-04      tim   apple  0.0
28 2017-01-04      tim   melon  3.0
29 2017-01-04      tim  orange  0.0
30 2017-01-04      jim   apple  2.0
31 2017-01-04      jim   melon  0.0
32 2017-01-04      jim  orange  0.0
33 2017-01-04      tom   apple  0.0
34 2017-01-04      tom   melon  1.0
35 2017-01-04      tom  orange  4.0