why cannot assign to function call

[Steven D'Aprano]

On Tue, 13 Jan 2009 23:06:58 +0000, Mark Wooding wrote:


> I'm going to move away from the formal semantics stuff and try a
> different tack.  Here's what I think is the defining property of
> pass-by-value (distilled from the formal approach I described earlier,
> but shorn of the symbolism):
> 
>   The callee's parameters are /new variables/, initialized /as if by
>   assignment/ from the values of caller's argument expressions.


Well, that's one way of looking at it, I guess. The problem is that when 
I do this:

x = "something"  # just an example, not necessarily a string
y = x

Does the line y=x create a new variable? Depends on what you mean by 
variable. If you mean a new name, symbol or whatever, then obviously yes. 

But if you mean a new value, namely the string "something", then not 
necessarily. In Pascal (and C?) those bytes are copied, giving two 
independent values which just happen to be equal. In Python, both x and y 
refer to the same string, and I choose the term deliberately. See, I'm 
not *entirely* against the use of referring to references, when 
appropriate. I think we just differ on when we think it's appropriate.

Which I guess brings us back to your earlier assertion (I think it was 
you) that people aren't confused about argument passing, they're confused 
about assignment. 

[...]

> But! (you claim) ...
> 
>> Python simply can't be pass-by-value, because it doesn't behave like
>> pass-by-value in other languages (particularly C and Pascal).
> 
> Ah! (say I) but assignment in C and Pascal looks different from the way
> it looks in C

I'm sorry, that confuses me. Assignment in C looks different from the way 
it looks in C? I guess the second C should be Python.

> -- and in exactly the same way that argument passing looks
> different.  And there, I think, I'm going to rest my case.
> 
> I'm sorry I took so long to distill these thoughts.  Thank you for
> putting up with my theoretical meanderings on the way.

And thank you for not kill-filing me when tempers were getting short.



-- 
Steven