Support dask arrays in datetime_to_numeric (#6556)

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

xarray/core/duck_array_ops.py

@@ -431,7 +431,14 @@ def datetime_to_numeric(array, offset=None, datetime_unit=None, dtype=float):
     # Compute timedelta object.
     # For np.datetime64, this can silently yield garbage due to overflow.
     # One option is to enforce 1970-01-01 as the universal offset.
-    array = array - offset
+
+    # This map_blocks call is for backwards compatibility.
+    # dask == 2021.04.1 does not support subtracting object arrays
+    # which is required for cftime
+    if is_duck_dask_array(array) and np.issubdtype(array.dtype, np.object):
+        array = array.map_blocks(lambda a, b: a - b, offset, meta=array._meta)
+    else:
+        array = array - offset
 
     # Scalar is converted to 0d-array
     if not hasattr(array, "dtype"):
@@ -517,10 +524,19 @@ def pd_timedelta_to_float(value, datetime_unit):
     return np_timedelta64_to_float(value, datetime_unit)
 
 
+def _timedelta_to_seconds(array):
+    return np.reshape([a.total_seconds() for a in array.ravel()], array.shape) * 1e6
+
+
 def py_timedelta_to_float(array, datetime_unit):
     """Convert a timedelta object to a float, possibly at a loss of resolution."""
-    array = np.asarray(array)
-    array = np.reshape([a.total_seconds() for a in array.ravel()], array.shape) * 1e6
+    array = asarray(array)
+    if is_duck_dask_array(array):
+        array = array.map_blocks(
+            _timedelta_to_seconds, meta=np.array([], dtype=np.float64)
+        )
+    else:
+        array = _timedelta_to_seconds(array)
     conversion_factor = np.timedelta64(1, "us") / np.timedelta64(1, datetime_unit)
     return conversion_factor * array
 

xarray/tests/test_duck_array_ops.py

@@ -675,39 +675,68 @@ def test_multiple_dims(dtype, dask, skipna, func):
     assert_allclose(actual, expected)
 
 
-def test_datetime_to_numeric_datetime64():
+@pytest.mark.parametrize("dask", [True, False])
+def test_datetime_to_numeric_datetime64(dask):
+    if dask and not has_dask:
+        pytest.skip("requires dask")
+
     times = pd.date_range("2000", periods=5, freq="7D").values
-    result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h")
+    if dask:
+        import dask.array
+
+        times = dask.array.from_array(times, chunks=-1)
+
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h")
     expected = 24 * np.arange(0, 35, 7)
     np.testing.assert_array_equal(result, expected)
 
     offset = times[1]
-    result = duck_array_ops.datetime_to_numeric(times, offset=offset, datetime_unit="h")
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(
+            times, offset=offset, datetime_unit="h"
+        )
     expected = 24 * np.arange(-7, 28, 7)
     np.testing.assert_array_equal(result, expected)
 
     dtype = np.float32
-    result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h", dtype=dtype)
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(
+            times, datetime_unit="h", dtype=dtype
+        )
     expected = 24 * np.arange(0, 35, 7).astype(dtype)
     np.testing.assert_array_equal(result, expected)
 
 
 @requires_cftime
-def test_datetime_to_numeric_cftime():
+@pytest.mark.parametrize("dask", [True, False])
+def test_datetime_to_numeric_cftime(dask):
+    if dask and not has_dask:
+        pytest.skip("requires dask")
+
     times = cftime_range("2000", periods=5, freq="7D", calendar="standard").values
-    result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h", dtype=int)
+    if dask:
+        import dask.array
+
+        times = dask.array.from_array(times, chunks=-1)
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h", dtype=int)
     expected = 24 * np.arange(0, 35, 7)
     np.testing.assert_array_equal(result, expected)
 
     offset = times[1]
-    result = duck_array_ops.datetime_to_numeric(
-        times, offset=offset, datetime_unit="h", dtype=int
-    )
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(
+            times, offset=offset, datetime_unit="h", dtype=int
+        )
     expected = 24 * np.arange(-7, 28, 7)
     np.testing.assert_array_equal(result, expected)
 
     dtype = np.float32
-    result = duck_array_ops.datetime_to_numeric(times, datetime_unit="h", dtype=dtype)
+    with raise_if_dask_computes():
+        result = duck_array_ops.datetime_to_numeric(
+            times, datetime_unit="h", dtype=dtype
+        )
     expected = 24 * np.arange(0, 35, 7).astype(dtype)
     np.testing.assert_array_equal(result, expected)