Support for nested indexing?

[davidbrochart]

What happened:

Nested indexing doesn't seem to be supported.

What you expected to happen:

Maybe it's not in the scope of this library, but it would be great if the mutable mapping felt more like a dictionary.

Minimal Complete Verifiable Example:

import pickle
import zlib

from zict import File, Func, LRU

l4 = File("cache/", mode="a")
l3 = Func(zlib.compress, zlib.decompress, l4)
l2 = Func(pickle.dumps, pickle.loads, l3)
l1 = LRU(100, l2)

l1["foo"] = {}
l1["foo"]
# {}

l1["foo"]["bar"] = {}
l1["foo"]["bar"]
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# KeyError: 'bar'

Anything else we need to know?:

I'd like to use zict in akernel to cache the execution of cells. It currently uses a dictionary for the cache, and I thought I would be able to swap if with a mutable mapping from zict.

Environment:

• Dask version: N/A
• Python version: 3.10
• Operating System: Ubuntu 21.04
• Install method (conda, pip, source): pip