gh-144191: Dataclasses single field ordering

[whyvineet]

Simplify single-field dataclass ordering comparisons
#144191

• Issue: Can dataclass ordering avoid tuple wrappers for single field comparisons? #144191

[bedevere-app]

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[johnslavik]

Thanks! You can wait for the green light from Eric before going forward or add tests for this even now. I'd probably change test_1_field_compare. And of course a news entry, too.

[bedevere-app]

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[whyvineet]

Hey @picnixz and @johnslavik, just a quick update: I ran a small local benchmark on a CPython 3.15 dev build to ensure this change doesn't cause any noticeable slowdown. In my tests, the match/case version was slightly faster in the single-field scenario and comparable in the multi-field case. Since this code executes only during class creation, I didn’t notice any adverse effects.

I’m happy to follow your advice on this. If you’d rather test it yourselves or revert to the if/else approach for clarity, just let me know, and I’ll update the PR accordingly.

[whyvineet]

Just for reference, here are the numbers I observed locally (CPython 3.15 dev build):

• Single-field case: match/case ~8–9% faster than if len(flds) == 1
• Multiple-field case: no meaningful difference (well under 1%)

These were consistent across repeated runs.

[picnixz]

Please share the benchmarking script and the way you ran it. Class creation matters when considering import time as well.

[whyvineet]

The script was executed using the freshly built interpreter. Each implementation (match/case vs if/else) is run back-to-back for a large number of iterations to reduce noise.

Script

import timeit
import sys
from dataclasses import dataclass

class MockField:
    def __init__(self, name):
        self.name = name

def _tuple_str(obj_name, flds):
    if not flds:
        return '()'
    return f'({",".join(f"{obj_name}.{f.name}" for f in flds)},)'

def match_impl(flds):
    match flds:
        case [single_fld]:
            self_expr = f"self.{single_fld.name}"
            other_expr = f"other.{single_fld.name}"
        case _:
            self_expr = _tuple_str("self", flds)
            other_expr = _tuple_str("other", flds)
    return self_expr, other_expr

def ifelse_impl(flds):
    if len(flds) == 1:
        self_expr = f"self.{flds[0].name}"
        other_expr = f"other.{flds[0].name}"
    else:
        self_expr = _tuple_str("self", flds)
        other_expr = _tuple_str("other", flds)
    return self_expr, other_expr

def run_benchmark():
    single_field = [MockField("value")]
    multi_field = [MockField("x"), MockField("y"), MockField("z")]
    iterations = 1_000_000

    # Just to make sure that I am using the local build
    print("Python:", sys.version)
    print("Executable:", sys.executable)
    print()

    print("Single-field case")
    print("match/case:", timeit.timeit(lambda: match_impl(single_field), number=iterations))
    print("if/else:", timeit.timeit(lambda: ifelse_impl(single_field), number=iterations))

    print("Multi-field case")
    print("match/case:", timeit.timeit(lambda: match_impl(multi_field), number=iterations))
    print("if/else:", timeit.timeit(lambda: ifelse_impl(multi_field), number=iterations))

    print("Real dataclass comparison (sanity check)")
    @dataclass(order=True)
    class A:
        x: int

    @dataclass(order=True)
    class B:
        x: int
        y: int
        z: int

    a1, a2 = A(1), A(2)
    b1, b2 = B(1, 2, 3), B(2, 3, 4)

    print("single-field:", timeit.timeit(lambda: a1 < a2, number=iterations))
    print("multi-field: ", timeit.timeit(lambda: b1 < b2, number=iterations))

if __name__ == "__main__":
    run_benchmark()