Python-сообщество

a=[(0,1),(0,2), (2,3)]
sorted(a,
       key=lambda (user_id, num_friends): num_friends,
       reverse=True)

  File "<input>", line 2
    key=lambda (user_id, num_friends): num_friends,
               ^
SyntaxError: invalid syntax

>>> a
[(0, 1), (0, 2), (2, 3)]
>>> sorted(a,key=lambda(user_id, num_friends):num_friends,reverse=True)
  File "<input>", line 1
    sorted(a,key=lambda(user_id, num_friends):num_friends,reverse=True)
                       ^
SyntaxError: invalid syntax

from operator import itemgetter
sort_a=sorted(a,key=itemgetter(1),reverse=True)

sort_a=sorted(a,key=lambda x: x[1],reverse=True)

>>> l = lambda x,y: y
>>> l(x=1,y=2)
2
>>> l(1,2)
2
>>> l((1,2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: <lambda>() missing 1 required positional argument: 'y'

>>> t = lambda (x, y): (x * 2, y * 2)
>>> t((1, 2))
(2, 4)
>>>

>>> t = lambda (x, y): (x * 2, y * 2)
  File "<stdin>", line 1
    t = lambda (x, y): (x * 2, y * 2)
               ^
SyntaxError: invalid syntax
>>>