How to do sub-sorting in python?

Question

Many thanks to the SO community for helping me with the previous problems I had encountered. Love the help here!

I have yet another problem now. I have a flat list of DNA sequences that have associated "Construct Number" and "Part Number. As things stand right now, from my previous code, I have it as a csv file which I open up, read in, and import as a list of dictionary objects. Everything is sorted by "Construct Number" already, but I need to then sort by "Part Number". (It's sort of like in Excel, where they say "First sort by and then sort by _."

Does anybody know how to get this done? Thus far, all I have written is this:

primers_list = open('primers-list.csv', 'rU')
primers_unsorted = csv.DictReader(primers_list)
for row in primers_unsorted:
    print(row)

A subset of the output thus far is just the following, for visualization of the data I'm working with:

{' Direction': 'fw primer', ' Construct Number': '1', ' Part Number': '2', 'Primer Sequence': 'AAGCGGCCGCTCGAGTCTAAgctcactcaaaggcggtaatcagataaaaaaaatccttag'}
{' Direction': 're primer', ' Construct Number': '1', ' Part Number': '1', 'Primer Sequence': 'attaccgcctttgagtgagcTTAGACTCGAGCGGCCGCTTTTTGACACCAGACCAACTGG'}
{' Direction': 'fw primer', ' Construct Number': '1', ' Part Number': '1', 'Primer Sequence': 'TTTAATTACTAACTTTATCTATGATAGATCCCGTCGTTTTACAACGTCGTGACTGGGAAA'}
{' Direction': 're primer', ' Construct Number': '1', ' Part Number': '2', 'Primer Sequence': 'AAAACGACGGGATCTATCATAGATAAAGTTAGTAATTAAACTTAAAAGTTGTTTAATGTC'}
{' Direction': 'fw primer', ' Construct Number': '2', ' Part Number': '2', 'Primer Sequence': 'gtaaatccaagttgtaataatactagagTAGCATAACCCCTTGGGGCCTCTAAACGGGTC'}
{' Direction': 're primer', ' Construct Number': '2', ' Part Number': '1', 'Primer Sequence': 'GGGGTTATGCTActctagtattattacaacttggatttaccacctttcttcgccttgatc'}
{' Direction': 'fw primer', ' Construct Number': '2', ' Part Number': '1', 'Primer Sequence': 'TACGACTCACTATAGGGAGAtactagagttaaggaggtaaaaaaaatgggtccggtcgtt'}
{' Direction': 're primer', ' Construct Number': '2', ' Part Number': '2', 'Primer Sequence': 'ttacctccttaactctagtaTCTCCCTATAGTGAGTCGTATTACTCTAGAAGCGGCCGCg'}
{' Direction': 'fw primer', ' Construct Number': '3', ' Part Number': '2', 'Primer Sequence': 'gtaaatccaagttgtaataatactagagTAGCATAACCCCTTGGGGCCTCTAAACGGGTC'}
{' Direction': 're primer', ' Construct Number': '3', ' Part Number': '1', 'Primer Sequence': 'GGGGTTATGCTActctagtattattacaacttggatttaccacctttcttcgccttgatc'}
{' Direction': 'fw primer', ' Construct Number': '3', ' Part Number': '1', 'Primer Sequence': 'TAACTATCACTATAGGGAGAtactagagttaaggaggtaaaaaaaatgggtccggtcgtt'}
{' Direction': 're primer', ' Construct Number': '3', ' Part Number': '2', 'Primer Sequence': 'ttacctccttaactctagtaTCTCCCTATAGTGATAGTTATTACTCTAGAAGCGGCCGCg'}

Answer 1

Another way:

import operator
primers_unsorted.sort(key=operator.itemgetter(' Construct Number', ' Part Number'))

for row in primers_unsorted:
    print(row)

Answer 2

If you want to do it block by block you can do something like:

a=0
while a<len(primers_list):
    b=a
    current_construct=primers_list['Construct Number']
    while primers_list[b]['Construct Number']==current_construct:
         b=b+1
    primers_list[a:b]=sorted( primers_list[a:b] , key = lambda e: (e[' Construct Number'],e[' Part Number']))
    a=b

which might be useful if the list is very long.

Answer 3

My final code is this, and it worked out perfectly:

primers_list = open('primers-list.csv', 'rU')
primers_unsorted = csv.DictReader(primers_list)
primers_sorted = sorted(primers_unsorted, key=operator.itemgetter('Construct Number', 'Part Number'))
for row in primers_sorted:
    print(row)

The key (pardon the pun) to this was to make use operator.itemgetter(...), which accepts as many arguments as needed. It's passed into the 'key' argument in sorted(...).

Many thanks to Eric for answering my question!