Dear,

I am using
$ python -V
Python 2.6.4
on Ubuntu 9.10

I met a serious bug in s.append(x) operation. If I append a list into another list, there is a change of content. In the following code, [2,-2,0,0] is replaced with [-2,-2,0,0] after s.append(x) operaton.

------- begin of code ------

total=[]
for i in range(4):
    for j in range(i):
        root=[0,0,0,0]
        for k in [2,-2]:
         for l in [2,-2]:
          root[i]=k
          root[j]=l
          total.append(root)
          print root
print total

--------------- end of code -----

Result: each element is correctly generated
[2, 2, 0, 0]
[-2, 2, 0, 0]
[2, -2, 0, 0]
[-2, -2, 0, 0]
[2, 0, 2, 0]
[-2, 0, 2, 0]
[2, 0, -2, 0]
[-2, 0, -2, 0]
[0, 2, 2, 0]
[0, -2, 2, 0]
[0, 2, -2, 0]
[0, -2, -2, 0]
[2, 0, 0, 2]
[-2, 0, 0, 2]
[2, 0, 0, -2]
[-2, 0, 0, -2]
[0, 2, 0, 2]
[0, -2, 0, 2]
[0, 2, 0, -2]
[0, -2, 0, -2]
[0, 0, 2, 2]
[0, 0, -2, 2]
[0, 0, 2, -2]
[0, 0, -2, -2]

But the total list is wrong
[[-2, -2, 0, 0], [-2, -2, 0, 0], [-2, -2, 0, 0], [-2, -2, 0, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [-2, 0, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [0, -2, -2, 0], [-2, 0, 0, -2], [-2, 0, 0, -2], [-2, 0, 0, -2], [-2, 0, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, -2, 0, -2], [0, 0, -2, -2], [0, 0, -2, -2], [0, 0, -2, -2], [0, 0, -2, -2]]

This is a bug in your code, rather than in Python.

A simpler example, for the purposes of explanation:

>>> root, total = [0], []
>>> total.append(root)
>>> total  # good so far
[[0]]
>>> root[0] = 1   # modify root
>>> total  # note that total changes here!
[[1]]
>>> total.append(root)
>>> total
[[1], [1]]

In effect, total contains two references to the same list.  Modify that list, and you'll see those changes reflected in `total` as well.

Thank you for kind explanation.

So s.append(x) just copies the pointer, not the actual value. It is a little tricky to know that.

> So s.append(x) just copies the pointer, not the actual value.

Yes, that's a reasonable way to think about it (though the term 'reference' seems to more popular than 'pointer' in this context).  It matches the implementation, too: internally, a list is represented as a resizable array of pointers to objects.  (For CPython, anyway;  alternative implementations of Python might differ.)

Implicit object copies are rare in Python.

comp.lang.python is a good place to ask questions about Python's object model.