Question

In Python, I have the following list of maps:

[{'CN': 'SC',
  'LB': 'g1k',
  'SM': 'HG1'},
 {'CN': 'SC',
  'LB': 'g2k',
  'SM': 'HG2'},
 {'CN': 'SC',
  'LB': 'g3k',
  'SM': 'HG3'}]

and I would like to obtain a map like this:

{ 'CN' : 'SC',
  'LB' : ['g1k', 'g2k', 'g3k'],
  'SM' : ['HG1', 'HG2', 'HG3']
}

What is the most pythonic way to do this?
Thanks!

p.s. Further, I intend to convert the new structure of data to JSON in order to be displayed in a web page.

Answer 1

>>> list_of_map = [{'CN': 'SC',
...   'LB': 'g1k',
...   'SM': 'HG1'},
...  {'CN': 'SC',
...   'LB': 'g2k',
...   'SM': 'HG2'},
...  {'CN': 'SC',
...   'LB': 'g3k',
...   'SM': 'HG3'}]
>>> from collections import defaultdict
>>> d = defaultdict(set)
>>> for map in list_of_map:
...     for k,v in map.items():
...         d[k].add(v)
... 
>>> d
defaultdict(<type 'set'>, {'LB': set(['g3k', 'g1k', 'g2k']), 'CN': set(['SC']), 'SM': set(['HG2', 'HG3', 'HG1'])})

This isn’t quite the data structure you wanted, but it’s pretty close and would be easy to change over if you really needed to. (I haven’t shown it here though as I think that sets are the way to go here instead of lists).

To get back to lists:

>>> back_to_lists = {k:list(v) for k,v in d.items()}
>>> back_to_lists
{'LB': ['g3k', 'g1k', 'g2k'], 'CN': ['SC'], 'SM': ['HG2', 'HG3', 'HG1']}

Or even:

>>> back_to_lists = {k:sorted(v) for k,v in d.items()}
>>> back_to_lists
{'LB': ['g1k', 'g2k', 'g3k'], 'CN': ['SC'], 'SM': ['HG1', 'HG2', 'HG3']}