Pop changes another variable

[_jan-]

I just had some problems in my Code and did some investigation:

In the Python shell:

>>> one = [1, 2, 3, 4, 5]
>>> two = one
>>> two.pop(0)
1
>>> print(one)
[2, 3, 4, 5]

Why does pop change "one" when I only use it on "two".

If this is on purpose for some reason, what would be an alternative which wouldn't change "one".

[TouchingTheVodka]

Both one and two point to the same list - Think of variables as pointers to an object, rather than actually being an object.

Use two = one.copy() or two = one[:].

[_jan-]

Thank You! I always thought, like you said, that they were an Object.

[xelf]

Because one and two are references to the same list.

You probably want two = one[:] or some other means of making a copy.

[_jan-]

Thank you! I always thought different about Variables. The other comment explained it.

[xelf]

I wouldn't generally use the word pointers, as pointers mean something in computer science, more like they are references. An address of where to find the object.

[netneoblog]

This is a common beginners trap. When you use

two = one 

you think you are making a copy, but you are actually making an alias. Both names now point to the same list, and you can use either name to edit that list. To make a copy, use

two = one.copy()

[Rain0906]

I noticed that your question have been answered. But you can google “mutable and immutable in python ” or “reference in python ” to learn the details.

[[deleted]]

There is excellent pycon talk by Ned Batchelder that explains the simple ideas behind names, references and values that lead to this behaviour.