Newbie question: Allocation vs references

[Stian Søiland]

On 2005-06-02 14:43:40, Jan Danielsson wrote:

> a = [ 'Foo', 'Bar' ]
> b = [ 'Boo', 'Far' ]
> q = [ a, b ]

>    Or, better yet, how do I store a and b in q, and then tell Python
> that I want a and b to point to new lists, without touching the contents
> in q?

There are several ways to create a copy of a list:

a1 = a[:]          # new copy, sliced from 0 to end
a2 = list(a)       # create a new list object out of any sequence
import copy
a3 = copy.copy(a)  # use the copy module


So you could do for example:

    q1 = [ list(a), list(b) ]
    q2 = [ a[:], b[:] ]
    q3 = [ list(x) for x in (a,b)]


Note that the copy module also has a function deepcopy that will make
copies at all levels. So if you had:

q = [a,b]
q1 = copy.deepcopy(q2)


every list in q1, even the inner "a" and "b" will be new copies. Note
that non-mutables such as "Foo" and "Bar" are NOT copied, but as they
cannot be changed, that doesn't matter.

-- 
Stian Søiland               Work toward win-win situation. Win-lose
Trondheim, Norway           is where you win and the other lose.
http://soiland.no/          Lose-lose and lose-win are left as an
                            exercise to the reader.  [Limoncelli/Hogan]
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