Iterating through list and comparing dicts inside of list

Question

I have a list like this:

[(datetime.date(2014, 4, 15), {'first_name': 'Mike', 'last_name': 'Jackson'}),  
(datetime.date(2014, 4, 15), {'first_name': 'Hannah', 'last_name': 'Jackson'}),
(datetime.date(2014, 4, 16), {'first_name': 'Tom', 'last_name': 'Jackson'},  
(datetime.date(2014, 4, 16), {'first_name': 'Macy', 'last_name': 'Jackson'})]

I want to iterate trough this list so that it compares these dicts to each other. Then I want to make own lists of the dicts that has the same date. How can I do this?

The outcome should be this:

List n

[(datetime.date(2014, 4, 15), {'first_name': 'Mike', 'last_name': 'Jackson'}),  
(datetime.date(2014, 4, 15), {'first_name': 'Hannah', 'last_name': 'Jackson'})]

List n+1

[(datetime.date(2014, 4, 16), {'first_name': 'Tom', 'last_name': 'Jackson'},  
(datetime.date(2014, 4, 16), {'first_name': 'Macy', 'last_name': 'Jackson'})]

Answer 1

Given a is your list, this solution will work:

from itertools import groupby
for k, g in groupby(sorted(a, key=lambda x: x[0]), key=lambda x: x[0]):
    print(list(g))

and produce this output

[(datetime.date(2014, 4, 15), {'first_name': 'Mike', 'last_name': 'Jackson'}),
 (datetime.date(2014, 4, 15), {'first_name': 'Hannah', 'last_name': 'Jackson'})]
[(datetime.date(2014, 4, 16), {'first_name': 'Tom', 'last_name': 'Jackson'}),
 (datetime.date(2014, 4, 16), {'first_name': 'Macy', 'last_name': 'Jackson'})]

Answer 2

There's a facility for this task called groupby. But it uses iterators and depends on the fact that items to be grouped together are already in order, so you might need to sort the input before applying it, depending on is "sortedness" of course.

from itertools import groupby

v = [ (datetime.date(2014, 4, 15), {'first_name': 'Mike', 'last_name': 'Jackson'}),
      (datetime.date(2014, 4, 15), {'first_name': 'Hannah', 'last_name': 'Jackson'}),
      (datetime.date(2014, 4, 16), {'first_name': 'Tom', 'last_name': 'Jackson'}),
      (datetime.date(2014, 4, 16), {'first_name': 'Macy', 'last_name': 'Jackson'}) ]
v.sort()   # to ensure that all equal datetimes are together in the list
result = [ list(i) for x, i in itertools.groupby(v, lambda a: a[0]) ]

result will then be this:

[ [ (datetime.date(2014, 4, 15), {'first_name': 'Mike', 'last_name': 'Jackson'}),
    (datetime.date(2014, 4, 15), {'first_name': 'Hannah', 'last_name': 'Jackson'}) ],
  [ (datetime.date(2014, 4, 16), {'first_name': 'Tom', 'last_name': 'Jackson'}),
    (datetime.date(2014, 4, 16), {'first_name': 'Macy', 'last_name': 'Jackson'}) ] ]

Which is what you wanted.