Python list assignation

Question

I've got this code

class coordenates:
 x = 0 
 y = 0 
coor = coordenates()
coor.x=0
coor.y=0
list = []
list.append(coor)
list.append(coor)

Now, the problem is that when I update

list[0].x=100

it is also modifing list[1].x somehow!

print str(list[0].x)
>> 100
print str(list[1].x)
>> 100

which must remain in 0 since I haven't update it. Is append() creating the same object pointing in the same position in memory in positions 0 and 1? why creating 2 different objects solves the problem?

• Klaus D.: You are appending the same object to the list twice. If you change it it will change at both indexes.
• juanpa.arrivillaga: append isn't creating the same object in the same memory position, you are explicitely appending the same object.

Answer 1

In your current code, x and y are class-level attributes. I suspect that you want them to be instance-level attributes. If so, set them in __init__():

class Coordinates:
 def __init__(self):
 self.x = 0 
 self.y = 0 

More important, if you append the same coor to the list twice, any mutation of the coor will be reflected in "both" coordinates (because the list is just holding a reference to the same underlying coordinate in both positions of the list). Maybe you want something like this instead, where you create two independent coordinate instances?

list = []
list.append(Coordinates())
list.append(Coordinates())

You can see an illustration of your problem with this code:

c = Coordinates()
cs = []
cs.append(c)
cs.append(c)
for c in cs:
 print id(c) # Both elements of the list refer to the same object.

• pbreach: Probably the best way. It might also be worth mentioning the copy module.
• Joe: So if I do temp= 0, list.append(temp), list.append(temp) list[0]=100 will modify both positions as well? btw amazing response! I implemented list.append(Coordinates()) as the way to go.
• FMc: @Joe Well ... that example is a bit different. You are assigning an entirely new object (an integer 100) to the [0] element. In the initial example, we were appending the same object (coor) to the list twice, and then mutating the underlying object.