Objects, lists and assigning values

[Manuel Graune]

Hello,

while trying to learn how to program using objects in python (up to now
simple scripts were sufficient for my needs) I stumbled over the
a problem while assigning values to an object.

The following piece of code shows what I intend to do:

<---snip--->

class new_class(objec t):
def __init__( self,
internal_list=[]):
self.internal_l ist= internal_list

external_list=[[b*a for b in xrange(1,5)] for a in xrange(1,5)]
print external_list

first_collectio n=[new_class() for i in xrange(4)]

temporary_list=[[] for i in xrange(4)]
for i in xrange(4):
for j in xrange(4):
temporary_list[i].append(externa l_list[i][j])
first_collectio n[i].internal_list= temporary_list[i]
#Now everything is as I want it to be:
for i in xrange(4):
print first_collectio n[i].internal_list
#Now I tried to get the same result without the temporary
#variable:

second_collecti on=[new_class() for i in xrange(4)]

for i in xrange(4):
for j in xrange(4):
second_collecti on[i].internal_list. append(external _list[i][j])

#Which obviously leads to a very different result:

for i in xrange(4):
print second_collecti on[i].internal_list

<---snip--->

Can someone explain to me, what's happening here and why the two
approaches do not lead to the same results? Thanks in Advance.

Regards,

Manuel


--
A hundred men did the rational thing. The sum of those rational choices was
called panic. Neal Stephenson -- System of the world
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[Gabriel Genellina]

En Thu, 05 Apr 2007 14:13:43 -0300, Manuel Graune <ma***********@ koeln.de>
escribió:

class new_class(objec t):
def __init__( self,
internal_list=[]):
self.internal_l ist= internal_list

All your instances share the *same* internal list, because default
arguments are evaluated only once (when the function is defined)
The usual way is to write:

class new_class(objec t):
def __init__(self, internal_list=N one):
if internal_list is None:
internal_list = []
self.internal_l ist= internal_list

See
http://effbot.org/pyfaq/why-are-defa...en-objects.htm

--
Gabriel Genellina

[Manuel Graune]

Hello Gabriel, hello William,

thanks to both of you for your answers. I seem to need a
better book about python.

Regards,

Manuel

"Gabriel Genellina" <ga*******@yaho o.com.arwrites:

class new_class(objec t):
def __init__(self, internal_list=N one):
if internal_list is None:
internal_list = []
self.internal_l ist= internal_list

See
http://effbot.org/pyfaq/why-are-defa...en-objects.htm

--
Gabriel Genellina

--
A hundred men did the rational thing. The sum of those rational choices was
called panic. Neal Stephenson -- System of the world
http://www.graune.org/GnuPG_pubkey.asc
Key fingerprint = 1E44 9CBD DEE4 9E07 5E0A 5828 5476 7E92 2DB4 3C99

[7stud]

On Apr 6, 1:23 am, Manuel Graune <manuel.gra...@ koeln.dewrote:

Hello Gabriel, hello William,

thanks to both of you for your answers. I seem to need a
better book about python.

What book are you reading?

[Manuel Graune]

"7stud" <bb**********@y ahoo.comwrites:

>
What book are you reading?

I worked my way through most of the online-docs. A bit to casual
obviously.

As printed desktop-reference I use a german book called
"Python ge-packt".
--
A hundred men did the rational thing. The sum of those rational choices was
called panic. Neal Stephenson -- System of the world
http://www.graune.org/GnuPG_pubkey.asc
Key fingerprint = 1E44 9CBD DEE4 9E07 5E0A 5828 5476 7E92 2DB4 3C99

[7stud]

On Apr 7, 2:52 pm, Manuel Graune <manuel.gra...@ koeln.dewrote:

"7stud" <bbxx789_0...@y ahoo.comwrites:

What book are you reading?

I worked my way through most of the online-docs. A bit to casual
obviously.

See the online tutorial's section on default function arguments here:

http://docs.python.org/tut/node6.htm...00000000000000

specifically the "Important Warning".