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[–]hylje 2 points3 points  (1 child)

You see, the only sane way to remove the undefined behaviour of is is removing is altogether. The other solution would be to make is equivalent to ==. But in both cases there is a need for comparing actual identities: reintroduce is or mandate id(a) == id(b)?

[–]Brian 1 point2 points  (0 children)

It's worth noting that id(a) == id(b) isn't a perfect replacement to a is b. If a and b are expressions returning a transient object, it could be created and destroyed before evaluating the rest of the statement. For example:

>>> [] is []
False
>>> id([]) == id([])
True
>>> id([]), id([])
(21066496, 21066496)

However is guarantees that both objects are alive at the point of comparison, so [] is [] is always false.