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.

[johnslavik]

This is how I'd approach it:

Suggested change

	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)
	match flds:
	# Special-case single field comparisons. See GH-144191.
	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)

[whyvineet]

Thanks! I’ve updated the logic using match/case. It’s more readable and makes the intent clearer.

[picnixz]

Match constructions are known to be way slower than bare ifs. Considering that people sometimes complain that dataclasses are slow, and could impact import times of large modules, can we have a little benchmark introduced by this change?

[johnslavik]

If this would slow down the function significantly, I take this back.
I haven't considered performance impact.
I'll see if I can put together a benchmark, but at earliest on the weekend.

[johnslavik]

A similarly idiomatic version with bare ifs:

if len(flds) == 1:
    (single_fld,) = flds
    self_expr = f"self.{single_fld.name}"
    other_expr = f"other.{single_fld.name}"
else:
    self_expr = _tuple_str('self', flds)
    other_expr = _tuple_str('other', flds)

But at this point it's just cosmetics. Feel free to ignore.

[bedevere-app]

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