Question

I have a large list l. I want to create a view from element 4 to 6. I can do it with sequence slice.

>>> l = range(10)
>>> lv = l[3:6]
>>> lv
[3, 4, 5]

However lv is a copy of a slice of l. If I change the underlying list, lv does not reflect the change.

>>> l[4] = -1
>>> lv
[3, 4, 5]

Vice versa I want modification on lv reflect in l as well. Other than that the list size are not going to be changed.

I’m not looking forward to build a big class to do this. I’m just hoping other Python gurus may know some hidden language trick. Ideally I hope it can be like pointer arithmetic in C:

int lv[] = l + 3;

Answer 1

There is no “list slice” class in the Python standard library (nor is one built-in). So, you do need a class, though it need not be big — especially if you’re content with a “readonly” and “compact” slice. E.g.:

import collections

class ROListSlice(collections.Sequence):

    def __init__(self, alist, start, alen):
        self.alist = alist
        self.start = start
        self.alen = alen

    def __len__(self):
        return self.alen

    def adj(self, i):
        if i<0: i += self.alen
        return i + self.start

    def __getitem__(self, i):
        return self.alist[self.adj(i)]

This has some limitations (doesn’t support “slicing a slice”) but for most purposes might be OK.

To make this sequence r/w you need to add __setitem__, __delitem__, and insert:

class ListSlice(ROListSlice):

    def __setitem__(self, i, v):
        self.alist[self.adj(i)] = v

    def __delitem__(self, i, v):
        del self.alist[self.adj(i)]
        self.alen -= 1

    def insert(self, i, v):
        self.alist.insert(self.adj(i), v)
        self.alen += 1