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appending to list

Alex Martelli aleax at aleax.it
Wed Oct 1 12:19:23 EDT 2003 
sud jag wrote:

> Hi,
>   Iam a newbie to python. So any help on this is
> deeply appreciated
> 
>   If I append an instance of a class to a list will
> the list have a copy of the instance or just a
> reference to that instance ? If it is only a reference
> is there something similar to deep copy in case of
> append ?

A reference.  Python never make copies unless you ASK
for a copy, be it deep or normal (shallow) [note that
deep copying is generally very expensive -- and rarely
needed].  No need for "something similar to deep copy
in case of append": just do

  thelist.append(copy.copy(theinstance))

or

  thelist.append(copy.deepcopy(theinstance))

depending on what exactly you need (after "import copy",
of course) -- or something more sophisticated if that
should be necessary.

An example of "more sophisticated"...: say that your
need is for all instances appended to 'thelist' to be
"snapshots" from the "outside" viewpoint, BUT it's
all right for them to share their own internals with
each other.  In this case, you may take advantage of
the fact that deepcopy takes an optional "memo" dict:
keep the memo (originally empty) around and pass it
to every call to deepcopy.  Of course, sophisticated
behavior needs a good grasp of what you're doing, so,
I'm not posting example code lest it tempt you, as a
newbie to python, into facing stuff that's still a bit
beyond your grasp; I'm just planting the seed of the
idea "if and when I do need deep copies, I can take
SOME limited control of the process, in certain cases,
via the ``memo'' dict that deepcopy accepts" -- it will
probably be quite a while before you actually NEED this
information, but, who knows...


Alex
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