Setting two arrays equal [duplicate]

Question

array1=[0,1,2]
array2=array1
array2[0]=234234
print array1

OUTPUT:

[234234, 1, 2]

Why does python change 'array1'? Shouldn't it just change array2? How can I prevent python from changing array1 when I change array2?

Answer 1

array1 and array2 are the same object. That's why changing either changes the other. If you want to copy the object, here's one way to do it:

array2 = array1[:]

See more on this here.

Answer 2

Use slice notation to copy like this

array2 = array1[:]

Or you can use list function

array2 = list(array1)

When you assign one list to another list, a new list will not be created but both the variables will be made to refer the same list. This can be confirmed with this program.

array1 = [1, 2, 3, 4]
array2 = array1
print id(array1), id(array2)

They both will print the same id. It means that they both are the same (If you are from C background you can think of them as pointers (In CPython implementation they really are pointers, other implementations choose to print unique ids - Please check kojiro's comment)). Read more about id here. When you do

array3 = array1[:]
array4 = list(array1)
print id(array1), id(array3), id(array4)

you ll get different ids, because new lists will be created in these cases.

Answer 3

array1 and array2 are two names for the same list, since that's how you set them. If you don't want this, copy the list using one of the following methods:

array2 = array1[:] 
array2 = list(array1)