Efficient implementation of deeply nested lists

[Peter Otten]

Kay Schluehr wrote:

> I want to manipulate a deeply nested list in a generic way at a
> determined place. Given a sequence of constant objects a1, a2, ..., aN
> and a variable x. Now construct a list from them recursively:
> 
> L = [a1, [a2, [....[aN, [x]]...]]
> 
> The value of x is the only one to be changed. With each value of x a
> new instance of L is needed so that we actually have a function:
> 
> L = lambda x: [a1, [a2, [....[aN, [x]]...]]
> 
> I'm seeking for an efficient implementation that does not recompute the
> whole list each time. Any ideas?

I'd say you are nesting the wrong way.

>>> items = make_list(range(10))
>>> def make_list(items):
...     return reduce(lambda *args: args,  items)
...
>>> def update_value(items, value):
...     return items[0], value
...
>>> items = make_list(range(10))
>>> items
(((((((((0, 1), 2), 3), 4), 5), 6), 7), 8), 9)
>>> update_value(items, 42)
(((((((((0, 1), 2), 3), 4), 5), 6), 7), 8), 42)

Obviously I must be missing something. What?

Peter