>From Programming in Python 3, 2nd edition (p. 22-23): >>> a = ["Retention", 3, None] >>> b = ["Retention", 3, None] >>> a is b False >>> b = a >>> a is b True
My current understanding is as follows: On the first two lines, two separate objects are defined, stored in two separate blocks of memory. These two objects just happen to have the same value, ["Retention", 3, None], stored in two separate locations. a and b, the object references (Variables are what I used to call these.), store these two separate memory locations. Thus a is b is false. However, when the line b = a is implemented, b now references the same object (memory location) as a, which now causes a is b to be true. Is my understanding correct? On the next page the author states (after giving a string example where a and b each are assigned the string "many paths", similar to the example above): "In some cases, comparing the identity of two strings or numbers--for example, using a is b--will return True, even if each has been assigned separately as we did here. This is because some implementations of Python will reuse the same object (since the value is the same and is immutable) for the sake of efficiency..." I ask: Which implementations of Python do this? In trying to make any code I write portable across as many platforms as possible, should I avoid using the identity operator, is (and its opposite, is not), except when I wish to compare to None? -- Thanks! boB _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
