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?
15
votes
Welcome to StackOverflow. Don't worry too much about the downvoters – some people think that every duplicate or beginner question should be downvoted. But besides being a duplicate this is a fine question: it's clear what the question is and it shows pithy code that clearly demonstrates the problem.
- kojiro
3 Answers
13
votes
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.
4
votes
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.
