feat(expr-ir): Impl DataFrame.unique

[dangotbanned]

Description

This'll be the first in a series of PRs that close gaps in non-Expr features.

Will be focusing on what is defined in DslPlan, and this one is for DslPlan::Distinct.

Related issues

• Child of feat(RFC): A richer Expr IR #2572
• oh-nodes...expr-ir/logical-plan

[dangotbanned]

I still want to have this, but it doesn't make sense now that it isn't used in the PR 😅

[dangotbanned]

Follow-up PR

Be stricter here.

Calling an Expr method on a Selector preserves the runtime type of Selector

narwhals/narwhals/_plan/_parse.py

Lines 147 to 161 in 9b9122b

	def _parse_into_selector(
	input: ColumnNameOrSelector | Expr, /, *, require_all: bool = True
	) -> Selector:
	if is_selector(input):
	selector = input
	elif isinstance(input, str):
	import narwhals._plan.selectors as cs
	selector = cs.by_name(input, require_all=require_all)
	elif is_expr(input):
	selector = input.meta.as_selector()
	else:
	msg = f"cannot turn {qualified_type_name(input)!r} into a selector"
	raise TypeError(msg)
	return selector

Some related bits

narwhals/narwhals/_plan/meta.py

Lines 83 to 88 in 9b9122b

	def as_selector(self) -> Selector:
	"""Try to turn this expression into a selector.
	Raises if the underlying expressions is not a column or selector.
	"""
	return self._ir.to_selector_ir().to_narwhals()

narwhals/narwhals/_plan/_guards.py

Lines 97 to 101 in 9b9122b

	def is_column_name_or_selector(
	obj: Any, *, allow_expr: bool = False
	) -> TypeIs[ColumnNameOrSelector]:
	tps = (str, _selectors().Selector) if not allow_expr else (str, _expr().Expr)
	return isinstance(obj, tps)