Question

I have several lists in python, and I would like to take only values which are in each list, is there any function to do it directly?

for example I have:

{'a','b','c','d','e'},{'a','g','c','d','h','e'}, {'i','b','m','d','e','a'}

and I want to make one list which contains

{'a','d','e'}

but i don’t know how many lists I actually have, cause it’s dependent on value ‘i’.

thanks for any help!

Answer 1

if the elements are unique and hashable (and order doesn’t matter in the result), you can use set intersection: e.g.:

 common_elements = list(set(list1).intersection(list2).intersection(list3))

This is functionally equivalent to:

 common_elements = list( set(list1) & set(list2) & set(list3) )

The & operator only works with sets whereas the the intersection method works with any iterable.

If you have a list of lists and you want the intersection of all of them, you can do this easily:

 common_elements = list( set.intersection( * map(set, your_list_of_lists) ) )

special thanks to DSM for pointing this one out

Or you could just use a loop:

 common_elements = set(your_list_of_lists[0])
 for elem in your_list_of_lists[1:]:
     common_elements = common_elements.intersection(elem)  #or common_elements &= set(elem) ...
 else:
     common_elements = list(common_elements)

Note that if you really want to get the order that they were in the original list, you can do that using a simple sort:

common_elements.sort( key = lambda x, your_list_of_lists[0].index(x) )

By construction, there is no risk of a ValueError being raised here.