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Aliasing [was Re: [Tutor] beginning to code]

Bill BILL_NOSPAM at whoknows.net
Wed Sep 27 00:29:32 EDT 2017 
Steve D'Aprano wrote:
> On Wed, 27 Sep 2017 02:03 am, Stefan Ram wrote:
>
>> Steve D'Aprano <steve+python at pearwood.info> writes:
>>> On Tue, 26 Sep 2017 03:26 am, Antoon Pardon wrote:
>>>> at that moment, but it still needed correction. If the assignment is
>>>> an alias operator then after the statements
>>> Here's some C++ code that demonstrates it. Apologies in advance if it isn't
>>> the most idiomatic C++ code.
>>    In C++, assignments and initializations are different
>>    concepts.
>>
>>> int& b = a;  // reference variable or alias
>>    This is an initialization, not an assignment.
> A pedantic difference that makes no difference to my argument.
>
> I see that you ignored the later assignment:
>
> b = 2;
>
> which also assigned to a. *That** is the fundamental point: b is certainly an
> alias for a, and assigning to b assigns to a.
>
> That's how aliases work in C++. That's how var parameters in Pascal work, and
> out parameters in Ada. That is what it means to say that "b is an alias to a".
>
> b is another name for the *variable* a, not just whatever value a happens to
> hold now.
>
> I say that assignment in Python is NOT an aliasing operation. Antoon claims I'm
> wrong, and his evidence is:
>
> a = []
> b = a  # Antoon says this is an alias operation
> b.append(1)
> assert a == [1]
>
>
> But that's not enough for the variable b to be an alias for the variable a.
>
> Antoon is correct that a and b are two different names for the same list, but
> the two variables are not aliases to each other because assignments to b do not
> affect a, and vice versa.
>
> A good test for aliasing is to take the source code and mechanically replace
> every occurrence of the alias (in the same scope of course) with the original,
> or vice versa, and see whether the meaning of the code changes.
>
> In C++, apart from the initial binding:
>
>      int& b = a;
>
> ("initialisation") you could now randomly swap a for b or b for a and the
> meaning of the code will not change.
>
> But in Python, if we try the same trick, the code *does* change:
>
> a = 1
> b = a
> b = 2
>
> *is not* the same as:
>
> a = 1
> b = a
> a = 2
>
>
> (1) In Pascal, Ada, C++ etc using a reference variable (or var parameter, or
> whatever terminology they use) makes the two names aliases to EACH OTHER, not
> to the value they are bound to.
>
> (2) In Python, Javascript, Ruby etc assignment can give you two names for the
> same object, but the names do not alias each other.
>
> The critical distinction here is whether the names refer to each other:
>
> a <---> b
>
> or whether they merely refer to the same value:
>
> a ---> [ value ] <--- b
>
>
> Python uses the second model. Var parameters in Pascal and references in C++ use
> the first. Since the term "aliasing" is well-established for the first, using
> it in Python *without making the difference clear* is wrong.
>
>

That is a very nice argument!  : )
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