I have a Python code, where I first define two lists, then I make them identical and do the same operation on them after they are identical - but the result is not the same:
test1 = [[[0],[0]]]*2
test2 = [[0]]*2
test2[0] = [[0],[0]]
test2[1] = [[0],[0]]
print 'They are identical?:',test1 == test2 # gives True
# Now that test1 == test2 do the same operation on test list 1 and 2:
test1[0][0] = 2
test2[0][0] = 2
print test1
print test2
This gives
[[2, [0]], [2, [0]]] # test1
[[2, [0]], [[0], [0]]] # test2
Can somebody explain the difference to me?
If x is a list,
x * 2
Returns two times the same elements from the list.
As lists are passed by references, this means:
>>> A = 2 * [[0]]
>>> B = [[0], [0]]
Will actually not have the same structure: both A[0] and A[1] point to the same list, while B[0] and B[1] point to two different lists.
And indeed, if you try:
>>> A[0] == A[1]
True
>>> A[0] is A[1]
True
>>> A[0].append(1)
>>> A
[[0, 1], [0, 1]]
While:
>>> B[0] == B[1]
True
>>> B[0] is B[1]
False
>>> B[0].append(1)
>>> B
[[0, 1], [0]]