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strange side effect with lists!?

Alex Martelli aleaxit at yahoo.com
Wed Oct 13 08:19:56 EDT 2004 
<Wolfgang.Stoecher at profactor.at> wrote:

> Hello,
> 
> I'm new to Python and playing around. I'm confused by the following 
> behaviour:
> 
> >>> l1 = [1] # define list
> >>> l2 = l1  # copy of l1 ?
> >>> l2,l1
> ([1], [1])
> >>> l2.extend(l1) # only l2 should be altered !?
> >>> l2,l1
> ([1, 1], [1, 1]) # but also l1 is altered!
> 
> So what is the policy of assignment? When is a copy of an object created?
> Where to find dox on this?

Bruno's answer seems very thorough so I'll just try to briefly summarize
the answers:

1. simple assignment (to a bare name, at least), per se, never
   implicitly copies objects, but rather it sets a reference to an
   object (_another_ reference if the object already had some).

2. a new object is created when you request such creation or perform
   operations that require it.  Lists are particularly rich in such
   operations (see later).  Simple assignment to a bare name is not
   an operation, per se -- it only affects the name, by making it refer
   to whatever object (new, or used;-) is on the righht of the '='.

3. I believe any decent book on Python will cover this in detail.

Now for ways to have a new list object L2 made, with just the same items
and in the same order as another list object L1 ("shallow copy"):

a. import copy; L2 = copy.copy(L1)

This works to shallow-copy _any_ copyable object L1; unfortunately you
do have to import copy first.  Module copy also exposes function
deepcopy, for those rare cases in which you wish to recursively also get
copies of all objects to which a "root object" refers (as items, or
attributes; there are some anomalies, e.g., copy.deepcopy(X) is X when X
is a class, or type...).

b. L2 = list(L1)

I find this is most often what I use - it works (making a new list
object) whatever kind of sequence, iterator, or other iterable L1 may
be.  It is also what I recommend you use unless you have some very
specific need best met otherwise.

c. various operations such as...:
   L2 = L1 * 1
   L2 = L1 + []
   L2 = L1[:]
i.e. "multiply by one", "concatenate the empty list", or "get a slice of
all items".  I'm not sure why, but the latter seems to be very popular,
even though it's neither as concise as L1*1 nor as clear and readable as
list(L1).


Alex
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