[issue1408] Inconsistence in multiply list

2007-11-09 Thread Georg Brandl 

Georg Brandl added the comment:

I'm sorry, this is no bug. List multiplication works by referencing,
there is no way to implement it differently in a straightforward way.

Note that in
 [a[:]] + [a[:]]
the expression a[:] is evaluated twice, yielding two independent
copies of a. In contrast,
 [a[:]] * 2
evaluates a[:] only once, before the list multiplication is done.
Because of the same reason,
 [deepcopy(a)] * 2
doesn't work as you want.

One way to do what you have in mind is
 [a[:] for i in range(2)]
which evaluates the a[:] once for each iteration of the list
comprehension loop.

--
nosy: +georg.brandl
resolution:  - invalid
status: open - closed

__
Tracker [EMAIL PROTECTED]
http://bugs.python.org/issue1408
__
___
Python-bugs-list mailing list 
Unsubscribe: 
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue1408] Inconsistence in multiply list

2007-11-08 Thread beco 

Changes by beco:


--
components: Interpreter Core
nosy: beco
severity: major
status: open
title: Inconsistence in multiply list
type: behavior
versions: Python 2.5

__
Tracker [EMAIL PROTECTED]
http://bugs.python.org/issue1408
__
___
Python-bugs-list mailing list 
Unsubscribe: 
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com

[issue1408] Inconsistence in multiply list

2007-11-08 Thread beco 

New submission from beco:

There is no way to create a big nested list without references using the
multiplication operator.

'*' is supposed to work like + ... + in this cases:

 a=[0, 0]
 b=[a[:]]+[a[:]]
 b
[[0, 0], [0, 0]]
 b[0][0]=1
 b
[[1, 0], [0, 0]]

Ok! Copy here, not reference. Mainly because we use [:] explicitly
expressing we want a copy.

 c=[a[:]]*2
 c
[[0, 0], [0, 0]]
 c[0][0]=2
 c
[[2, 0], [2, 0]]

Inconsistence here. It is supposed to be clear and copy, not reference
in between.

Consequence: there is no clear way to create a nested list of, lets say,
60x60, using multiplications.

Even when using this, we cannot deal with the problem:

 import copy
 d=[copy.deepcopy(a[:])]*2
 d
[[0, 0], [0, 0]]
 d[0][0]=3
 d
[[3, 0], [3, 0]]

Workaround:
 from numpy import *
 a=zeros((2,2),int)
 a
array([[0, 0],
   [0, 0]])
 b=a.tolist()
 b
[[0, 0], [0, 0]]
 b[0][0]=4
 b 
[[4, 0], [0, 0]]

And that is the expected behaviour.

Thanks.

__
Tracker [EMAIL PROTECTED]
http://bugs.python.org/issue1408
__
___
Python-bugs-list mailing list 
Unsubscribe: 
http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com