always the same object

[Uwe Mayer]

Hi,

I am using struct.unpack() quite often. unpack() returns a tuple, however a
tuple with always the same id(), which is AFAIK the memory location of the
structure.

I found myself working with above tuple, initialising other data structures,
etc... and when I tried to change one list element, suddenly all list
elements change => so they all contain the identical object

Now what are the rules when Python re-uses an old object (as with
struct.unpack() ) and when does it create new objects?
And what ist the proposed solution for dealing with this situation, i.e. how
to I create a true copy of what struct.unpack() returns me?

Thanks in advance
Ciao
Uwe

[Peter Otten]

Re: always the same object

Uwe Mayer wrote:
[color=blue]
> Hi,
>
> I am using struct.unpack() quite often. unpack() returns a tuple, however
> a tuple with always the same id(), which is AFAIK the memory location of
> the structure.
>
> I found myself working with above tuple, initialising other data
> structures, etc... and when I tried to change one list element, suddenly
> all list elements change => so they all contain the identical object
>
> Now what are the rules when Python re-uses an old object (as with
> struct.unpack() ) and when does it create new objects?
> And what ist the proposed solution for dealing with this situation, i.e.
> how to I create a true copy of what struct.unpack() returns me?[/color]

Most likely your problems have nothing to do with struct.unpack() , as it
returns a tuple containing - as far as I know - only immutable objects. I
suppose you are using the same list multiple times later in your script,
and making a shallow copy

otherList = list(someList)

should suffice to remedy the observed "change once, see anywhere"
experience. However, this is hard to tell without any sourcecode. So in the
future, please take the time to compose a minimal script reproducing the
observed behaviour along with a short description of what the script is
supposed to do.

Peter

[Jeff Epler]

Re: always the same object

If you included some code, maybe we'd be able to help you.
[color=blue][color=green][color=darkred]
>>> struct.unpack(" 2i", " "*8)[/color][/color][/color]
(538976288, 538976288)

struct.unpack returned a tuple. The tuple happens to be immutable, and
all the things it contains are immutable. So there's no way to change
ever change the value of this object. As far as I know, this would be
true for any struct format you care to choose.

(You could change the "value" of an immutable object if it contains some
mutable objects, like this:
a = ([],)
b = ([],)
assert a == b
a[0].append(0)
assert a != b
a and b go from equal to unequal, even though a and b are both
immutable)

Jeff

[John Roth]

Re: always the same object


"Uwe Mayer" <merkosh@hadiko .de> wrote in message
news:bukbuo$rqp $1@news.rz.uni-karlsruhe.de...[color=blue]
> Hi,
>
> I am using struct.unpack() quite often. unpack() returns a tuple, however[/color]
a[color=blue]
> tuple with always the same id(), which is AFAIK the memory location of the
> structure.
>
> I found myself working with above tuple, initialising other data[/color]
structures,[color=blue]
> etc... and when I tried to change one list element, suddenly all list
> elements change => so they all contain the identical object[/color]

The biggest problem people have with lists is attempting to
reuse one rather than starting with a new one.

For example, this ***WILL NOT*** work:

class Fubar:
aList = [None, None, None]

def addToList(self, something):
aList[:3] = something

on the other hand, this will work:

class Good:
def __init__(self):
self.aList = [None, None, None]

def addToList(self, something):
aList[:3] = something

[color=blue]
> Now what are the rules when Python re-uses an old object (as with
> struct.unpack() ) and when does it create new objects?[/color]

As far as lists are concerned, list literals ([...]) always return a
new list.
[color=blue]
> And what ist the proposed solution for dealing with this situation, i.e.[/color]
how[color=blue]
> do I create a true copy of what struct.unpack() returns me?[/color]

See above.

John Roth[color=blue]
>
> Thanks in advance
> Ciao
> Uwe[/color]