How to compare a list of lists/sets in python?

python list compare set tuples
tang picture tang · May 24, 2011 · Viewed 57k times · Source

What is the easiest way to compare the 2 lists/sets and output the differences? Are there any built in functions that will help me compare nested lists/sets?

Inputs:

First_list = [['Test.doc', '1a1a1a', 1111], 
              ['Test2.doc', '2b2b2b', 2222],  
              ['Test3.doc', '3c3c3c', 3333]
             ]  
Secnd_list = [['Test.doc', '1a1a1a', 1111], 
              ['Test2.doc', '2b2b2b', 2222], 
              ['Test3.doc', '8p8p8p', 9999], 
              ['Test4.doc', '4d4d4d', 4444]]

Expected Output: 

Differences = [['Test3.doc', '3c3c3c', 3333],
               ['Test3.doc', '8p8p8p', 9999], 
               ['Test4.doc', '4d4d4d', 4444]]

Answer

dr jimbob picture dr jimbob · May 24, 2011

So you want the difference between two lists of items.

first_list = [['Test.doc', '1a1a1a', 1111], 
              ['Test2.doc', '2b2b2b', 2222], 
              ['Test3.doc', '3c3c3c', 3333]]
secnd_list = [['Test.doc', '1a1a1a', 1111], 
              ['Test2.doc', '2b2b2b', 2222], 
              ['Test3.doc', '8p8p8p', 9999], 
              ['Test4.doc', '4d4d4d', 4444]]

First I'd turn each list of lists into a list of tuples, so as tuples are hashable (lists are not) so you can convert your list of tuples into a set of tuples:

first_tuple_list = [tuple(lst) for lst in first_list]
secnd_tuple_list = [tuple(lst) for lst in secnd_list]

Then you can make sets:

first_set = set(first_tuple_list)
secnd_set = set(secnd_tuple_list)

EDIT (suggested by sdolan): You could have done the last two steps for each list in a one-liner:

first_set = set(map(tuple, first_list))
secnd_set = set(map(tuple, secnd_list))

Note: map is a functional programming command that applies the function in the first argument (in this case the tuple function) to each item in the second argument (which in our case is a list of lists).

and find the symmetric difference between the sets:

>>> first_set.symmetric_difference(secnd_set) 
set([('Test3.doc', '3c3c3c', 3333),
     ('Test3.doc', '8p8p8p', 9999),
     ('Test4.doc', '4d4d4d', 4444)])

Note first_set ^ secnd_set is equivalent to symmetric_difference.

Also if you don't want to use sets (e.g., using python 2.2), its quite straightforward to do. E.g., with list comprehensions:

>>> [x for x in first_list if x not in secnd_list] + [x for x in secnd_list if x not in first_list]
[['Test3.doc', '3c3c3c', 3333],
 ['Test3.doc', '8p8p8p', 9999],
 ['Test4.doc', '4d4d4d', 4444]]

or with the functional filter command and lambda functions. (You have to test both ways and combine).

>>> filter(lambda x: x not in secnd_list, first_list) + filter(lambda x: x not in first_list, secnd_list)

[['Test3.doc', '3c3c3c', 3333],
 ['Test3.doc', '8p8p8p', 9999],
 ['Test4.doc', '4d4d4d', 4444]]

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