[pygtk] "Rolling Up" Calculations

[Andrew Rowland]

I have a group of information that is related in a hierarchical structure.
 I display the structure in a treeview.  I am trying to roll up various
calculations where, the sum of the lower levels becomes the unit value of
the next higher level.  Below is, hopefully, a helpful example of how the
calculations should roll up.

Description                        Quantity    Unit Cost    Total Cost
System                                      1             21.42
 21.42
   Subsystem 1                        1              12.45          12.45
       Assembly 1                      2                4.85            9.70
           Part 1                            1                0.10
   0.10
           Part 2                            2                1.00
   2.00
           Part 3                            5                0.55
   2.75
       Assembly 2                      1                1.70            1.70
           Part 4                            2                0.55
   1.10
           Part 5                            3                0.20
   0.60
   Subsystem 2                        1                8.97            8.97
       Assembly 3                      1                8.97            8.97
           Subassembly 1            1                8.97            8.97
               Part 6                        1                2.98
  2.98
               Part 7                        1                5.99
  5.99

Clearly, things need to be calculated in the correct order starting with
Part and working up to System.  I am struggling with a good way to "walk"
up the tree making all the calculations at one level before continuing to
the next higher level.  I'm sure it's not as difficult as I've convinced
myself it is.

Thanks in advance for any helpful pointers.

Andrew
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