Fix first, last again

flox/__init__.py

@@ -2,7 +2,7 @@
 # flake8: noqa
 """Top-level module for flox ."""
 from . import cache
-from .aggregations import Aggregation  # noqa
+from .aggregations import Aggregation, Scan  # noqa
 from .core import groupby_reduce, groupby_scan, rechunk_for_blockwise, rechunk_for_cohorts  # noqa
 
 

flox/core.py

@@ -170,7 +170,9 @@ def _is_minmax_reduction(func: T_Agg) -> bool:
 
 
 def _is_first_last_reduction(func: T_Agg) -> bool:
-    return isinstance(func, str) and func in ["nanfirst", "nanlast", "first", "last"]
+    if isinstance(func, Aggregation):
+        func = func.name
+    return func in ["nanfirst", "nanlast", "first", "last"]
 
 
 def _get_expected_groups(by: T_By, sort: bool) -> T_ExpectIndex:
@@ -1642,7 +1644,12 @@ def dask_groupby_agg(
     #       This allows us to discover groups at compute time, support argreductions, lower intermediate
     #       memory usage (but method="cohorts" would also work to reduce memory in some cases)
     labels_are_unknown = is_duck_dask_array(by_input) and expected_groups is None
-    do_simple_combine = not _is_arg_reduction(agg) and not labels_are_unknown
+    do_grouped_combine = (
+        _is_arg_reduction(agg)
+        or labels_are_unknown
+        or (_is_first_last_reduction(agg) and array.dtype.kind != "f")
+    )
+    do_simple_combine = not do_grouped_combine
 
     if method == "blockwise":
         #  use the "non dask" code path, but applied blockwise
@@ -1698,7 +1705,7 @@ def dask_groupby_agg(
 
         tree_reduce = partial(
             dask.array.reductions._tree_reduce,
-            name=f"{name}-reduce",
+            name=f"{name}-simple-reduce",
             dtype=array.dtype,
             axis=axis,
             keepdims=True,
@@ -1733,14 +1740,20 @@ def dask_groupby_agg(
             groups_ = []
             for blks, cohort in chunks_cohorts.items():
                 cohort_index = pd.Index(cohort)
-                reindexer = partial(reindex_intermediates, agg=agg, unique_groups=cohort_index)
+                reindexer = (
+                    partial(reindex_intermediates, agg=agg, unique_groups=cohort_index)
+                    if do_simple_combine
+                    else identity
+                )
                 reindexed = subset_to_blocks(intermediate, blks, block_shape, reindexer)
                 # now that we have reindexed, we can set reindex=True explicitlly
                 reduced_.append(
                     tree_reduce(
                         reindexed,
-                        combine=partial(combine, agg=agg, reindex=True),
-                        aggregate=partial(aggregate, expected_groups=cohort_index, reindex=True),
+                        combine=partial(combine, agg=agg, reindex=do_simple_combine),
+                        aggregate=partial(
+                            aggregate, expected_groups=cohort_index, reindex=do_simple_combine
+                        ),
                     )
                 )
                 # This is done because pandas promotes to 64-bit types when an Index is created
@@ -1986,8 +1999,13 @@ def _validate_reindex(
     expected_groups,
     any_by_dask: bool,
     is_dask_array: bool,
+    array_dtype: Any,
 ) -> bool | None:
     # logger.debug("Entering _validate_reindex: reindex is {}".format(reindex))  # noqa
+    def first_or_last():
+        return func in ["first", "last"] or (
+            _is_first_last_reduction(func) and array_dtype.kind != "f"
+        )
 
     all_numpy = not is_dask_array and not any_by_dask
     if reindex is True and not all_numpy:
@@ -1997,7 +2015,7 @@ def _validate_reindex(
             raise ValueError(
                 "reindex=True is not a valid choice for method='blockwise' or method='cohorts'."
             )
-        if func in ["first", "last"]:
+        if first_or_last():
             raise ValueError("reindex must be None or False when func is 'first' or 'last.")
 
     if reindex is None:
@@ -2008,9 +2026,10 @@ def _validate_reindex(
         if all_numpy:
             return True
 
-        if func in ["first", "last"]:
+        if first_or_last():
             # have to do the grouped_combine since there's no good fill_value
-            reindex = False
+            # Also needed for nanfirst, nanlast with no-NaN dtypes
+            return False
 
         if method == "blockwise":
             # for grouping by dask arrays, we set reindex=True
@@ -2412,12 +2431,19 @@ def groupby_reduce(
     if method == "cohorts" and any_by_dask:
         raise ValueError(f"method={method!r} can only be used when grouping by numpy arrays.")
 
