Can I create a "view" on a Python list?
arrays, list, python, slice
Solution
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
Problem
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; ```