How to do this "join" for pairing in Pandas?

[Rex]

1. Given following CSV file

     $cat data.csv>     

     ID,City,Zip,Flag

     1,A,95126,0

     2,A,95126,1

     3,A,95126,1

     4,B,95124,0

     5,B,95124,1

     6,C,95124,0

     7,C,95127,1

     8,C,95127,0

     9,C,95127,1

(a) where "ID" above is a primary key (unique), 

(b) for each "City" and "Zip" combination, there is one ID in max with Flag=0; while it can contain multiple IDs with Flag=1 for each "City" and "Zip" combination.

(c) Flag can be 0 or 1

2. For each ID with Flag=0, we want to pair it with another ID with Flag=1 but with the same City - Zip. If one cannot find another paired ID with Flag=1 and matched City - Zip, we just delete that record.

Here is the expected result:

     ID,City,Zip,Flag

     1,A,95126,0

     2,A,95126,1

     4,B,95124,0

     5,B,95124,1

     7,C,95127,1

     8,C,95127,0

Any valuable tips how to do this pairing in Pandas?

Great thanks!