# Gotcha Question

Akira Kiyomiya akira.kiyomiya at autodesk.com
Wed Mar 8 20:46:07 CET 2000

```Well, there was a gotcha question from Learning Python form O'Reilly at p64
and it is really a gotcha question since it got me....

Could someone explain why it is in dummy's language?   Maybe I am not used
to Python stuff.

1.
>>> L = [1, 2, 3]
>>> M = ['X', 'L', 'Y']    #embed a reference to L
>>> M
['X', [1, 2, 3], 'Y']

>>> L[1] = 0     #changes M too
>>> M
['X', [1, 0, 3], 'Y']    #yeah, I can understand it.   Easy.

>>> L = [1, 2, 3]
>>> M = ['X', L[:], 'Y']    #embed a copy of L
>>> L[1] = 0     #only changes L, not M
>>> L
[1, 0, 3]
>>>M
['X', [1, 2, 3], 'Y']      #Why??  I know it only changed L, not M.   But
why is it?

2.
>>>L = [4, 5, 6]
>>> X = L * 4
>>>Y = [L] * 4
>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]      # yeah, it is easy
>>>Y
[[4, 5, 6], [4, 5, 6], [4, 5, 6], [4, 5, 6]]   # yeah, it is easy

>>> L[1] = 0;   # impacts y but not x
>>> X
[4, 5, 6, 4, 5, 6, 4, 5, 6, 4, 5, 6]      # why?
>>>Y
[[4, 0, 6], [4, 0, 6], [4, 0, 6], [4, 0, 6]]   # yeah, I understand this.

```