Slice in python - is it a copy or just a pointer?

Question

>>> a = [3, 2]
>>> a[0:1][0] = 1
>>> a
[3, 2]

>>> a[0:1] = [1]
>>> a
[1, 2]

What does a[0:1] mean?

1. If it's a pointer to the range of a, then a[0:1][0] = 1 should change the value of a.
2. If it's a copy of the range of a, then a[0:1] = [1] shouldn't change the value of a.

I think the result of the two is inconsistent with each other. Could you please help me work out the problem?

Answer 1

Internally, this is a big difference:

>>> a = [3, 2]
>>> a[0:1][0] = 1

is a shorthand for

temp = a[0:1]
temp[0] = 1

and is internally expressed as

a.__getitem__(slice(0, 1)).__setitem__(0, 1)

resp.

temp = a.__getitem__(slice(0, 1))
temp.__setitem__(0, 1)

so it accesses a part of the list, making a separate object, and doing an assignment on this object, which is then dropped.

On the other hand,

>>> a[0:1] = [1]

does

a.__setitem__(slice(0, 1), [1])

which just operates on the original object.

So, while looking similar, these expressions are distinct on what they mean.

Let's test that:

class Itemtest(object):
    def __init__(self, name):
        self.name = name
    def __repr__(self):
        return self.name
    def __setitem__(self, item, value):
        print "__setitem__", self, item, value
    def __getitem__(self, item):
        print "__getitem__", self, item
        return Itemtest("inner")

a = Itemtest("outer")
a[0:1] = [4]
temp = a[0:1]
temp[0] = 4
a[0:1][0] = 4

outputs

__setitem__ outer slice(0, 1, None) [4]
__getitem__ outer slice(0, 1, None)
__setitem__ inner 0 4
__getitem__ outer slice(0, 1, None)
__setitem__ inner 0 4