understanding list scope

[Alex]

Hi all!

I have a problem understanding the behaviour of this snippet:

data_set = ({"param":"a"}, {"param":"b"},{ "param":"c" })

for i in range(len(data_ set)):
ds = data_set[:]
data = ds[i]
if i == 1: data['param'] = "y"
if i == 2: data['param'] = "x"

print data_set


This script print out:
({'param': 'a'}, {'param': 'y'}, {'param': 'x'})

Why? I'm coping data_set in ds so why data_set is changed?

Thanks in advance.

Alex

[George Sakkis]

Re: understanding list scope

On Sep 21, 8:51 am, Alex <metallourla... @gmail.comwrote :

Hi all!
>
I have a problem understanding the behaviour of this snippet:
>
data_set = ({"param":"a"}, {"param":"b"},{ "param":"c" })
>
for i in range(len(data_ set)):
    ds = data_set[:]
    data = ds[i]
    if i == 1: data['param'] = "y"
    if i == 2: data['param'] = "x"
>
print data_set
>
This script print out:
({'param': 'a'}, {'param': 'y'}, {'param': 'x'})
>
Why? I'm coping data_set in ds so why data_set is changed?

Because you're doing a shallow copy: http://docs.python.org/lib/module-copy..html

George

[alex23]

Re: understanding list scope

On Sep 21, 10:51 pm, Alex <metallourla... @gmail.comwrote :

Why? I'm coping data_set in ds so why data_set is changed?

You're making a copy of the ds tuple, which has the -same- contents as
the original. To create copies of the contents as well, try the
deepcopy function from the copy module.

As an aside, you're also trying to make a copy of ds for each
iteration of the loop, which is unnecessary in this case. Here's a
slightly better example of your code:

>>from copy import deepcopy
>>data_set = ({"param":"a"}, {"param":"b"},{ "param":"c" })
>>ds = deepcopy(data_s et)
>>for i, data in enumerate(ds):

.... if i == 1: data['param'] = "y"
.... if i == 2: data['param'] = "x"
....

>>print data_set

({'param': 'a'}, {'param': 'b'}, {'param': 'c'})

>>print ds

({'param': 'a'}, {'param': 'y'}, {'param': 'x'})

Although your use of a tuple full of dicts for data_set is kinda
strange... Tuples are generally used when you want a structured data
element, in which case you'd just address each element directly rather
than iterate through it:

>>ds = deepcopy(data_s et)
>>ds[1]['param'] = "y"
>>ds[2]['param'] = "x"

[Alex]

Re: understanding list scope

On 21 Set, 15:07, George Sakkis <george.sak...@ gmail.comwrote:

On Sep 21, 8:51 am, Alex <metallourla... @gmail.comwrote :
>
>
>

Hi all!

>

I have a problem understanding the behaviour of this snippet:

>

data_set = ({"param":"a"}, {"param":"b"},{ "param":"c" })

>

for i in range(len(data_ set)):
ds = data_set[:]
data = ds[i]
if i == 1: data['param'] = "y"
if i == 2: data['param'] = "x"

>

print data_set

>

This script print out:
({'param': 'a'}, {'param': 'y'}, {'param': 'x'})

>

Why? I'm coping data_set in ds so why data_set is changed?

>
Because you're doing a shallow copy:http://docs.python.org/lib/module-copy.html
>
George

Thanks a lot. It was giving me and headache!

[Peter Pearson]

Re: understanding list scope

On Sun, 21 Sep 2008 06:17:36 -0700 (PDT), Alex wrote:

On 21 Set, 15:07, George Sakkis <george.sak...@ gmail.comwrote:

>On Sep 21, 8:51 am, Alex <metallourla... @gmail.comwrote :

[snip]

I have a problem understanding the behaviour of this snippet:

[snip]

>Because you're doing a shallow copy:
>http://docs.python.org/lib/module-copy.html

[snip]

Thanks a lot. It was giving me and headache!

FWIW, since I started following this newsgroup, I've noticed
that I no longer have those crises that revolve around the depth
of a copy. I conjecture that they resulted from non-pythonic
style. Try following this newsgroup for a while, and you might
see a lot of startlingly elegant ways of doing things.

--
To email me, substitute nowhere->spamcop, invalid->net.

[mbezzi@landi.it]

Re: understanding list scope

On Sep 21, 7:48 pm, Peter Pearson <ppear...@nowhe re.invalidwrote :

FWIW, since I started following this newsgroup, I've noticed
that I no longer have those crises that revolve around the depth
of a copy.  I conjecture that they resulted from non-pythonic
style.  Try following this newsgroup for a while, and you might
see a lot of startlingly elegant ways of doing things.
>
--
To email me, substitute nowhere->spamcop, invalid->net.

I know the code is really ugly but I just make a quick and dirty
snippet to ask for help.
I should have wrapped the code between <bad code></bas codetags ;-)

Alex