Create two immutable objects with the same value in Python

Question

Is it possible in Python to create two immutable objects with the same value?

So that you understand what I mean, here are some examples:

>>> a = 13
>>> b = 13
>>> a is b
True


>>> a = 13
>>> b = 26/2
>>> a is b
True


>>> a = 13
>>> b = int.__new__(int, 13)
>>> a is b
True


>>> a = 13
>>> b = int("13")
>>> a is b
True

Is it possible to create a and b with the same value but a is b to return False? Just learning.... :D

Were you asking just for your own enlightenment, or did you have a use case in mind? Ordinarily you shouldn't care if it's two objects or one. — Mark Ransom
– Mark Ransom, Commented Nov 16, 2011 at 22:37
@Mark Ransom: I was reviewing Python's data model and playing with some code for a better understanding when my brain tripped on the above :D. There is no use case. — ElenaT
– ElenaT, Commented Nov 17, 2011 at 20:47

Tim Pietzcker · Accepted Answer · 2011-11-16 21:25:23Z

5

Sure, just choose a value that is too large to be cached:

>>> a = 256
>>> b = 256
>>> a is b
True
>>> a = 257
>>> b = 257
>>> a is b
False
>>> a = "hey"
>>> b = "hey"
>>> a is b
True
>>> a = "hey!"
>>> b = "hey!"
>>> a is b
False

Only small integers and short strings will be cached (and this is implementation-dependent, so you shouldn't rely on that anyway). is should only be used for testing object identity, never for testing equality.

edited Nov 16, 2011 at 21:25

answered Nov 16, 2011 at 21:20

Tim Pietzcker

337k59 gold badges520 silver badges572 bronze badges

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

3 Comments

ElenaT Over a year ago

I somehow thought that because the objects are immutable they are reused (somewhat like String literals in Java) when in fact it's caching. Hmmm... Interesting. Just one quick question: So the only way I can be sure (no matter the implementation) that a is b is always True is to have a = 13 then b = a. Correct?

Tim Pietzcker Over a year ago

Exactly, because then they both refer to the same object (in this case an integer). This of course works for all objects (after a = b = 257, a is b returns True).

Greg Hewgill Over a year ago

@ElenaT: Avoid relying on the behaviour of is for scalar values. See Python “is” operator behaves unexpectedly with integers for more information.

Collectives™ on Stack Overflow

Create two immutable objects with the same value in Python

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related