how to stop python from "binding" variables that stand for lists

Question

In python, the code

x = 0
y = x
y = 1
print x

returns "0" while the code

x = [0]
y = x
y[0] = 1
print x

returns "[1]". Why does python treat lists so differently from integers and what can I do to force it to treat the bottom "x" as it does the top "x"? It seems like the '='s in the respective second lines mean different things - the top one only affects y while the bottom somehow binds x to y. So maybe I need to use a different symbol in the bottom code?

No, they are doing the exact same thing. Read this: https://nedbatchelder.com/text/names.html — Daniel Roseman, Dec 27 '17 at 11:56
There are no strings involved here (except for the conversion done by `print`), what you have is a *list*. `y = x` on a *mutable* object like a list is just a reference assignment, the object they reference is the same. — cdarke, Dec 27 '17 at 11:56
But the `=`s in "the respective third lines" **are doing smth totally different**. — CristiFati, Dec 27 '17 at 11:57
@CristiFati ok that makes sense. So how do I make python see x and y as different lists. Because I want to be able to manipulate y without changing x. — Tim kinsella, Dec 27 '17 at 12:00
@Timkinsella Check the answers for [How to clone or copy a list?](https://stackoverflow.com/q/2612802/2063361) — Moinuddin Quadri, Dec 27 '17 at 12:02
`y = x[:]`. But I'm sure this topic is also covered by many [SO](https://stackoverflow.com) questions. — CristiFati, Dec 27 '17 at 12:02

score 1 · Accepted Answer · answered Dec 27 '17 at 12:18

a = [20, 21]
b = [20, 21]

print(a is b) # False
print(a == b) # True
print(id(a) != id(b)) # True

a and b have the same value, but do not refer to the same object.

x = [1]
y = x

print(x is y) # True
print(x == y) # True
print(id(x) == id(y)) # True

x and y have the same value and refer to the same object.

i = [0]
j = i[:] # Copy of i

print(i is j) # False
print(i == j) # True
print(id(i) != id(j)) # True

i and j have the same value, but do not refer to the same object.

how to stop python from "binding" variables that stand for lists

1 Answers1