Question

I have 2 lists of instances:

list1
list2

each instance contains variables such as id, name, etc…

I am iterating through list2, and I want to find entries that don’t exist in list1.

eg..

for entry in list2:
  if entry.id in list1:
    <do something> 

I’m hoping to find a way to do this without a douple for loop. Is there an easy way?

Answer 1

I might do something like:

set1 = set((x.id,x.name,...) for x in list1)
difference = [ x for x in list2 if (x.id,x.name,...) not in set1 ]

where ... is additional (hashable) attibutes of the instance — You need to include enough of them to make it unique.

This takes your O(N*M) algorithm and turns it into an O(max(N,M)) algorithm.