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

I'm guessing that the lambda you're creating in the list comprehension is bound to the variable i which eventually ends up at 5. Thus, when you evaluate the lambdas after the fact, they're all bound to 5 and end up calculating 25. The same thing is happening with num in your second example. When you evaluate the lambda inside the loop it's num hasn't changed so you get the right value. After the loop, num is 5...

I'm not quite sure what you're going for, so I'm not sure how to suggest a solution. How about this?

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

This gives me the expected output:

1
4
9
16
25

Another way to think of this is that a lambda "captures" its lexical environment at the point where it is created. So, if you give it num it doesn't actually resolve that value until its invoked. This is both confusing and powerful.