+    if not is_duck_array(array):
+        array = np.asarray(array)
+
     reindex = _validate_reindex(
-        reindex, func, method, expected_groups, any_by_dask, is_duck_dask_array(array)
+        reindex,
+        func,
+        method,
+        expected_groups,
+        any_by_dask,
+        is_duck_dask_array(array),
+        array.dtype,
     )
 
-    if not is_duck_array(array):
-        array = np.asarray(array)
     is_bool_array = np.issubdtype(array.dtype, bool)
     array = array.astype(np.intp) if is_bool_array else array
 
@@ -2601,7 +2627,7 @@ def groupby_reduce(
 
         # TODO: clean this up
         reindex = _validate_reindex(
-            reindex, func, method, expected_, any_by_dask, is_duck_dask_array(array)
+            reindex, func, method, expected_, any_by_dask, is_duck_dask_array(array), array.dtype
         )
 
         if TYPE_CHECKING:

tests/conftest.py

@@ -10,11 +10,12 @@
     suppress_health_check=[HealthCheck.filter_too_much, HealthCheck.too_slow],
 )
 settings.register_profile(
-    "local",
+    "default",
     max_examples=300,
     suppress_health_check=[HealthCheck.filter_too_much, HealthCheck.too_slow],
     verbosity=Verbosity.verbose,
 )
+settings.load_profile("default")
 
 
 @pytest.fixture(

tests/test_core.py

@@ -613,6 +613,33 @@ def test_dask_reduce_axis_subset():
         )
 
 
+@pytest.mark.parametrize("group_idx", [[0, 1, 0], [0, 0, 1], [1, 0, 0], [1, 1, 0]])
+@pytest.mark.parametrize(
+    "func",
+    [
+        # "first", "last",
+        "nanfirst",
+        "nanlast",
+    ],
+)
+@pytest.mark.parametrize(
+    "chunks",
+    [
+        None,
+        pytest.param(1, marks=pytest.mark.skipif(not has_dask, reason="no dask")),
+        pytest.param(2, marks=pytest.mark.skipif(not has_dask, reason="no dask")),
+        pytest.param(3, marks=pytest.mark.skipif(not has_dask, reason="no dask")),
+    ],
+)
+def test_first_last_useless(func, chunks, group_idx):
+    array = np.array([[0, 0, 0], [0, 0, 0]], dtype=np.int8)
+    if chunks is not None:
+        array = dask.array.from_array(array, chunks=chunks)
+    actual, _ = groupby_reduce(array, np.array(group_idx), func=func, engine="numpy")
+    expected = np.array([[0, 0], [0, 0]], dtype=np.int8)
+    assert_equal(actual, expected)
+
+
 @pytest.mark.parametrize("func", ["first", "last", "nanfirst", "nanlast"])
 @pytest.mark.parametrize("axis", [(0, 1)])
 def test_first_last_disallowed(axis, func):
@@ -1563,18 +1590,36 @@ def test_validate_reindex_map_reduce(
     dask_expected, reindex, func, expected_groups, any_by_dask
 ) -> None:
     actual = _validate_reindex(
-        reindex, func, "map-reduce", expected_groups, any_by_dask, is_dask_array=True
+        reindex,
+        func,
+        "map-reduce",
+        expected_groups,
+        any_by_dask,
+        is_dask_array=True,
+        array_dtype=np.dtype("int32"),
     )
     assert actual is dask_expected
 
     # always reindex with all numpy inputs
     actual = _validate_reindex(
-        reindex, func, "map-reduce", expected_groups, any_by_dask=False, is_dask_array=False
+        reindex,
+        func,
+        "map-reduce",
+        expected_groups,
+        any_by_dask=False,
+        is_dask_array=False,
+        array_dtype=np.dtype("int32"),
     )
     assert actual
 
     actual = _validate_reindex(
-        True, func, "map-reduce", expected_groups, any_by_dask=False, is_dask_array=False
+        True,
+        func,
+        "map-reduce",
+        expected_groups,
+        any_by_dask=False,
+        is_dask_array=False,
+        array_dtype=np.dtype("int32"),
     )
     assert actual
 
