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Date: Fri, 5 Jun 2015 09:04:51 +1000
Subject: Re: Can Python function return multiple data?
From: Chris Angelico <rosuav@gmail.com>
Cc: "python-list@python.org" <python-list@python.org>
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Xref: csiph.com comp.lang.python:92101

On Fri, Jun 5, 2015 at 8:52 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> On Fri, 5 Jun 2015 04:38 am, Grant Edwards wrote:
>
>>>> But, discussing pass-by-this vs. pass-by-that without also discussing
>>>> the semantics of the assignment operator is rather pointless.
>>>
>>> No, that's a red-herring.
>>
>> I don't think so.  The reason that many people seem to confused about
>> Python's argument passing is that they don't understand what
>> assignment does.
>
>
> I don't see the connection, but do explain, I'm interested.

The connection is that argument passing is, in Python and in many
other languages, a form of assignment, and follows all the same rules
as assignment follows. (It's a kind of magic assignment that spans two
namespaces, but that has nothing to do with pass-by-X questions.)
These two will exhibit the same behaviour:

# Option 1
x = some_object()
y = x
... do something with y ...
... compare y and x

# Option 2
def do_stuff(y):
    ... do something with y ...

x = some_object()
do_stuff(x)
... compare y and x


Python's object semantics go far deeper than just argument passing,
but everyone's confusions about pass-by-X are always about function
calls.

Whether this makes a real difference to anything is a separate question.

ChrisA