How do I create a list of Python lambdas (in a list comprehension/for loop)?

Question

I want to create a list of lambda objects from a list of constants in Python; for instance:

listOfNumbers = [1,2,3,4,5]
square = lambda x: x * x
listOfLambdas = [lambda: square(i) for i in listOfNumbers]

This will create a list of lambda objects, however, when I run them:

for f in listOfLambdas:
    print f(),

I would expect that it would print

1 4 9 16 25

Instead, it prints:

25 25 25 25 25

It seems as though the lambdas have all been given the wrong parameter. Have I done something wrong, and is there a way to fix it? I'm in Python 2.4 I think.

EDIT: a bit more of trying things and such came up with this:

listOfLambdas = []
for num in listOfNumbers:
    action = lambda: square(num)
    listOfLambdas.append(action)
    print action()

Prints the expected squares from 1 to 25, but then using the earlier print statement:

for f in listOfLambdas:
    print f(),

still gives me all 25s. How did the existing lambda objects change between those two print calls?

Related question: Why results of map() and list comprehension are different?

Answer 1

You have:

listOfLambdas = [lambda: i*i for i in range(6)]

for f in listOfLambdas:
    print f()

Output:

25
25
25
25
25
25

You need currying! Aside from being delicious, use this default value "hack".

listOfLambdas = [lambda i=i: i*i for i in range(6)]

for f in listOfLambdas:
    print f()

Output:

0
1
4
9
16
25

Note the i=i. That's where the magic happens.