Values for a list variable is changing despite there being no explicit changes

Question

The variable 'operations' which is a list is changing after the for loop is run despite their being no line that explicitly says for its value to change. Here is my code:

validOperations = ['(', ')', '^', '*', '/', '+', '-']
operations = ['+', '*', '/']
newOp = operations  

for y in range(len(newOp) - 1):
        for z in range(len(newOp) - 1):
                if(validOperations.index(newOp[z]) > validOperations.index(newOp[z+1])):
                        oldVal = newOp[z]
                        newOp[z] = newOp[z+1]
                        newOp[z+1] = newOp[z]
                        print(newOp)
                        print(operations)

What can I do to make it to where the value of operations stays constant?

`newOp = operations` doesn't make a copy. See http://nedbatchelder.com/text/names.html — user2357112, Nov 16 '16 at 20:18

score 2 · Answer 1 · edited May 23 '17 at 11:53

2

newOP is not a copy of operatoions. newOp is an alias for operations. Which means that when newOP changes, operations changes as well. You need to tell Python explicitly to copy your list:

newOp = operations[:]

If your list is more than one level deep in nesting however, using slice notation will fail. You must use copy.deepcopy instead.

edited May 23 '17 at 11:53

Community

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answered Nov 16 '16 at 20:19

Christian Dean

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Note: This works in this case, but for more complex structures, you need to use [`deepcopy`](https://docs.python.org/2/library/copy.html) – TemporalWolf Nov 16 '16 at 20:20
1

@TemporalWolf Yes. I'll metion that. Thanks – Christian Dean Nov 16 '16 at 20:20
Thanks for responding, I have been working with this for a couple of days and not finding an answer was driving me crazy. – Sawyer Powell Nov 16 '16 at 20:31

Values for a list variable is changing despite there being no explicit changes

1 Answers1