unexpected behavior: did i create a pointer?

[gu]

hi to all!
after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

<code>
a = {}

for x in range(10):
for y in range(10):
a[x,y] = "0"

copyOfA = a

def functionA(x,y):
print a[x,y],
copyOfA[x,y] = "*"
print a[x,y],copyOfA[x,y]
for x in range(10):
for y in range(10):
functionA(x,y)

</code>
now, in the second "for" cycle and in functionA() i only 'touch' copyOfA
(altering it). as i don't touch the variable "a", i expect it not to be
affected by any change, but copyOfA acts like a pointer to a and
altering copyOfA's values result in altering the values of "a", so the
result that i expect is:
0 0 *
0 0 *
0 0 *
0 0 *
[..]

but i get:
0 * *
0 * *
0 * *
0 * *
[..]

what's going on?
thanks in advance.

[Gabriel Genellina]

En Fri, 07 Sep 2007 05:07:03 -0300, gu <pi********@gma il.comescribiï¿ ½:

after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

<code>
a = {}

for x in range(10):
for y in range(10):
a[x,y] = "0"

copyOfA = a

copyOfA is *NOT* a copy - it's just another name pointing to the SAME
object as a.
Python will never copy anything unless told explicitely.

Read this <http://effbot.org/zone/python-objects.htm>

--
Gabriel Genellina

[Roel Schroeven]

gu schreef:

copyOfA = a

now, in the second "for" cycle and in functionA() i only 'touch' copyOfA
(altering it).

copyOfA isn't a copy of a; it's a different name bound to the same
object as a. You can verify that: id(a) and id(copyOfA) will return the
same value.

To make a copy of a (assuming a is a dict), you can do:

copyOfA = dict(a)

or

copyOfA = a.copy()

or more generally

import copy
copyOfA = copy.copy(a)
Cheers,
Roel

--
If I have been able to see further, it was only because I stood
on the shoulders of giants. -- Isaac Newton

Roel Schroeven

[Wildemar Wildenburger]

gu wrote:

hi to all!
after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

[snip FAQ]

Yes, basically you *created* a pointer. That's all that python has:
pointers.

When saying

>>a = AnyOldObject()
b = a

then 'a' and 'b' are different /names/ for the /very same/ object (try
"a is b", or "id(a)==id(b)") .

This is really a FAQ (once a week or so?), but for the life of me I
can't find the right words for a google query.
TO THE TROOP: What keywords would you attach to that question?

/W

[Steven D'Aprano]

On Fri, 07 Sep 2007 11:46:38 +0200, Wildemar Wildenburger wrote:

gu wrote:

>hi to all!
after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

[snip FAQ]

Yes, basically you *created* a pointer. That's all that python has:
pointers.

No, you are confusing the underlying C implementation with Python. Python
doesn't have any pointers. CPython is implemented with pointers. PyPy,
being written entirely in Python, is implemented with Python objects like
lists and dicts. Jython, being implemented in Java, probably isn't
implemented with pointers either -- although of course the underlying
Java compiler might be. IronPython and Python for .Net, I have no idea
how they work. Could be magic for all I know. (Probably necromancy.)

Naturally, regardless of whether you are using CPython, IronPython, PyPy
or some other variety of Python, the objects available to you include
ints, floats, strings, lists, dicts, sets and classes... but not pointers.

Nor does it include "peek" and "poke" commands for reading and writing
into random memory locations. Python is not C, and it is not Basic, nor
is it Forth or Lisp or assembler, and you shouldn't hammer the round peg
of Python objects into the square hole of C pointers.

--
Steven.

[Marc 'BlackJack' Rintsch]

On Fri, 07 Sep 2007 10:40:47 +0000, Steven D'Aprano wrote:

Nor does it include "peek" and "poke" commands for reading and writing
into random memory locations.

I guess `ctypes` offers tools to write `peek()` and `poke()`. :-)

Ciao,
Marc 'BlackJack' Rintsch

[Peter Otten]

Am Fri, 07 Sep 2007 10:40:47 +0000 schrieb Steven D'Aprano:

Python doesn't have any pointers.

Thinking of python variables or "names" as pointers should
get you a long way when trying to understand python's behaviour.

