Offer a fixture for unifying DataArray & Dataset tests

(stacked on #8512, worth reviewing after that's merged)

Some tests are literally copy & pasted between DataArray & Dataset tests. This change allows them to use a single test. Not everything will work — sometimes we want to check specifics — but sometimes they will...

xarray/tests/conftest.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 import numpy as np
 import pandas as pd
 import pytest
@@ -77,3 +79,44 @@ def da(request, backend):
         return da
     else:
         raise ValueError
+
+
+@pytest.fixture(params=[Dataset, DataArray])
+def type(request):
+    return request.param
+
+
+@pytest.fixture(params=[1])
+def d(request, backend, type) -> DataArray | Dataset:
+    """
+    For tests which can test either a DataArray or a Dataset.
+    """
+    result: DataArray | Dataset
+    if request.param == 1:
+        ds = Dataset(
+            dict(
+                a=(["x", "y"], np.arange(16).reshape(8, 2)),
+                b=(["y", "z"], np.arange(12, 32).reshape(2, 10).astype(np.float64)),
+            ),
+            dict(
+                x=("x", np.linspace(0, 1.0, 8)),
+                y=range(2),
+                z=("z", np.linspace(0, 1.0, 10)),
+                w=("y", ["a", "b"]),
+            ),
+        )
+        if type == DataArray:
+            result = ds["a"].assign_coords(w=ds.coords["w"])
+        elif type == Dataset:
+            result = ds
+        else:
+            raise ValueError
+    else:
+        raise ValueError
+
+    if backend == "dask":
+        return result.chunk()
+    elif backend == "numpy":
+        return result
+    else:
+        raise ValueError

xarray/tests/test_rolling.py

@@ -36,6 +36,31 @@ def compute_backend(request):
         yield request.param
 
 
+@pytest.mark.parametrize("func", ["mean", "sum"])
+@pytest.mark.parametrize("min_periods", [1, 20])
+def test_cumulative(d, func, min_periods) -> None:
+    # One dim
+    result = getattr(d.cumulative("x", min_periods=min_periods), func)()
+    expected = getattr(d.rolling(x=d["x"].size, min_periods=min_periods), func)()
+    assert_identical(result, expected)
+
+    # Multiple dim
+    result = getattr(d.cumulative(["x", "y"], min_periods=min_periods), func)()
+    expected = getattr(
+        d.rolling(x=d["x"].size, y=d["y"].size, min_periods=min_periods),
+        func,
+    )()
+    assert_identical(result, expected)
+
+
+def test_cumulative_vs_cum(d) -> None:
+    result = d.cumulative("x").sum()
+    expected = d.cumsum("x")
+    # cumsum drops the coord of the dimension; cumulative doesn't
+    expected = expected.assign_coords(x=result["x"])
+    assert_identical(result, expected)
+
+
 class TestDataArrayRolling:
     @pytest.mark.parametrize("da", (1, 2), indirect=True)
     @pytest.mark.parametrize("center", [True, False])
@@ -485,29 +510,6 @@ def test_rolling_exp_keep_attrs(self, da, func) -> None:
         ):
             da.rolling_exp(time=10, keep_attrs=True)
 
-    @pytest.mark.parametrize("func", ["mean", "sum"])
-    @pytest.mark.parametrize("min_periods", [1, 20])
-    def test_cumulative(self, da, func, min_periods) -> None:
-        # One dim
-        result = getattr(da.cumulative("time", min_periods=min_periods), func)()
-        expected = getattr(
-            da.rolling(time=da.time.size, min_periods=min_periods), func
-        )()
-        assert_identical(result, expected)
-
-        # Multiple dim
-        result = getattr(da.cumulative(["time", "a"], min_periods=min_periods), func)()
-        expected = getattr(
-            da.rolling(time=da.time.size, a=da.a.size, min_periods=min_periods),
-            func,
-        )()
-        assert_identical(result, expected)
-
-    def test_cumulative_vs_cum(self, da) -> None:
-        result = da.cumulative("time").sum()
-        expected = da.cumsum("time")
-        assert_identical(result, expected)
-
 
 class TestDatasetRolling:
     @pytest.mark.parametrize(
@@ -832,25 +834,6 @@ def test_raise_no_warning_dask_rolling_assert_close(self, ds, name) -> None:
         expected = getattr(getattr(ds.rolling(time=4), name)().rolling(x=3), name)()
         assert_allclose(actual, expected)
 
-    @pytest.mark.parametrize("func", ["mean", "sum"])
-    @pytest.mark.parametrize("ds", (2,), indirect=True)
-    @pytest.mark.parametrize("min_periods", [1, 10])
-    def test_cumulative(self, ds, func, min_periods) -> None:
-        # One dim
-        result = getattr(ds.cumulative("time", min_periods=min_periods), func)()
-        expected = getattr(
-            ds.rolling(time=ds.time.size, min_periods=min_periods), func
-        )()
-        assert_identical(result, expected)
-
-        # Multiple dim
-        result = getattr(ds.cumulative(["time", "x"], min_periods=min_periods), func)()
-        expected = getattr(
-            ds.rolling(time=ds.time.size, x=ds.x.size, min_periods=min_periods),
-            func,
-        )()
-        assert_identical(result, expected)
-
 
 @requires_numbagg
 class TestDatasetRollingExp: