[Tutor] python dictionaries (copy by reference or copy by value?)

[Steven D'Aprano]

On Sat, May 02, 2015 at 04:25:41PM -0700, Alex McFerron wrote:
> trying to understand why this is true
[...]
> question: if y=x from step 2 (the copy job)  is just creating a pointer y
> that points to the same thing as x then why when i set x = {} in step 5
> does that also not cause y to equal {}?
> 
> what am i not understanding about python dictionaries?

Python, like most modern languages, is neither copy-by-value nor 
copy-by-reference.

Python uses *exactly* the same name-binding mechanism for all values, 
regardless of type, and all names, whether they are global variables, 
function parameters, or anything else.

I discuss this question as it applies to function arguments here:

http://import-that.dreamwidth.org/1130.html

but the same reasoning applies to any other assignment. The important 
thing to remember is that

y = x

doesn't make a copy of x, it just makes y refer to the same object as x 
currently refers to. This is *not the same* as making y refer to x 
(that is, y and x are permanently linked to be alternative names for 
the same variable). Can you see the difference? If not, I'm happy to 
respond with more detail.


-- 
Steve