Mutable variables and memory management in Python

Question

I am trying to understand how Python memory management works. If I have an mutable variable, say a list:

x = ['x1', 'x2']
print(id(x))

then I will get a certain memory address. If I now modify x say x.append('x3'), the memory address is the same as before because lists are mutable data types. If, however, I change x with x = x + ['x3'] and I print the address I get a different one. Why it is so? I mean, the x variable has still the same name and all I have done is to modify its content. Why does Python change the address of the variable in this case?

because it creates a new variable that it assigns to name x. You did not "just modify it's content". To modify the content do x += ['x3'] — Jean-François Fabre
– Jean-François Fabre ♦, Commented May 10, 2020 at 11:37

Jean-François Fabre · Accepted Answer · 2020-05-10 11:41:14Z

1

when doing:

x = x + ['x3']

you're creating a new list which is assigned to x. The previous list held under the name x is lost and garbage collected (unless it was stored somewhere else).

The creation of a new element, even stored under the same name, results in a new identifier.

Also note that if you want to append x3 (or another list) that is highly inefficient because of the copy of the original list to create another one. In your case, x.append('x3') is the fastest way, and extend() or x += ['x3','x4','x5'] is the fastest way to append to (and mutate) an existing list with multiple elements at once.

(also note that += mutates the list but when used on immutable types like tuple, it still creates another tuple)

answered May 10, 2020 at 11:41

Jean-François Fabre♦

141k24 gold badges179 silver badges246 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Halil İbrahim Yıldırım · Accepted Answer · 2020-05-10 11:40:47Z

0

Variables are memory references in Python. When you say x = x + ['x3'] it merges x and ['x3'] and build a new list. From now on, x is pointing the new object built. That's why you get a different id. Meanwhile ['x1', 'x2'] object is deleted because nothing is pointing that object because x is pointing the new object.

answered May 10, 2020 at 11:40

Halil İbrahim Yıldırım

4002 silver badges9 bronze badges

Collectives™ on Stack Overflow

Mutable variables and memory management in Python

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related