为什么具有相同值的可变对象在Python中具有不同的ID

Question

Thank you for your valuable time, I have just started learning Python. I came across Mutable and Immutable objects. As far as I know mutable objects can be changed after their creation.

a = [1,2,3]
print(id(a))
45809352
a = [3,2,1]
print(id(a))
52402312

Then why id of the same list "a" gets changed when its values are changed.

Answer 1

your interpretation is incorrect.

When you assign a new list to a , you change its reference.

On the other hand you could do:

a[:] = [3,2,1]

and then the reference would not change.

Answer 2

mutable means that the content of the object is changed. for example a.append(4) actually make a equal to [1, 2, 3, 4] , while on the contrary, appending to a string (which is immutable) does not change it, it creates a new one.

However, when you re-assign, you create a new object and assign it to a , you don't alter the existing content of a . The previous content is lost (unless refered-to by some other variable)

Answer 3

If you change a list, its id doesn't change. But you may do things that instead create a new list, and then it will also have a new id.

Eg,

>>> l=[]
>>> id(l)
140228658969920
>>> l.append(3)  # Changes l
>>> l
[3]
>>> id(l)
140228658969920  # Same ID
>>> l = l + [4]  # Computes a new list that is the result of l + [4], assigns that
>>> l
[3, 4]
>>> id(l)
140228658977608  # ID changed

Answer 4

When you do

a = [3, 2, 1]

1. You unlink the list of [1, 2, 3] from variable a .
2. Create a new list [3, 2, 1] then assign it to a variable.

Answer 5

Being immutable doesn't mean you assign a new object, it means your original object can be changed "in place" for example via .append()

>>> my_list = [1,2,3]
>>> id(my_list)
140532335329544
>>> my_list.append(5)
>>> id(my_list)
140532335329544
>>> my_list[3] = 4
>>> my_list
[1, 2, 3, 4]
>>> id(my_list)
140532335329544