>>> funcs = []
>>> for x in xrange(3):
>>> funcs.append(lambda: x)
>>> sum(f() for f in funcs)
6
Were you expecting 3? Lambdas (besides having such an awfully long and difficult to type name :P) capture only the "name" of x, so when the for loop changes x later, the lambda sees the new value--so the sum is (2 + 2 + 2) instead of (1 + 2 + 3).
What you can do, instead (and I do it often enough that I wish lambdas created their own lexical closure....)
>>> funcs = []
>>> for x in xrange(3):
>>> funcs.append(lambda x=x: x)
>>> sum(f() for f in funcs)
3
By using x as a default argument, you force the x inside the lambda to be the value it was when the lambda was created. It kinda sucks, and always felt to me like a deficiency of the implementation leaking up into the language :[
