[andipro]

I was very surprised about the result of following code:

list0 = [u"Zero",u"One",u"Two"]
list1 = list0
list1.insert(0,list1.pop(1))
print list1
print list0

Result:
>>>
[u'One', u'Zero', u'Two']
[u'One', u'Zero', u'Two']
>>>
Why does the origin list (list0) also become changed?

[bogdan.galiceanu]

Because list1 points to list0, so to speak. If you want to prevent list0 from changing:

list0 = [u"Zero",u"One",u"Two"]
list1 = list(list0)
list1.insert(0,list1.pop(1))
print list1
print list0

Result:
[u'One', u'Zero', u'Two']
[u'Zero', u'One', u'Two']

[andipro]

That’s very interesting.

I never have seen such effect in other program languages, I think.
When writing an assigning like X = Y afterwards it should not make any difference for X what happens with Y

But thanks for your alternative code example!

[igordsm]

You could also use the slice operator with the default values:

list0 = [u"Zero",u"One",u"Two"]
list1 = list0[:]
list1.insert(0,list1.pop(1))
print list1
print list0

To learn more about lists in python visit http://effbot.org/zone/python-list.htm

[pedroagb]

Hi guys, I think I have a similar problem (python noob here).
I was trying to do a list of lists this way:

>>> list1=[[0]*2]*3
>>> list1
[[0, 0], [0, 0], [0, 0]]

but when I try to set an element, all elements of the same index change too:

>>> list1[0][1]=1
>>> list1
[[0, 1], [0, 1], [0, 1]]

But this works as I would expect:

>>> list2=[[0 for j in range(2)] for i in range(3)]
>>> list2
[[0, 0], [0, 0], [0, 0]]
>>> list2[0][1]=1
>>> list2
[[0, 1], [0, 0], [0, 0]]

Is it because [0]*n creates n references to the same object, not n different objects? I thought it was the short way to initialize an array...
Can someone please explain? Thanks in advance!

[aya42]

Originally Posted by pedroagb

Is it because [0]*n creates n references to the same object, not n different objects? I thought it was the short way to initialize an array...

[x]*n will create 'n' copies of 'x' if 'x' is of an immutable type, or 'n' references to 'x' if 'x' is of a mutable type.

Immutable types include: boolean, complex, float, int, long, none, string, tuple, type, unicode.

Mutable types include: class, dictionary, function, instance, list, module.

[pedroagb]

hum... so if I follow you right, when I do [[0]*2]*3 it fails because while in [0]*2 the multiplication operates on an immutable type (int 0 - actually why isnt [0] considered a list too?), in [[0]*2]*3 it does no longer so (cause now it operates on a list [0,0])
That will take a while to soak in, but I can see it makes sense...
Anyway, thanks for your reply, it was helpful