understanding python variable assignment [duplicate]

Question

I'm trying to teach myself python (and programming in general) and am a little confused about variable assignment. I understand that if I have

>>> a = [1,2,3]
>>> b = a

that b refers to the same object in memory as a does. So if I wanted to create a new list, b, with the same values as a currently has, how would I achieve that?

Also, consider this example:

>>> a = [1, 2, 3]
>>> b = a
>>> x = a[1]
>>> a[1] = 4
>>> print a, b, x
[1, 4, 3] [1, 4, 3] 2

I see from this example, that x is a new object but b points to a. Could someone explain to me what is going on here, why x is a new object but b isn't?

Answer 1

Consider this example:

In [20]: a = [[1], [2], [3]]

In [21]: b = a

In [22]: x = a[1]

In [23]: a
Out[23]: [[1], [2], [3]]

In [24]: b
Out[24]: [[1], [2], [3]]

In [25]: x
Out[25]: [2]

In [26]: a[1][0] = 4

In [27]: a
Out[27]: [[1], [4], [3]]

In [28]: b
Out[28]: [[1], [4], [3]]

In [29]: x
Out[29]: [4]

The difference here is that when we tinkered around with a[1] we did so by modifying it instead of telling a[1] to refer to a whole new thing.

In your case, when you told x to refer to whatever a[1] refers to, it picked up a reference to some concrete thing, whatever was in a[1] at the time, in your case a specific integer.

Later when you told a[1] to change, it did change. But the thing it used to refer to did not stop existing (because x was still there referring to it).

By saying x = a[1] you are not saying x shall always refer to whatever a[1] refers to.

You are saying x shall refer to whatever a[1] refers to at this moment of assignment.

The same is true for b also, it's just that the "concrete" thing that b was told to refer to was a whole list -- which can having changing contents.

Another example:

a = [1, 2, 3,]
b = a

print a, b

# Prints
#[1, 2, 3]
#[1, 2, 3]

a = [4, 5, 6]

print a, b

# Prints
#[4, 5, 6]
#[1, 2, 3]