On Thursday, March 5, 2015 at 2:27:12 PM UTC-8, rand...@fastmail.us wrote: > It's been brought up on Stack Overflow that the "in" operator (on > tuples, and by my testing on dict and list, as well as dict lookup) uses > object identity as a shortcut, and returns true immediately if the > object being tested *is* an element of the container. However, the > contains operation does not specify whether object identity or equality > is to be used. In effect, the built-in container types use a hybrid > test: "a is b or a == b". > > My question is, is this a *correct* implementation of the operator, or > are objects "supposed to" use a basis of equality for these tests?
I would argue that if `a is b` then it is obvious that `a == b`, so if all you care about is whether or not `a == b`, then it shouldn't matter if `a is b` is checked first. It could greatly speed up comparisons for objects that are expensive to compare. I would also argue that the "in" operator *SHOULD* be using equality of value. Otherwise, if it only used equality of identity, testing if user input is valid by seeing if it is "in" a list of valid inputs wouldn't work. Testing identity in this case would *never* be true unless you were dealing with a select number of integers. >>> i = input() 5 >>> i is 5 True >>> i = intput() 999999 >>> i is 999999 False >>> i in [1, 2, 3, 999999] True -- https://mail.python.org/mailman/listinfo/python-list
