Multiple statements in list compherensions in Python?

Question

Is it possible to have something like:

list1 = ...

currentValue = 0
list2 = [currentValue += i, i for i in list1]

I tried that but didn\'t wor

Answer 1

As pjz said, you can use functions, so here you can use a closure to keep track of the counter value:

# defines a closure to enclose the sum variable
def make_counter(init_value=0):
    sum = [init_value]
    def inc(x=0):
        sum[0] += x
        return sum[0]
    return inc

Then you do what you want with list1:

list1 = range(5)  # list1 = [0, 1, 2, 3, 4]

And now with just two lines, we get list2:

counter = make_counter(10)  # counter with initial value of 10
list2 = reduce(operator.add, ([counter(x), x] for x in list1))

In the end, list2 contains:

[10, 0, 11, 1, 13, 2, 16, 3, 20, 4]

which is what you wanted, and you can get the value of the counter after the loop with one call:

counter()  # value is 20

Finally, you can replace the closure stuff by any kind of operation you want, here we have an increment, but it's really up to you. Note also that we use a reduce to flatten list2, and this little trick requires you to import operator before calling the line with the reduce:

import operator