Nested List Question

[Newsfeeds]

Hello All,

Could anyone tell me why this code produces the output it does?

noAdjacencies = 2
gridsPerAdj = 3
rows = 4
columns = 5


gridSystemId = [[None]*columns]*rows
for row in range(rows):
for column in range(columns):
gridSystemId[row][column] = "%d-%d" % (row,column)

print gridSystemId

Produces:

[['3-0', '3-1', '3-2', '3-3', '3-4'], ['3-0', '3-1', '3-2', '3-3', '3-4'],
['3-0
', '3-1', '3-2', '3-3', '3-4'], ['3-0', '3-1', '3-2', '3-3', '3-4']]

Rather than:

[['0-0', '0-1', '0-2', '0-3', '0-4'], ['1-0', '1-1', '1-2', '1-3', '1-4'],
['2-0
', '2-1', '2-2', '2-3', '2-4'], ['3-0', '3-1', '3-2', '3-3', '3-4']]

Thanks for your help,
Chris M.



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[Alex Martelli]

Re: Nested List Question

Newsfeeds <chris.mccoy@sp irentcom.com> wrote:
[color=blue]
> Hello All,
>
> Could anyone tell me why this code produces the output it does?[/color]
...[color=blue]
> gridSystemId = [[None]*columns]*rows[/color]

You've made gridSystemID a list of `rows` references to the SAME "inner"
list, so the behavior you observe is the only possible one.

If you want copies instead, ASK for copies...:

gridSystemId = [ [None]*columns for x in xrange(rows) ]


Alex

[Chris McCoy]

Re: Nested List Question

Thank you! I've been banging my head against the wall!

Chris M.

"Alex Martelli" <aleax@mail.com cast.net> wrote in message
news:1h5f4t2.1y v785h120xalfN%a leax@mail.comca st.net...[color=blue]
> Newsfeeds <chris.mccoy@sp irentcom.com> wrote:
>[color=green]
>> Hello All,
>>
>> Could anyone tell me why this code produces the output it does?[/color]
> ...[color=green]
>> gridSystemId = [[None]*columns]*rows[/color]
>
> You've made gridSystemID a list of `rows` references to the SAME "inner"
> list, so the behavior you observe is the only possible one.
>
> If you want copies instead, ASK for copies...:
>
> gridSystemId = [ [None]*columns for x in xrange(rows) ]
>
>
> Alex
>[/color]



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[Roman Suzi]

Pychecker Re: Nested List Question

On Thu, 3 Nov 2005, Chris McCoy wrote:
[color=blue]
> Thank you! I've been banging my head against the wall!
>
> Chris M.[/color]
[color=blue][color=green]
>> gridSystemId = [[None]*columns]*rows[/color]
>
> You've made gridSystemID a list of `rows` references to the SAME "inner"
> list, so the behavior you observe is the only possible one.
>
> If you want copies instead, ASK for copies...:
>
> gridSystemId = [ [None]*columns for x in xrange(rows) ][/color]


Interesting, could not pychecker recognize such situations in Python
code and give warnings?


Sincerely yours, Roman Suzi
--
rnd@onego.ru =\= My AI powered by GNU/Linux RedHat 7.3

[Chris McCoy]

Re: Pychecker Re: Nested List Question

It may, but I haven't been using Pychecker yet. I'm still fairly new to
Python.

Thanks,
Chris M.

"Roman Suzi" <rnd@onego.ru > wrote in message
news:mailman.55 .1130998328.187 01.python-list@python.org ...[color=blue]
> On Thu, 3 Nov 2005, Chris McCoy wrote:
>[color=green]
>> Thank you! I've been banging my head against the wall!
>>
>> Chris M.[/color]
>[color=green][color=darkred]
>>> gridSystemId = [[None]*columns]*rows[/color]
>>
>> You've made gridSystemID a list of `rows` references to the SAME "inner"
>> list, so the behavior you observe is the only possible one.
>>
>> If you want copies instead, ASK for copies...:
>>
>> gridSystemId = [ [None]*columns for x in xrange(rows) ][/color]
>
>
> Interesting, could not pychecker recognize such situations in Python
> code and give warnings?
>
>
> Sincerely yours, Roman Suzi
> --
> rnd@onego.ru =\= My AI powered by GNU/Linux RedHat 7.3
>[/color]



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[Fredrik Lundh]

Re: Nested List Question

"Newsfeeds" <chris.mccoy@sp irentcom.com> wrote:
[color=blue]
> Could anyone tell me why this code produces the output it does?[/color]

http://www.python.org/doc/faq/programming.html#how-do-i-create-a-multidimensional-list

has the full story.

</F>

[Mike Meyer]

Re: Pychecker Re: Nested List Question

Roman Suzi <rnd@onego.ru > writes:[color=blue]
> On Thu, 3 Nov 2005, Chris McCoy wrote:[color=green][color=darkred]
>>> gridSystemId = [[None]*columns]*rows[/color]
>> You've made gridSystemID a list of `rows` references to the SAME "inner"
>> list, so the behavior you observe is the only possible one.
>> If you want copies instead, ASK for copies...:
>> gridSystemId = [ [None]*columns for x in xrange(rows) ][/color]
> Interesting, could not pychecker recognize such situations in Python
> code and give warnings?[/color]

Well, it could always just issue warnings everytime it saw a list
multiplied by something. But that would get annoying in the cases
where that idiom doesn't have problems - which is naturally most such
usages. Your example included one such, which is why the translation
wasn't to:

gridSystemId = [[None for y in xrange(columns)] for x in xrange(rows)] WRONG

[None] * columns doesn't have problems. Nor does any other immutable
object. So to avoid spurious warnings, pychecker would have to know
whether the objects in the list were immutable or not. It could guess
that if the objects are builtin types, but not for other types.

<mike
--
Mike Meyer <mwm@mired.or g> http://www.mired.org/home/mwm/
Independent WWW/Perforce/FreeBSD/Unix consultant, email for more information.