Deepcopy on nested referenced lists created by list multiplication does not work

Question

As much as I love Python, the reference and deepcopy stuff sometimes freaks me out.

Why does deepcopy not work here:

>>> import copy
>>> a = 2*[2*[0]]
>>> a
[[0, 0], [0, 0]]
>>> b = copy.deepcopy(a)
>>> b[0][0] = 1
>>> b
[[1, 0], [1, 0]]     #should be: [[1, 0], [0, 1]]
>>> 

I am using a numpy array as a workarround which I need later on anyway. But I really had hoped that if I used deepcopy I would not have to chase any unintended references any more. Are there any more traps where it does not work?

Answer 1

It doesn't work because you are creating an array with two references to the same array.

An alternative approach is:

[[0]*2 for i in range(2)]

Or the more explicit:

[[0 for j in range(2)] for i in range(2)]

This works because it creates a new array on each iteration.

Are there any more traps where it does not work?

Any time you have an array containing references you should be careful. For example [Foo()] * 2 is not the same as [Foo() for i in range(2)]. In the first case only one object is constructed and the array contains two references to it. In the second case, two separate objects are constructed.