sequence unpacking in argument list

>>> def foo(a, (b,c)): print a,b,c
...
>>> foo(1,(2,3))
1 2 3
>>> foo(1,())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in foo
ValueError: need more than 0 values to unpack

This feature has probably really confused you if you ever mistakenly put parenthesis around the parameters to a lambda.  In that case the lambda expression will take only one argument, and that argument will be a tuple.

>>> a = lambda (a,b): a+b

>>> a(1,2)

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

TypeError: <lambda>() takes exactly 1 argument (2 given)

>>> a((1,2))
3