feat: add support for median

[AlessandroMiola]

What type of PR is this? (check all applicable)

• 💾 Refactor
• ✨ Feature
• 🐛 Bug Fix
• 🔧 Optimization
• 📝 Documentation
• ✅ Test
• 🐳 Other

Related issues

• Closes feat: Expr.median / Series.median #1190

Checklist

• Code follows style guide (ruff)
• Tests added
• Documented the changes

If you have comments or can explain your changes, please do so below.

WIP: I would need to better dig into details for pyarrow and dask. Already opened the PR to (also) get advices and/or guidance:)

My understanding per now (might be missing bits, though) is the following:

• neither pyarrow nor dask do implement a "proper" median; otoh, they respectively implement approximate_median and median_approximate (dask's median_approximate fails tests when npartitions=2, though)
• relying on already implemented quantile(s) would come with the following issues: for pyarrow, the issue of having an interpolation parameter which would not quite fit the nw.median() signature; for dask, the issue of having to hardcode a "linear" interpolation strategy and having to xfail tests as well when encountering dask_lazy_p2_constructor.

Also, need to revise docstrings so as to comply with #1000 for median.

[FBruzzesi]

Hey @AlessandroMiola thanks for the effort! This already looks very promising.

neither pyarrow nor dask do implement a "proper" median; otoh, they respectively implement approximate_median and median_approximate

I think that's good enough as long as we document that results may slightly differ between backends because of the difference of underlying algorithms used

dask's median_approximate fails tests when npartitions=2, though

This may need some investigation

Also, need to revise docstrings so as to comply with #1000 for median.

This is much appreciated 😁

I have a couple of additional considerations:

• Polars on string series returns None, pandas raises an error, I don't know about others - should we check that the input is numeric and limit the functionality to that? cc @MarcoGorelli
• Can we check that all algorithms ignore nulls/nans?

[AlessandroMiola]

Thanks for your help @FBruzzesi! :) I'll try to address all of your comments!

[MarcoGorelli]

nice, looks really good!

regarding

Polars on string series returns None,

Polars does indeed raise here for Expr:

In [10]: df
Out[10]:
shape: (3, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ str │
╞═════╪═════╡
│ 1   ┆ f   │
│ 2   ┆ a   │
│ 3   ┆ x   │
└─────┴─────┘

In [11]: df.select(pl.median('b'))
---------------------------------------------------------------------------
InvalidOperationError                     Traceback (most recent call last)
Cell In[11], line 1
----> 1 df.select(pl.median('b'))

File ~/scratch/.venv/lib/python3.12/site-packages/polars/dataframe/frame.py:9010, in DataFrame.select(self, *exprs, **named_exprs)
   8910 def select(
   8911     self, *exprs: IntoExpr | Iterable[IntoExpr], **named_exprs: IntoExpr
   8912 ) -> DataFrame:
   8913     """
   8914     Select columns from this DataFrame.
   8915
   (...)
   9008     └──────────────┘
   9009     """
-> 9010     return self.lazy().select(*exprs, **named_exprs).collect(_eager=True)

File ~/scratch/.venv/lib/python3.12/site-packages/polars/lazyframe/frame.py:2050, in LazyFrame.collect(self, type_coercion, predicate_pushdown, projection_pushdown, simplify_expression, slice_pushdown, comm_subplan_elim, comm_subexpr_elim, cluster_with_columns, collapse_joins, no_optimization, streaming, engine, background, _eager, **_kwargs)
   2048 # Only for testing purposes
   2049 callback = _kwargs.get("post_opt_callback", callback)
-> 2050 return wrap_df(ldf.collect(callback))

InvalidOperationError: `median` operation not supported for dtype `str`

I find it a bit odd that Series.median doesn't raise for Polars, and think it'd be ok for us to just always raise

[MarcoGorelli]

does pl.median([*column_names]) work across different polars versions? if we can avoid branching, that'll keep complexity down

[AlessandroMiola]

it does :)

[AlessandroMiola]

I find it a bit odd that Series.median doesn't raise for Polars, and think it'd be ok for us to just always raise

A (possibly silly) question on my side for clarification. Should we make it so that it passes tests like the ones below (thus raising a custom error within the underlying backends) or should we just raise where it natively does not while keeping native behaviour as is?
I would have gone the first way, but I'm all ears! :D

data = {
    "a": [3, 8, 2, None],
    "b": [5, 5, None, 7],
    "z": [7.0, 8, 9, None],
    "s": ["f", "a", "x", "x"],
}

@pytest.mark.parametrize(
    "expr", [nw.col("s").median(), nw.median("s")]
)
def test_median_expr_raises_on_str(constructor: Constructor, expr: nw.Expr) -> None:
    df = nw.from_native(constructor(data))
    with pytest.raises(
        TypeError,
        match="`median` operation not supported for non-numeric input type."
    ):
        df.select(expr)

@pytest.mark.parametrize(("col"), [("s")])
def test_median_series_raises_on_str(
    constructor_eager: Any,
    col: str,
) -> None:
    series = nw.from_native(constructor_eager(data), eager_only=True)[col]
    with pytest.raises(
        TypeError,
        match="`median` operation not supported for non-numeric input type."
    ):
        series.median()

Please correct me if I'm completely off-road! Thanks

[FBruzzesi]

A (possibly silly) question on my side for clarification. Should we make it so that it passes tests like the ones below (thus raising a custom error within the underlying backends) or should we just raise where it natively does not while keeping native behaviour as is? I would have gone the first way, but I'm all ears! :D

Hey @AlessandroMiola , I would personally opt for the second option: you could create a InvalidOperationError class in _exceptions.py and raise that one. For series it would be even possible to do so in narwhals.Series so that the type checking and raise does not get duplicated. For narwhals.Expr I would not know how to do it directly though, since datatype is not know until computation in polars.

So maybe first option 😂