[Python-Dev] implementation of copy standard lib

[Simon Brunning]

On 8/16/05, Martijn Brouwer <e.a.m.brouwer at alumnus.utwente.nl> wrote:
> Well, I guess making a deep copy of an object is important in many case.
> Just try this:
> A=[]
> B=A
> A.append("foo")
> print B
> 
> Guess the output. It is:
> ['foo']

I remember thinking that this behavior was odd when I learned Java
many years ago. (Java shares this behavior, and I learned it before
Python. In fact, I think that many OO languages behave this way -
JavaScript, C#, and so on.) But these days, it's 2nd nature.

I also thought that it might be dangerous, but now I know that it
hardly ever is. (When is it dangerous? Well, if you are trying to
build an immutable class, you have to be careful not to pass out
references to mutable attributes.)
 
> In many cases this is undesirable, so you need to make a deep copy using
> something as B=copy.deepcopy(A).

This is where we differ. I know that it's *occasionally* useful to be
able to copy or deep-copy an object - that's why the copy module is
there. But I find that it's not "many cases" - it's actually rather
rare.

Shallow copies of dictionaries and (especially) lists are a little
less rare, but you can make those without the copy module:

new_list = list(old_list) # or old_list[:]
new_dict = dict(old_dict) # or old_dict.copy()

-- 
Cheers,
Simon B,
simon at brunningonline.net,
http://www.brunningonline.net/simon/blog/