Question

Why does the following behave unexpectedly in Python?

>>> a = 256 >>> b = 256 >>> a is b True           # This is an expected result >>> a = 257 >>> b = 257 >>> a is b False          # What happened here? Why is this False? >>> 257 is 257 True           # Yet the literal numbers compare properly 

I am using Python 2.5.2. Trying some different versions of Python, it appears that Python 2.3.3 shows the above behaviour between 99 and 100.

Based on the above, I can hypothesize that Python is internally implemented such that ‘small’ integers are stored in a different way than larger integers and the is operator can tell the difference. Why the leaky abstraction? What is a better way of comparing two arbitrary objects to see whether they are the same when I don’t know in advance whether they are numbers or not?

Answer 1

Take a look at this:

>>> a = 256 >>> b = 256 >>> id(a) == id(b) True >>> a = 257 >>> b = 257 >>> id(a) == id(b) False 

Here’s what I found in the documentation for "Plain Integer Objects":

The current implementation keeps an array of integer objects for all integers between -5 and 256. When you create an int in that range you actually just get back a reference to the existing object.

So, integers 256 are identical, but 257 are not. This is a CPython implementation detail, and not guaranteed for other Python implementations.