Python integer caching

问题:

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

回答1:

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)

回答2:

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