Pint array: strip and reattach appropriate units

Closes #163

flox/aggregations.py

@@ -3,6 +3,7 @@
 import copy
 import warnings
 from functools import partial
+from typing import Callable
 
 import numpy as np
 import numpy_groupies as npg
@@ -114,6 +115,7 @@ def __init__(
         dtypes=None,
         final_dtype=None,
         reduction_type="reduce",
+        units_func: Callable | None = None,
     ):
         """
         Blueprint for computing grouped aggregations.
@@ -156,6 +158,8 @@ def __init__(
             per reduction in ``chunk`` as a tuple.
         final_dtype : DType, optional
             DType for output. By default, uses dtype of array being reduced.
+        units_func : pint.Unit
+            units for the output
         """
         self.name = name
         # preprocess before blockwise
@@ -187,6 +191,8 @@ def __init__(
         # The following are set by _initialize_aggregation
         self.finalize_kwargs = {}
         self.min_count = None
+        self.units_func = units_func
+        self.units = None
 
     def _normalize_dtype_fill_value(self, value, name):
         value = _atleast_1d(value)
@@ -235,17 +241,44 @@ def __repr__(self):
     final_dtype=np.intp,
 )
 
+
+def identity(x):
+    return x
+
+
+def square(x):
+    return x**2
+
+
+def raise_units_error(x):
+    raise ValueError(
+        "Units cannot supported for prod in general. "
+        "We can only attach units when there are "
+        "equal number of members in each group. "
+        "Please strip units and then reattach units "
+        "to the output manually."
+    )
+
+
 # note that the fill values are the result of np.func([np.nan, np.nan])
 # final_fill_value is used for groups that don't exist. This is usually np.nan
-sum_ = Aggregation("sum", chunk="sum", combine="sum", fill_value=0)
-nansum = Aggregation("nansum", chunk="nansum", combine="sum", fill_value=0)
-prod = Aggregation("prod", chunk="prod", combine="prod", fill_value=1, final_fill_value=1)
+sum_ = Aggregation("sum", chunk="sum", combine="sum", fill_value=0, units_func=identity)
+nansum = Aggregation("nansum", chunk="nansum", combine="sum", fill_value=0, units_func=identity)
+prod = Aggregation(
+    "prod",
+    chunk="prod",
+    combine="prod",
+    fill_value=1,
+    final_fill_value=1,
+    units_func=raise_units_error,
+)
 nanprod = Aggregation(
     "nanprod",
     chunk="nanprod",
     combine="prod",
     fill_value=1,
     final_fill_value=dtypes.NA,
+    units_func=raise_units_error,
 )
 
 
@@ -262,6 +295,7 @@ def _mean_finalize(sum_, count):
     fill_value=(0, 0),
     dtypes=(None, np.intp),
     final_dtype=np.floating,
+    units_func=identity,
 )
 nanmean = Aggregation(
     "nanmean",
@@ -271,6 +305,7 @@ def _mean_finalize(sum_, count):
     fill_value=(0, 0),
     dtypes=(None, np.intp),
     final_dtype=np.floating,
+    units_func=identity,
 )
 
 
@@ -296,6 +331,7 @@ def _std_finalize(sumsq, sum_, count, ddof=0):
     final_fill_value=np.nan,
     dtypes=(None, None, np.intp),
     final_dtype=np.floating,
+    units_func=square,
 )
 nanvar = Aggregation(
     "nanvar",
@@ -306,6 +342,7 @@ def _std_finalize(sumsq, sum_, count, ddof=0):
     final_fill_value=np.nan,
     dtypes=(None, None, np.intp),
     final_dtype=np.floating,
+    units_func=square,
 )
 std = Aggregation(
     "std",
@@ -316,6 +353,7 @@ def _std_finalize(sumsq, sum_, count, ddof=0):
     final_fill_value=np.nan,
     dtypes=(None, None, np.intp),
     final_dtype=np.floating,
+    units_func=identity,
 )
 nanstd = Aggregation(
     "nanstd",
@@ -329,10 +367,14 @@ def _std_finalize(sumsq, sum_, count, ddof=0):
 )
 
 
-min_ = Aggregation("min", chunk="min", combine="min", fill_value=dtypes.INF)
-nanmin = Aggregation("nanmin", chunk="nanmin", combine="nanmin", fill_value=np.nan)
-max_ = Aggregation("max", chunk="max", combine="max", fill_value=dtypes.NINF)
-nanmax = Aggregation("nanmax", chunk="nanmax", combine="nanmax", fill_value=np.nan)
+min_ = Aggregation("min", chunk="min", combine="min", fill_value=dtypes.INF, units_func=identity)
+nanmin = Aggregation(
+    "nanmin", chunk="nanmin", combine="nanmin", fill_value=np.nan, units_func=identity
+)
+max_ = Aggregation("max", chunk="max", combine="max", fill_value=dtypes.NINF, units_func=identity)
+nanmax = Aggregation(
+    "nanmax", chunk="nanmax", combine="nanmax", fill_value=np.nan, units_func=identity
+)
 
 
 def argreduce_preprocess(array, axis):
@@ -420,10 +462,14 @@ def _pick_second(*x):
     final_dtype=np.intp,
 )
 
