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Chris Angelico <rosuav at gmail.com>: > On Fri, Jul 7, 2017 at 1:21 AM, Marko Rauhamaa <marko at pacujo.net> wrote: >> What I'm looking for is snippets of Python code that illustrate the >> difference. >> >> That's how you can illustrate the difference between the "==" and "is" >> operators: >> >> >>> ["a"] is ["a"] >> False >> >>> ["a"] == ["a"] >> True > > When you have an address, you can use that to locate the thing. You are pulling that out of your hat (or a dictionary). Python doesn't define the concept at all (yet it manages to locate every useful thing there is). > In C, that's pointer dereferencing. If I give you the id of a Python > object, can you locate that object and find out something about it? If > you can't, it's not an address. > > And you have to make this work in Python (the language), not just > CPython (the interpreter). I'll consider an answer satisfactory if it > runs on the Big Four - CPython, Jython, IronPython, and PyPy - and > mainly, you need to show that it works in Jython, because that's the > most different. I don't follow you. A code example, please. >> Unfortunately, when I try it, I get: >> >> >>> a is a >> Traceback (most recent call last): >> File "<stdin>", line 1, in <module> >> NameError: name 'a' is not defined > > And oh how terrible, Python doesn't define any other operators on > unassigned names either. The 'is' operator is looking at OBJECT > identity, so you need to have an OBJECT. If you don't understand that > part of Python's object model, I recommend learning from Ned > Batchelder: It's enough to point to the Language Reference if you have a link. >>> First part is implied by Python's execution model, >> >> [Citation needed] > > https://docs.python.org/3/reference/executionmodel.html Sorry, which sentence? >>> and the second by the definition of the `is` operator. >> >> [Citation needed] > > https://docs.python.org/3/reference/expressions.html#is-not That one in its entirety: The operators "is" and "is not" test for object identity: x is y is true if and only if x and y are the same object. Object identity is determined using the id() function. x is not y yields the inverse truth value. That simply defines the identity with whatever is returned by the id() function. The id() function, in turn, is defined to be: 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. which doesn't clarify anything. > Just don't try explaining to a novice programmer that way. It's a > waste of everyone's time. I believe the concept of an object is among the more difficult things for novice programmers to get. Marko

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