perf(python): read union case fields directly instead of rebuilding Array

[MangelMaxime]

Summary

When working on Scriptorium, I discovered that Python generated code is slower than JS for union-heavy workloads.

One of the main reasons is that union case field reads (x.fields[i], emitted for every pattern match) went through Union.fields, a @property that rebuilds a full Array from getattr() calls on every access. Even if JavaScript does a similar thing, in that case it stores fields once at construction, while Python rebuilds it every time.

In my codebase, it resulted in union.fields being called (~8M calls) when profiling.

Amounting to ~5sec of the run time.

Before:

def sum(t: Tree) -> int32:
    match t.tag:
        case 1:
            return sum(t.fields[0]) + sum(t.fields[1])
        case _:
            return t.fields[0]

After:

def sum(t: Tree) -> int32:
    match t.tag:
        case 1:
            return sum(cast(Tree_Node, t).left_) + sum(cast(Tree_Node, t).right_)
        case _:
            return cast(Tree_Leaf, t).value_

The drawback I found from this change is that we now need to cast the union to the case class type before reading its fields. Otherwise, Pyright will complain that the "field property" (ex left_, item1, etc. ) is not defined on the union type.

Benchmark

I made a small benchmark to test the performance difference between the two approaches using something like the following code:

type Tree =
    | Leaf of value: int
    | Node of left: Tree * right: Tree

let rec sum (t: Tree) : int =
    match t with
    | Leaf v -> v
    | Node (l, r) -> sum l + sum r

Results:

nodes	before	after	speedup
2,047	2.24s	0.22s	10.2x
65,535	3.86s	0.56s	6.8x
2,097,151	4.51s	0.70s	6.4x

[github-actions]

Python Type Checking Results (Pyright)

Metric	Value
Total errors	34
Files with errors	4
Excluded files	4
New errors	✅ No

Excluded files with errors (4 files)

These files have known type errors and are excluded from CI. Remove from pyrightconfig.ci.json as errors are fixed.

File	Errors	Status
temp/tests/Python/test_hash_set.py	18	Excluded
temp/tests/Python/test_applicative.py	12	Excluded
temp/tests/Python/test_nested_and_recursive_pattern.py	2	Excluded
temp/tests/Python/fable_modules/thoth_json_python/encode.py	2	Excluded