Elements in common between lists

Question

How do I compare elements from 2 lists, and return the amount of elements in common ? For example:

a=[1, 2, 3, 4, 5]
b=[6, 7, 8, 9, 10, 1, 2, 11, 22]

This should return 2, because there are only 2 elements in common.

Answer 1

Just make the intersection of the two lists and then see the number of elements.

comum=len(set(a).intersection(b))

Answer 2

Set a variable on a repeated worthwhile 0 who will serve as an accountant:
>>> num_repetido = 0

Loop the list to see if any of the numbers repeat and then add in the repeated num_variable:

 for num in a:
  if num in b:
   num_repetido += 1

And finally print on the screen the final value of the variable on a repeated:
>>> print(num_repetido)

It is worth noting that there are several ways to do this, as the friend above put it, everything depends on what you need. Hug.