Python integer caching

6
votes

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!

x = 257
y = 257
assert x is y

x = 257            
y = 257 + 0
assert x is not y
2
pythoncaching
do not use is to check for equality ... basically ever ... it is litterally the memory location variable ... python prestores a small number of constants that will just happen to work in some cases ... - Joran Beasley
Check this answer out; it explains why is seems to work strangely sometimes! stackoverflow.com/questions/132988/… - Peter Wang

2 Answers

2
votes

integers are non mutable so any operation to change them results in a new memory location

>>> a =9876
>>> id(a)
38478552
>>> a+=1
>>> id(a)
38478576
>>> a+=0
>>> id(a)
38478528

is is checking the actual memory location of the object ... and should basically never be used to check for value equality (although it may arbitrarily work on some small subset of cases)

1
votes

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