List slicing and making a copy

Question

While reading about slicing a list; i got stuck here:-

a = [1, 3, 5]
b = a[:]
a[:] = [x**2 for x in a]
a[:] = [0]
print(b) # output --> [1,3,5]

And this:-

a = [1, 3, 5]
b = a
a[:] = [x**2 for x in a]
a[:] = [0]
print(b) # output --> [0]

I know that b = a[:] is making a copy of list a but then what b=a is doing in the second example? And when printing the outputs, in the first case b doesn't get modified but get modified in second one. What is the reason for this behavior? I am not asking about how to do slicing, but wondering why both the codes mentioned are behaving strangely and differently.

In the second case you set `b` as a reference to `a`, so any changes to `a` also effect `b` — user3483203, Apr 23 '18 at 04:57
what do you mean by reference exactly? In what ways can i use references? — Vicrobot, Apr 23 '18 at 04:59

Richard Inglis · Accepted Answer · 2018-04-23T05:17:51.083

3

b = a is an assignment by reference: it makes the variable b point at the same list that variable a is pointing to. So when you update the contents of that list on the next line, with a[:] = ... then both a and b are pointing to the updated list.

If the next line had been a = [x**2 for x in a] (instead of a[:] = ...) that would have created a new list [1,9,25] and assigned the variable a to point at it, leaving b still pointing at the original list.

edited Apr 23 '18 at 05:17

answered Apr 23 '18 at 05:00

Richard Inglis

5,888
2
33
37

So what is the use of such references? – Vicrobot Apr 23 '18 at 05:04
@Ankit it saves memory, plus a number of other things that I'm sure a google search would let you know about. – user3483203 Apr 23 '18 at 05:05
1

Pass-by reference is very quick and memory-efficient: you only change one memory location instead of many. But there are side-effects that you need to be aware of - hence the examples you're looking at. – Richard Inglis Apr 23 '18 at 05:06
@Ankit for more info go to https://stackoverflow.com/questions/2612802/how-to-clone-or-copy-a-list – Apr 23 '18 at 05:08
Also https://stackoverflow.com/a/986145/98806 – Richard Inglis Apr 23 '18 at 05:08
2

@Ankit don't overthink this- you're a beginner, so for now, just stick to the rule of NOT doing this: `a = b`. – Benjamin James Apr 23 '18 at 05:15

score 1 · Answer 2 · answered Apr 23 '18 at 04:57

When you do b = a, you simply creating a reference to the variable a and this reference is named b. A reference is just another name for the same object. You are not creating a copy of the variable a. Therefore, if you modify b, it modifies a as well and vice versa.

However, if you do b = a[:], you are creating a shallow copy of the variable a and assigning it to a new variable called b. Since b is a copy, modifying either one of them, won't change the other.

That is precisely why you get two different answers when printing b at the end.

List slicing and making a copy

2 Answers2