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A “frozen” dictionary for Python

(lwn.net)
191 points jwilk | 1 comments | 11 Dec 25 09:51 UTC | HN request time: 0.198s | source
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drhagen ◴[11 Dec 25 11:42 UTC] No.46230181[source]▶
If this gets wide enough use, they could add an optimization for code like this:

    n = 1000
    a = {}
    for i in range(n):
        a[i] = str(i)
    a = frozendict(a)  # O(n) operation can be turned to O(1)
It is relatively easy for the JIT to detect the `frozendict` constructor, the `dict` input, and the single reference immediately overwritten. Not sure if this would ever be worth it.
replies(1): >>46230599 #
_flux ◴[11 Dec 25 12:33 UTC] No.46230599[source]▶
Wouldn't ref-counting CPython already know that a has a single reference, allowing this optimization without needing any particular smart JIT?
replies(3): >>46231093 #>>46231158 #>>46236475 #
1. zbentley ◴[11 Dec 25 13:27 UTC] No.46231093[source]▶
I think GP was talking about optimizing away the O(N) call on the last line. The GC will take care of removing the reference to the old (mutable) dict, but constructing a new frozendict from a mutable dict would, in the current proposal, be an O(N) shallow copy.

There are also potentially other optimizations that could be applied (not specific to dict/frozendict) to reduce the memory overhead on operations like "a = f(a)" for selected values of "f".