tang - 6 months ago 23

Python Question

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

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]]
```