Как посчитать количество общих элементов в списках?

#coding: utf-8
a = [1, 4, 5, 6, 9]
b = [2, 1, 5, 10, 12]
result = []
 
for i in b:
    if i in a:
        result.append(i)
print(len(result))

sum(1 for i in a if i in b)

len(set(a).intersection(b))
# немного короче
len(set(a) & set(b))

a = [1, 1, 1]
b = [1]

#!/usr/bin/env python
#coding: utf-8
 
a = [1,1,1]
b = [1]
 
 
s = [1,2,3,1,1]
c = [4,3,1,5]
 
 
def somelenList(a,b):
  counter = 1
  if len(a) > len(b):
    for x in a:
      if b.count(x):
        counter+=1
  else:
    for x in b:
      if a.count(x):
        counter+=1
  return counter
 
 
print 'All: %d' % somelenList(a,b), a, b
print 'All: %d' % somelenList(s,c), s, c

>>> a = [1,1,1]
>>> b = [1]
>>> print [id(x) for x in a], id(b[0])
[24860600, 24860600, 24860600] 24860600
>>>

[1, 1, 1, 1, 1, 1]
[1, 1, 1, 1]

def some(a,b):
  t=[]
  counter=0
  t.extend(a)
  t.extend(b)
  for x in (set(a) & set(b)):
    if t.count(x):
      counter+=t.count(x)
  return counter

>>> a = [1, 1, 1, 1, 1, 1, 2]
>>> b = [1, 1, 1, 1, 2, 3]
>>> 
>>> def f(lst1, lst2):
...     n = 0
...     s = set(lst1) & set(lst2)
...     for lst in lst1, lst2:
...         for i in lst:
...             if i in s:
...                 n += 1
...     return n
... 
>>> f(a, b)
12
>>>