As long as you keep in mind that python doesn't have pointers to pointers,
and no pointer arithmetic either...

Peter

[Grant Edwards]

On 2007-09-07, Peter Otten <__*******@web. dewrote:

Am Fri, 07 Sep 2007 10:40:47 +0000 schrieb Steven D'Aprano:

>Python doesn't have any pointers.

Thinking of python variables or "names" as pointers should
get you a long way when trying to understand python's behaviour.

But thinking of them as names bound to objects will get you
further (and get you there faster). ;)

As long as you keep in mind that python doesn't have pointers
to pointers, and no pointer arithmetic either...

--
Grant Edwards grante Yow! Hello... IRON
at CURTAIN? Send over a
visi.com SAUSAGE PIZZA! World War
III? No thanks!

[Wildemar Wildenburger]

Steven D'Aprano wrote:

On Fri, 07 Sep 2007 11:46:38 +0200, Wildemar Wildenburger wrote:

>gu wrote:

>>hi to all!
after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

[snip FAQ]

Yes, basically you *created* a pointer. That's all that python has:
pointers.

No, you are confusing the underlying C implementation with Python.

I do not, as I have no clue of the C implementation :).
I just thought I'd go along with the analogy the OP created as that was
his mindset and it would make things easier to follow if I didn't try to
forcibly change that.
Please note that I had intended for the word 'created' to be in quotes
rather than asterisks. I bit myself after I read my mistake online but
then thought "Who cares?". I should have, maybe :).
And yes, I will admit that going along with that analogy isn't the best
way to explain it. Grant Edwards in reply to Peter Otten makes it much
clearer, I guess.

/W

[Steve Holden]

Steven D'Aprano wrote:

On Fri, 07 Sep 2007 11:46:38 +0200, Wildemar Wildenburger wrote:

>gu wrote:

>>hi to all!
after two days debugging my code, i've come to the point that the
problem was caused by an unexpected behaviour of python. or by lack of
some information about the program, of course! i've stripped down the
code to reproduce the problem:

[snip FAQ]

Yes, basically you *created* a pointer. That's all that python has:
pointers.

No, you are confusing the underlying C implementation with Python. Python
doesn't have any pointers. CPython is implemented with pointers. PyPy,
being written entirely in Python, is implemented with Python objects like
lists and dicts. Jython, being implemented in Java, probably isn't
implemented with pointers either -- although of course the underlying
Java compiler might be. IronPython and Python for .Net, I have no idea
how they work. Could be magic for all I know. (Probably necromancy.)

Naturally, regardless of whether you are using CPython, IronPython, PyPy
or some other variety of Python, the objects available to you include
ints, floats, strings, lists, dicts, sets and classes... but not pointers.

Nor does it include "peek" and "poke" commands for reading and writing
into random memory locations. Python is not C, and it is not Basic, nor
is it Forth or Lisp or assembler, and you shouldn't hammer the round peg
of Python objects into the square hole of C pointers.

This seems to be obscuring the real issue for the sake of hammering an
error in vocabulary. gu said """Yes, basically you *created* a pointer.
That's all that python has: pointers.""" and you took issue with that,
ultimately saying """the objects available to you include ints, floats,
strings, lists, dicts, sets and classes... but not pointers""".

You are both right, and you are also both wrong.

Python does indeed provide a rich set of strongly typed object classes,
and so clearly to say "Python only has pointers" is strictly an error.
However, this overlooks the fact that *all* names in Python, and *all*
items in container objects (lists, tuples, dicts ...) hold references to
objects.

I mention this because it *is* significant to the semantics of
assignment. From the point of view of a C programmer Python might
actually look quite like a language in which all data objects had to be
malloc'd, and the only variables were pointers to. Assignment (binding)
in Python creates copies of references to objects, not copies of the
objects themselves.

I have deliberately omitted discussion of the automatic dereferencing
that takes place in Python, thereby allowing us to treat names as though
they contained objects, but the fact remains that names *don't* contain
objects, they contain references to objects. If you want to regard a
reference and a pointer as two different things then I guess that's your
nit to pick. But I don't think the original assertions quite justified
your scathing remarks about "peek" and "poke".

regards
Steve
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
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