@@ -1584,19 +1629,37 @@ def test_validate_reindex() -> None:
     for method in methods:
         with pytest.raises(NotImplementedError):
             _validate_reindex(
-                True, "argmax", method, expected_groups=None, any_by_dask=False, is_dask_array=True
+                True,
+                "argmax",
+                method,
+                expected_groups=None,
+                any_by_dask=False,
+                is_dask_array=True,
+                array_dtype=np.dtype("int32"),
             )
 
     methods: list[T_Method] = ["blockwise", "cohorts"]
     for method in methods:
         with pytest.raises(ValueError):
             _validate_reindex(
-                True, "sum", method, expected_groups=None, any_by_dask=False, is_dask_array=True
+                True,
+                "sum",
+                method,
+                expected_groups=None,
+                any_by_dask=False,
+                is_dask_array=True,
+                array_dtype=np.dtype("int32"),
             )
 
         for func in ["sum", "argmax"]:
             actual = _validate_reindex(
-                None, func, method, expected_groups=None, any_by_dask=False, is_dask_array=True
+                None,
+                func,
+                method,
+                expected_groups=None,
+                any_by_dask=False,
+                is_dask_array=True,
+                array_dtype=np.dtype("int32"),
             )
             assert actual is False
 
@@ -1608,6 +1671,7 @@ def test_validate_reindex() -> None:
             expected_groups=np.array([1, 2, 3]),
             any_by_dask=False,
             is_dask_array=True,
+            array_dtype=np.dtype("int32"),
         )
 
     assert _validate_reindex(
@@ -1617,6 +1681,7 @@ def test_validate_reindex() -> None:
         expected_groups=np.array([1, 2, 3]),
         any_by_dask=True,
         is_dask_array=True,
+        array_dtype=np.dtype("int32"),
     )
     assert _validate_reindex(
         None,
@@ -1625,8 +1690,24 @@ def test_validate_reindex() -> None:
         expected_groups=np.array([1, 2, 3]),
         any_by_dask=True,
         is_dask_array=True,
+        array_dtype=np.dtype("int32"),
+    )
+
+    kwargs = dict(
+        method="blockwise",
+        expected_groups=np.array([1, 2, 3]),
+        any_by_dask=True,
+        is_dask_array=True,
     )
 
+    for func in ["nanfirst", "nanlast"]:
+        assert not _validate_reindex(None, func, array_dtype=np.dtype("int32"), **kwargs)  # type: ignore[arg-type]
+        assert _validate_reindex(None, func, array_dtype=np.dtype("float32"), **kwargs)  # type: ignore[arg-type]
+
+    for func in ["first", "last"]:
+        assert not _validate_reindex(None, func, array_dtype=np.dtype("int32"), **kwargs)  # type: ignore[arg-type]
+        assert not _validate_reindex(None, func, array_dtype=np.dtype("float32"), **kwargs)  # type: ignore[arg-type]
+
 
 @requires_dask
 def test_1d_blockwise_sort_optimization():

tests/test_properties.py

@@ -9,6 +9,7 @@
 pytest.importorskip("cftime")
 
 import dask
+import hypothesis.extra.numpy as npst
 import hypothesis.strategies as st
 import numpy as np
 from hypothesis import assume, given, note
@@ -19,6 +20,7 @@
 
 from . import assert_equal
 from .strategies import by_arrays, chunked_arrays, func_st, numeric_arrays
+from .strategies import chunks as chunks_strategy
 
 dask.config.set(scheduler="sync")
 
@@ -208,3 +210,16 @@ def test_first_last(data, array: dask.array.Array, func: str) -> None:
         first, *_ = groupby_reduce(array, by, func=func, engine="flox")
         second, *_ = groupby_reduce(array, by, func=mate, engine="flox")
         assert_equal(first, second)
+
+
+@given(data=st.data(), func=st.sampled_from(["nanfirst", "nanlast"]))
+def test_first_last_useless(data, func):
+    shape = data.draw(npst.array_shapes())
+    by = data.draw(by_arrays(shape=shape[slice(-1, None)]))
+    chunks = data.draw(chunks_strategy(shape=shape))
+    array = np.zeros(shape, dtype=np.int8)
+    if chunks is not None:
+        array = dask.array.from_array(array, chunks=chunks)
+    actual, groups = groupby_reduce(array, by, axis=-1, func=func, engine="numpy")
+    expected = np.zeros(shape[:-1] + (len(groups),), dtype=array.dtype)
+    assert_equal(actual, expected)