-first = Aggregation("first", chunk=None, combine=None, fill_value=0)
-last = Aggregation("last", chunk=None, combine=None, fill_value=0)
-nanfirst = Aggregation("nanfirst", chunk="nanfirst", combine="nanfirst", fill_value=np.nan)
-nanlast = Aggregation("nanlast", chunk="nanlast", combine="nanlast", fill_value=np.nan)
+first = Aggregation("first", chunk=None, combine=None, fill_value=0, units_func=identity)
+last = Aggregation("last", chunk=None, combine=None, fill_value=0, units_func=identity)
+nanfirst = Aggregation(
+    "nanfirst", chunk="nanfirst", combine="nanfirst", fill_value=np.nan, units_func=identity
+)
+nanlast = Aggregation(
+    "nanlast", chunk="nanlast", combine="nanlast", fill_value=np.nan, units_func=identity
+)
 
 all_ = Aggregation(
     "all",
@@ -483,6 +529,7 @@ def _initialize_aggregation(
     dtype,
     array_dtype,
     fill_value,
+    array_units,
     min_count: int | None,
     finalize_kwargs,
 ) -> Aggregation:
@@ -547,4 +594,8 @@ def _initialize_aggregation(
         agg.dtype["intermediate"] += (np.intp,)
         agg.dtype["numpy"] += (np.intp,)
 
+    if array_units is not None and agg.units_func is not None:
+        import pint
+
+        agg.units = agg.units_func(pint.Quantity([1], units=array_units))
     return agg

flox/core.py

@@ -24,6 +24,7 @@
     generic_aggregate,
 )
 from .cache import memoize
+from .pint_compat import _reattach_units, _strip_units
 from .xrutils import is_duck_array, is_duck_dask_array, isnull
 
 if TYPE_CHECKING:
@@ -1702,6 +1703,8 @@ def groupby_reduce(
     by_is_dask = tuple(is_duck_dask_array(b) for b in bys)
     any_by_dask = any(by_is_dask)
 
+    array, *bys, units = _strip_units(array, *bys)
+
     if method in ["split-reduce", "cohorts"] and any_by_dask:
         raise ValueError(f"method={method!r} can only be used when grouping by numpy arrays.")
 
@@ -1803,7 +1806,9 @@ def groupby_reduce(
         fill_value = np.nan
 
     kwargs = dict(axis=axis_, fill_value=fill_value, engine=engine)
-    agg = _initialize_aggregation(func, dtype, array.dtype, fill_value, min_count, finalize_kwargs)
+    agg = _initialize_aggregation(
+        func, dtype, array.dtype, fill_value, units[0], min_count, finalize_kwargs
+    )
 
     if not has_dask:
         results = _reduce_blockwise(
@@ -1862,4 +1867,7 @@ def groupby_reduce(
 
     if _is_minmax_reduction(func) and is_bool_array:
         result = result.astype(bool)
+
+    units[0] = agg.units
+    result, *groups = _reattach_units(result, *groups, units=units)
     return (result, *groups)

tests/__init__.py

@@ -24,6 +24,13 @@
 except ImportError:
     xr_types = ()  # type: ignore
 
+try:
+    import pint
+
+    pint_types = pint.Quantity
+except ImportError:
+    pint_types = ()  # type: ignore
+
 
 def _importorskip(modname, minversion=None):
     try:
@@ -46,6 +53,7 @@ def LooseVersion(vstring):
 
 
 has_dask, requires_dask = _importorskip("dask")
+has_pint, requires_pint = _importorskip("pint")
 has_xarray, requires_xarray = _importorskip("xarray")
 
 
@@ -95,6 +103,14 @@ def assert_equal(a, b, tolerance=None):
         xr.testing.assert_identical(a, b)
         return
 
+    if has_pint and isinstance(a, pint_types) or isinstance(b, pint_types):
+        assert isinstance(a, pint_types)
+        assert isinstance(b, pint_types)
+        assert a.units == b.units
+
+        a = a.magnitude
+        b = b.magnitude
+
     if tolerance is None and (
         np.issubdtype(a.dtype, np.float64) | np.issubdtype(b.dtype, np.float64)
     ):

tests/test_core.py

@@ -31,6 +31,7 @@
     has_dask,
     raise_if_dask_computes,
     requires_dask,
+    requires_pint,
 )
 
 labels = np.array([0, 0, 2, 2, 2, 1, 1, 2, 2, 1, 1, 0])
@@ -1321,3 +1322,39 @@ def test_negative_index_factorize_race_condition():
         for f in func
     ]
     [dask.compute(out, scheduler="threads") for _ in range(5)]
+
+
+@requires_pint
+@pytest.mark.parametrize("func", ["all", "count", "sum", "var"])
+@pytest.mark.parametrize("chunk", [True, False])
+def test_pint(chunk, func):
+    import pint
+
+    if chunk:
+        d = dask.array.array([1, 2, 3])
+    else:
+        d = np.array([1, 2, 3])
+    q = pint.Quantity(d, units="m")
+
+    actual, _ = groupby_reduce(q, [0, 0, 1], func=func)
+    expected, _ = groupby_reduce(q.magnitude, [0, 0, 1], func=func)
+
+    units = None if func in ["count", "all"] else getattr(np, func)(q).units
+    if units is not None:
+        expected = pint.Quantity(expected, units=units)
+    assert_equal(expected, actual)
+
+
+@requires_pint
+@pytest.mark.parametrize("chunk", [True, False])
+def test_pint_prod_error(chunk):
+    import pint
+
+    if chunk:
+        d = dask.array.array([1, 2, 3])
+    else:
+        d = np.array([1, 2, 3])
+    q = pint.Quantity(d, units="m")
+
+    with pytest.raises(ValueError):
+        groupby_reduce(q, [0, 0, 1], func="prod")