Python integer caching

Question

In the following python script, why the second assert goes through (i.e., when adding 0 to 257 and stores the result in y, then x and y become different objects)? Thanks!

Answer 1

When you use is, you are checking whether or not the two objects point to the same memory location. If they do, then the result is True. Otherwise, the result is False.

To check if the values are equivalent, use ==, e.g. assert x == y. Alternatively, to assert that they are not equal, use !=, e.g. assert x != y.

x = 257
y = 257


>>> id(x)
4576991320

>>> id(y)
4542900688

>>> x is y
False

x = 257
y = 257 + 0

>>> id(x)
4576991368

>>> id(y)
4576991536