[Tutor] Grouping based on attributes of elements in a List

[Peter Otten]

ranjan das wrote:

> I have the following list
> 
> List=[( 'G1', 'CFS', 'FCL', 'R1' ),('G3', 'LOOSEFREIGHT', 'MIXEDLCL',
> 'R9'),
> ('G4', 'CFS', 'FCL', 'R10' ), ('G2',  'LOOSEFREIGHT', 'LCL', 'R4' ),
> ('G1',
> 'CFS', 'FCL', 'R2' ), ('G2', 'LOOSEFREIGHT', 'LCL', 'R5')  ]
> 
> 
> now I want to group this elements of List  first by index [1] that is (CFS
> and LOOSEFREIGHT ) together and for those elements which are grouped
> together for LOOSEFREIGHT, i want to further divide them into different
> groups based on index[2] that is (LCL or MIXEDLCL)
> 
> 
> So essentially i want them grouped into different lists and my solution
> should be  of the form
> 
> New_List=[ [ ( 'G1', 'CFS', 'FCL', 'R1' ), ('G1', 'CFS', 'FCL', 'R2' ),
> ('G4', 'CFS', 'FCL', 'R10' ) ], [ ('G2',  'LOOSEFREIGHT', 'LCL', 'R4' ),
> ('G2', 'LOOSEFREIGHT', 'LCL', 'R5' )], [ ('G3', 'LOOSEFREIGHT',
> 'MIXEDLCL', 'R9')] ]
> 
> How do I do it?
> 
> I managed to do divide them into different lists based on index [1]
> however
> I was not able to further divide them  based on index [2]
> 
> Any help is appreciated

Assuming you have a function 

def group_list(items, key):
    # ...

that takes a list and a function to extract the key you can apply that 
recursively first on the initial list, then on all partial lists

def group_recursive(items, keys):
    if not keys:
        return items
    firstkey = keys[0]
    rest = keys[1:]
    items = group_list(items, firstkey)
    if rest:
        return [group_recursive(item, rest) for item in items]
    return items

if __name__ == "__main__":
    from operator import itemgetter
    import json

    items = [
        ('G1', 'CFS', 'FCL', 'R1'),
        ('G3', 'LOOSEFREIGHT', 'MIXEDLCL', 'R9'),
        ('G4', 'CFS', 'FCL', 'R10'),
        ('G2', 'LOOSEFREIGHT', 'LCL', 'R4'),
        ('G1', 'CFS', 'FCL', 'R2' ),
        ('G2', 'LOOSEFREIGHT', 'LCL', 'R5')
        ]
    groups = group_recursive(items, [itemgetter(1), itemgetter(2)])
    print json.dumps(groups, indent=2)

One way to implement group_list() is to sort the items and then use 
itertools.groupby() to create the sublists. Another is to put the items into 
a dictionary with key(item) as the dict key and a list of items as the 
value.