Best way to find the intersection of multiple sets

Question

I have a list of sets   setlist    s1 s2 s3       I want s1 n s2 n s3      I can write a function to do it by performing a series of pairwise s1 intersection s2   etc   Is there a recommended  better  or built-in way

User · Answer

Clearly set intersection is what you want here  but in case you ever need a generalisation of  take the sum of all these    take the product of all these    take the xor of all these   what you are looking for is the reduce function   from operator import and  from functools import reduce print reduce and     1 2 3   2 3 4   3 4 5          3    or  print reduce  lambda x y  x amp y     1 2 3   2 3 4   3 4 5          3

User · Answer

As of 2 6  set intersection takes arbitrarily many iterables     gt  gt  gt  s1   set  1  2  3    gt  gt  gt  s2   set  2  3  4    gt  gt  gt  s3   set  2  4  6    gt  gt  gt  s1  amp  s2  amp  s3 set  2    gt  gt  gt  s1 intersection s2  s3  set  2    gt  gt  gt  sets    s1  s2  s3   gt  gt  gt  set intersection  sets  set  2

User · Answer

I believe the simplest thing to do is   assuming three sets set1    1 2 3 4 5  set2    2 3 8 9  set3    2 10 11 12    intersection set4   set1  amp  set2  amp  set3  set4 will be the intersection of set1   set2  set3 and will contain the value 2  print set4   set  2

User · Answer

Here I m offering a generic function for multiple set intersection trying to take advantage of the best method available   def multiple set intersection  sets          Return multiple set intersection         try          return set intersection  sets      except TypeError    this is Python  lt  2 6 or no arguments         pass      try  a set  sets 0      except IndexError    no arguments         return set     return empty set      return reduce a set intersection  sets 1      Guido might dislike reduce  but I m kind of fond of it

User · Answer

If you don t have Python 2 6 or higher  the alternative is to write an explicit for loop   def set list intersection set list     if not set list      return set     result   set list 0    for s in set list 1        result  amp   s   return result  set list    set  1  2    set  1  3    set  1  4    print set list intersection set list    Output  set  1     You can also use reduce   set list    set  1  2    set  1  3    set  1  4    print reduce lambda s1  s2  s1  amp  s2  set list    Output  set  1     However  many Python programmers dislike it  including Guido himself      About 12 years ago  Python aquired lambda  reduce    filter   and map    courtesy of  I believe  a Lisp hacker who missed them and submitted working patches  But  despite of the PR value  I think these features should be cut from Python 3000       So now reduce    This is actually the one I ve always hated most  because  apart from a few examples involving   or    almost every time I see a reduce   call with a non-trivial function argument  I need to grab pen and paper to diagram what s actually being fed into that function before I understand what the reduce   is supposed to do  So in my mind  the applicability of reduce   is pretty much limited to associative operators  and in all other cases it s better to write out the accumulation loop explicitly

User · Answer

From Python version 2 6 on you can use multiple arguments to set intersection    like  u   set intersection s1  s2  s3    If the sets are in a list  this translates to   u   set intersection  setlist    where  a list is list expansion  Note that set intersection is not a static method  but this uses the functional notation to apply intersection of the first set with the rest of the list  So if the argument list is empty this will fail

User · Answer

Jean-Fran  ois Fabre set intesection  list of sets  answer is definetly the most Pyhtonic and is rightly the accepted answer   For those that want to use reduce  the following will also work   reduce set intersection  list of sets

[python] Best way to find the intersection of multiple sets?

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