I have a list of ~3000 items. Let's call it `listA`. 
And another list with 1,000,000 items. Let's call it `listB`. 

I want to check how many items of `listA` belong in `listB`. For example to get an answer like `436`. 

The obvious way is to have a nested loop looking for each item, but this is slow, especially due to the size of the lists. 

What is the fastest and/or Pythonic way to get the number of the items of one list belonging to another?