How do I operate on the actual object, not a copy, in a python for loop?

Question

let's say I have a list

a = [1,2,3]

I'd like to increment every item of that list in place. I want to do something as syntactically easy as

for item in a:
    item += 1

but in that example python uses just the value of item, not its actual reference, so when I'm finished with that loop a still returns [1,2,3] instead of [2,3,4]. I know I could do something like

a = map(lambda x:x+1, a)

but that doesn't really fit into my current code and I'd hate to have to rewrite it :-\

Answer 1

Here ya go:

# Your for loop should be rewritten as follows:
for index in xrange(len(a)):
    a[index] += 1

Incidentally, item IS a reference to the item in a, but of course you can't assign a new value to an integer. For any mutable type, your code would work just fine:

>>> a = [[1], [2], [3], [4]]
>>> for item in a: item += [1]
>>> a
[[1,1], [2,1], [3,1], [4,1]]

Answer 2

In python integers (and floats, and strings, and tuples) are immutable so you have the actual object (and not a copy), you just can't change it.

What's happening is that in the line: item += 1 you are creating a new integer (with a value of item + 1) and binding the name item to it.

What you want to do, is change the integer that a[index] points to which is why the line a[index] += 1 works. You're still creating a new integer, but then you're updating the list to point to it.

As a side note:

for index,item  in enumerate(a):
    a[index] = item + 1

... is slightly more idiomatic than the answer posted by Triptych.

Answer 3

Instead of your map-based solution, here's a list-comprehension-based solution

a = [item + 1 for item in a]