Here's the current message:

Python 2.5.2 (r252:60911, Apr 21 2008, 11:12:42) 
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> [foo] = [2, 3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack

It would be good if the message of the ValueError contained information
about how many values were expected and how many values were given.