Is a new object allocated in memory each time an immutable object is changed?

Question

Consider the following example:

(Python 3.7.2)
>>> a = '1'
>>> id(a)
4364850768
>>> a += '2'
>>> id(a)
4365285672
>>> a += '3'
>>> id(a)
4365285672

My understanding is that when a character is appended to string a, a new object is created and the contents of the old object are copied over with the new character added. This does seem to be the case when 2 is appended as the ids change, but this does not seem to be the case when 3 is appended. I'm looking for some clarification on why the ids don't change the second time.

A nitpick: immutable objects don't change, by definition. You are correct that a new object is created. — Karl Knechtel, Apr 15 '19 at 14:04

score 2 · Accepted Answer · answered Apr 15 '19 at 13:31

A new object is indeed created, but it looks like Python is recycling the ids. That's an implementation detail - it's best not to rely on id().

To demonstrate this, keep another copy of '12' around:

>>> id(a)
4377270112
>>> a = '1'
>>> id(a)
4376257360
>>> a += '2'
>>> id(a)
4377270152
>>> b = a 
>>> a += '3'
>>> id(a)
4377270032
>>> id(b)
4377270152

Is a new object allocated in memory each time an immutable object is changed?

1 Answers1