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devel / comp.lang.python / RE: To clarify how Python handles two equal objects

Re: To clarify how Python handles two equal objects

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From: nntp.mbourne@spamgourmet.com (Mark Bourne)
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by: Mark Bourne - Sun, 15 Jan 2023 13:14 UTC

Jen Kris wrote:
> Avi,
>
> Your comments go farther afield than my original question, but you made some interesting additional points. For example, I sometimes work with the C API and sys.getrefcount may be helpful in deciding when to INCREF and DECREF. But that’s another issue.
>
> The situation I described in my original post is limited to a case such as x = y where both "x" and "y" are arrays – whether they are lists in Python, or from the array module – and the question in a compiled C extension is whether the assignment can be done simply by "x" taking the pointer to "y" rather than moving all the data from "y" into the memory buffer for "x" which, for a wide array, would be much more time consuming than just moving a pointer. The other advantage to doing it that way is if, as in my case, we perform a math operation on any element in "x" then Python expects that the same change to be reflected in "y." If I don’t use the same pointers then I would have to perform that operation twice – once for "x" and once for "y" – in addition to the expense of moving all the data.
>
> The answers I got from this post confirmed that it I can use the pointer if "y" is not re-defined to something else during the lifespan of "x." If it is then "x" has to be restored to its original pointer. I did it that way, and helpfully the compiler did not overrule me.

I haven't done much with C extensions, but I don't think you'd need to
do anything with "x" in that case. If something else is assigned to
"y", "x" would still be a reference to the original object - why would
it need to be "restored" to anything? Unless I've misunderstood what's
going on here...

--
Mark.

RE: To clarify how Python handles two equal objects

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by: <avi.e.gross@gmail.com> - Sun, 15 Jan 2023 18:13 UTC

<<< Frank Millman>>> My 'aha' moment came when I understood that a python object has only three properties - a type, an id, and a value. It does *not* have a name.

Yes, Frank, it is a bit like how some people need to wrap their minds around a concept like an anonymous function. It has no name and for many purposes needs no name, but at the same time can be bound to one or more names if needed.

Some people will encounter a situation where you need to pass a function to be called such as a key function for determining the sort order and they will create a new function first the usual way with a name and then pass it along by name. That works fine but the function then persists while not being used again and even choosing a name can have consequences if the same name is already in use and so on.

Experienced programmers might use some kind of lambda expression in-line as the function as an object is then created, and passed along as a reference to be used and probably discarded once nothing refers to it.

Similar things can happen if an object is created as part of a larger complex such as a list or deque of them. Each item can be referenced unambiguously without having a formal name. You can pop off the next one an use it or ask to take the fifth. It may acquire and lose names as the program runs but sometimes can have many names or none.

Getting people to see that an object exists whether it has no name and to speak about them with that understanding can be a problem. We as humans seem to think we are our names. But I know my names have changed over the years partially because I moved between countries and I have several additional names used just for special purposes, and yet for some purposes I am simply a number. If I raise my class in a lecture hall where my name is not known, they may point at me or ask the guy in the blue short to talk. At the Department of Motor Vehicles, my name, like everyone else, is NEXT!

The real point is what Python does, not what other situations require. Two objects are equal does not always mean what you think or want it to mean. What we are discussing here is two objects loosely of type "name" that are being compared not for whether THEY are themselves equal as in the same symbols composing them and perhaps living in the same namespace. The question was about two objects that contained references to another object or even to different parts of another object. As it happens, that can be a deeper question in which some parts are not a required aspect of the language and may be part of one of many possible implementations. And, as noted, the names can be detached from that reference too so a link is not permanent.

Have we beaten this one to death yet?

-----Original Message-----
From: Python-list <python-list-bounces+avi.e.gross=gmail.com@python.org> On Behalf Of Frank Millman
Sent: Sunday, January 15, 2023 12:47 AM
To: python-list@python.org
Subject: Re: To clarify how Python handles two equal objects

--
https://mail.python.org/mailman/listinfo/python-list

Subject	Author
Re: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Greg Ewing
Re: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Roel Schroeven
Re: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Stefan Ram
Re: To clarify how Python handles two equal objects	Peter J. Holzer
Re: To clarify how Python handles two equal objects	Richard Damon
RE: To clarify how Python handles two equal objects	<avi.e.gross
Re: To clarify how Python handles two equal objects	Axel Reichert
Re: To clarify how Python handles two equal objects	Stefan Ram
RE: To clarify how Python handles two equal objects	<avi.e.gross
RE: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Bob van der Poel
Re: To clarify how Python handles two equal objects	Stefan Ram
Re: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Peter J. Holzer
RE: To clarify how Python handles two equal objects	<avi.e.gross
RE: To clarify how Python handles two equal objects	<avi.e.gross
RE: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Mark Bourne
Re: To clarify how Python handles two equal objects	Chris Angelico
Re: To clarify how Python handles two equal objects	Jen Kris
Re: To clarify how Python handles two equal objects	Chris Angelico
Re: To clarify how Python handles two equal objects	Roel Schroeven
Re: To clarify how Python handles two equal objects	Frank Millman
RE: To clarify how Python handles two equal objects	<avi.e.gross