Remove method in Python

Question

I have the following code:

tree = {'nodes':[1,2,3],'root':[1]}
nodes = tree['nodes']
nodes.remove(2)
print(tree['nodes'])
print(nodes)

The output is the following:

[1, 3]
[1, 3]

My question may be stupid but I do not understand why remove method caused that tree variable has changed too?

I thought that when I create a new variable like nodes in the example above, then any method applied on this variable will affect only this variable.

From this example, I can conclude that it had an impact on a tree variable too.

Is it related to the global and local variables somehow?

Answer 1

Both nodes and tree['nodes'] are referring to the same block of memory. It means they are same. By changing any of them, both will be affected.

To avoid this, you can use copy .

from copy import copy

nodes = copy(tree['nodes'])

In this case, they are referring to different memory blocks so they are completely separated.

Also take a look at this link , it might be useful for better clue.

Answer 2

Its actually not a stupid question.

Most people like myself who have foundations built in languages like C++ have a little difficulty with this at first when you start learning Python , or even Java .

Unlike C++, in Python, everything is returned as a reference.

So even if you are accessing a part of the object via another variable, you still are accessing the same part of the object.

Read this to get more context around this.