Patch dask-expr var logic in dask-cudf

ci/build_docs.sh

@@ -41,9 +41,6 @@ mkdir -p "${RAPIDS_DOCS_DIR}/libcudf/html"
 mv html/* "${RAPIDS_DOCS_DIR}/libcudf/html"
 popd
 
-# TODO: Remove this once dask-expr works in the 10min notebook
-export DASK_DATAFRAME__QUERY_PLANNING=False
-
 rapids-logger "Build Python docs"
 pushd docs/cudf
 make dirhtml

python/dask_cudf/dask_cudf/expr/_expr.py

@@ -1,7 +1,11 @@
 # Copyright (c) 2024, NVIDIA CORPORATION.
+import functools
 
 from dask_expr._cumulative import CumulativeBlockwise
-from dask_expr._reductions import Var
+from dask_expr._expr import Expr, VarColumns
+from dask_expr._reductions import Reduction, Var
+
+from dask.dataframe.core import is_dataframe_like, make_meta, meta_nonempty
 
 ##
 ## Custom expression patching
@@ -25,19 +29,95 @@ def _kwargs(self) -> dict:
 CumulativeBlockwise._kwargs = PatchCumulativeBlockwise._kwargs
 
 
-# This patch accounts for differences between
-# numpy and cupy behavior. It may make sense
-# to move this logic upstream.
-_dx_reduction_aggregate = Var.reduction_aggregate
+# The upstream Var code uses `Series.values`, and relies on numpy
+# for most of the logic. Unfortunately, cudf -> cupy conversion
+# is not supported for data containing null values. Therefore,
+# we must implement our own version of Var for now. This logic
+# is mostly copied from dask-cudf.
+
+
+class VarCudf(Reduction):
+    # Uses the parallel version of Welford's online algorithm (Chan '79)
+    # (http://i.stanford.edu/pub/cstr/reports/cs/tr/79/773/CS-TR-79-773.pdf)
+    _parameters = ["frame", "skipna", "ddof", "numeric_only", "split_every"]
+    _defaults = {
+        "skipna": True,
+        "ddof": 1,
+        "numeric_only": False,
+        "split_every": False,
+    }
+
+    @functools.cached_property
+    def _meta(self):
+        return make_meta(
+            meta_nonempty(self.frame._meta).var(
+                skipna=self.skipna, numeric_only=self.numeric_only
+            )
+        )
+
+    @property
+    def chunk_kwargs(self):
+        return dict(skipna=self.skipna, numeric_only=self.numeric_only)
+
+    @property
+    def combine_kwargs(self):
+        return {}
+
+    @property
+    def aggregate_kwargs(self):
+        return dict(ddof=self.ddof)
+
+    @classmethod
+    def reduction_chunk(cls, x, skipna=True, numeric_only=False):
+        kwargs = {"numeric_only": numeric_only} if is_dataframe_like(x) else {}
+        if skipna or numeric_only:
+            n = x.count(**kwargs)
+            kwargs["skipna"] = skipna
+            avg = x.mean(**kwargs)
+        else:
+            # Not skipping nulls, so might as well
+            # avoid the full `count` operation
+            n = len(x)
+            kwargs["skipna"] = skipna
+            avg = x.sum(**kwargs) / n
+        if numeric_only:
+            # Workaround for cudf bug
+            # (see: https://github.com/rapidsai/cudf/issues/13731)
+            x = x[n.index]
+        m2 = ((x - avg) ** 2).sum(**kwargs)
+        return n, avg, m2
+
+    @classmethod
+    def reduction_combine(cls, parts):
+        n, avg, m2 = parts[0]
+        for i in range(1, len(parts)):
+            n_a, avg_a, m2_a = n, avg, m2
+            n_b, avg_b, m2_b = parts[i]
+            n = n_a + n_b
+            avg = (n_a * avg_a + n_b * avg_b) / n
+            delta = avg_b - avg_a
+            m2 = m2_a + m2_b + delta**2 * n_a * n_b / n
+        return n, avg, m2
+
+    @classmethod
+    def reduction_aggregate(cls, vals, ddof=1):
+        vals = cls.reduction_combine(vals)
+        n, _, m2 = vals
+        return m2 / (n - ddof)
 
 
-def _reduction_aggregate(*args, **kwargs):
-    result = _dx_reduction_aggregate(*args, **kwargs)
-    if result.ndim == 0:
-        # cupy will sometimes produce a 0d array, and
-        # we need to convert it to a scalar.
-        return result.item()
-    return result
+def _patched_var(
+    self, axis=0, skipna=True, ddof=1, numeric_only=False, split_every=False
+):
+    if axis == 0:
+        if hasattr(self._meta, "to_pandas"):
+            return VarCudf(self, skipna, ddof, numeric_only, split_every)
+        else:
+            return Var(self, skipna, ddof, numeric_only, split_every)
+    elif axis == 1:
+        return VarColumns(self, skipna, ddof, numeric_only)
+    else:
+        raise ValueError(f"axis={axis} not supported. Please specify 0 or 1")
 
 
-Var.reduction_aggregate = staticmethod(_reduction_aggregate)
+Expr.var = _patched_var

python/dask_cudf/dask_cudf/tests/test_reductions.py

@@ -84,3 +84,13 @@ def test_rowwise_reductions(data, op):
             check_exact=False,
             check_dtype=op not in ("var", "std"),
         )
+
+
+@pytest.mark.parametrize("skipna", [True, False])
+def test_var_nulls(skipna):
+    # Copied from 10min example notebook
+    # See: https://github.com/rapidsai/cudf/pull/15347
+    s = cudf.Series([1, 2, 3, None, 4])
+    ds = dask_cudf.from_cudf(s, npartitions=2)
+    dd.assert_eq(s.var(skipna=skipna), ds.var(skipna=skipna))
+    dd.assert_eq(s.std(skipna=skipna), ds.std(skipna=skipna))