Finding the instance reference of an object

Steven D'Aprano steve at
Fri Oct 31 03:05:45 CET 2008

On Fri, 31 Oct 2008 13:58:13 +1300, greg wrote:

> Dale Roberts wrote:
>> Okay, you can have it that way, but every time you explain to someone
>> that Python passes "By Value", you will have to add the additional
>> baggage that, oh, by the way, there is a completely different meaning
>> for "value" in Python than what you are used to.
> For what it's worth, I happen to agree that telling someone that Python
> passes parameters "by value" without being sure they understand exactly
> what "by value" means, 

"By value" is very simple. It means a copy of the value is passed to the 

Using "call by value" to mean "pass a copy of a reference to the value" 
is an abuse of terminology, because that's exactly what happens in call 
by reference: a copy of the reference to the value is passed.

> is not a good idea -- not because the term isn't
> well-defined, but because of the widespread confusion out there about
> it.

Well duh. All these people trying to insist that after executing x=1 the 
value of x is some arbitrary memory location are certainly confused.

> But equally I wouldn't tell someone that it's *not* by value, because if
> they do happen to correctly understand what it means, that will confuse
> them just as much.

But it isn't call by value. Why wouldn't you tell people it is when the 
value isn't being copied?

> So my recommendation is just to tell them that it works by assigning the
> result of evaluating the actual parameter to the formal parameter.

And 99% of them will just say "What?".

That's not an explanation, it's obfuscation at its worst. What *is* the 
result of evaluating the actual parameter? Is it a reference to the 
object or the object itself? Is it a copy of the object? A shallow copy? 
A deep copy? Not a copy at all? What implications for behaviour does it 
have? What does it mean?

> If they understand how assignment works in Python, that tells them all
> they need to know.



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