Question

I’m very new to Python, so sorry for the probably simple question.
(Although, I spent now 2 hours to find an answer)

I simplified my code to illustrate the problem:

side=[5]
eva=side
print(str(side) + " side before")
print(str(eva) + " eva before")
eva.remove(5)
print(str(side) + " side after")
print(str(eva) + " eva after")

This yields:

[5] side before
[5] eva before
[] side after
[] eva after

Why does the remove command also affects the list ‘side’? What can I do to use a copy of ‘side’, without modifying the list?

Thank you very much

Edit: Thank you very much for the good and comprehensible answers!

Answer 1

Python has “things” and “names for things”. When you write

side = [5]

you make a new thing [5], and give it the name side. When you then write

eva = side

you make a new name for side. Assignments are just giving names to things! There’s still only one thing [5], with two different names.

If you want a new thing, you need to ask for it explicitly. Usually you would do copy.copy(thing), although in the case of lists there’s special syntax thing[:].

_{FYI “things” are usually called “objects”; “names” are usually called “references”.}