Skip Montanaro <s...@pobox.com>: > The use of "is" or "is not" is the right thing to do when the object > of the comparison is known to be a singleton.
Object identity stirs a lot of passions on this forum. I'm guessing the reason is that it is not defined very clearly (<URL: https://docs.python.org/3/library/functions.html#id>): id(object) Return the “identity” of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same id() value. CPython implementation detail: This is the address of the object in memory. The "is" relation can be defined trivially through the id() function: X is Y iff id(X) == id(Y) What remains is the characterization of the (total) id() function. For example, we can stipulate that: X = Y assert(id(X) == id(Y)) # assignment preserves identity (assuming X and Y are not modified in other threads or signal handlers). We know further that: i = id(X) time.sleep(T) assert(i == id(X)) # the identity does not change over time def f(x, y): return id(x) == id(y) assert(f(X, X)) # parameter passing preserves the identity def f(x): return x assert(id(f(X) == id(X))) # the return statement preserves the identity assert(id((X0, X1, ..., X)[k]) == id(Xk)) # tuple composition and projection preserve the identity i = id(X) X.Y assert(i == id(X)) # reference does not change the identity and so on. The nice thing about these kinds of formal definitions is that they make no metaphysical reference to "objects" or "lifetime" (or the CPython implementation). They can also be converted into implementation conformance statements and test cases. A much tougher task is to define when id(X) != id(Y). After all, all of the above would be satisfied by this implementation of id(): def id(x): return 0 The nonidentity will probably have to be defined separately for each builtin datatype. For example, for integers and strings we know only that: assert(X == Y or id(X) != id(Y)) # inequality implies nonidentity Marko -- https://mail.python.org/mailman/listinfo/python-list
