Merge pull request #71 from csiro-coasts/timedelta-fill-value

Timedelta fill value

Author: mx-moth

docs/releases/development.rst

@@ -2,4 +2,6 @@
 Next release (in development)
 =============================
 
-* ...
+* Fixed an issue with ``_FillValue`` / ``missing_value``
+  and variables with non-float types such as ``timedelta64``
+  (:pr:`71`)

src/emsarray/masking.py

@@ -14,6 +14,7 @@
 
 import numpy as np
 import xarray as xr
+from xarray.core.dtypes import maybe_promote
 
 from emsarray import utils
 from emsarray.types import Pathish
@@ -71,7 +72,6 @@ def mask_grid_dataset(
     # file system, at the added expense of having to recombine the dataset
     # afterwards.
     for key, data_array in dataset.data_vars.items():
-        logger.debug("DataArray %s", key)
         masked_data_array = mask_grid_data_array(mask, data_array)
         variable_path = work_path / f"{key}.nc"
         mfdataset_names.append(variable_path)
@@ -130,19 +130,28 @@ def mask_grid_data_array(mask: xr.Dataset, data_array: xr.DataArray) -> xr.DataA
     try:
         fill_value = find_fill_value(data_array)
     except ValueError:
+        logger.debug(
+            "Data array %r has no valid fill value, leaving as is",
+            data_array.name)
         return data_array
 
     # Loop through each possible mask
     for mask_name, mask_data_array in mask.data_vars.items():
         # If every dimension of this mask exists in the data array, apply it
         if dimensions >= set(mask_data_array.dims):
+            logger.debug(
+                "Masking data array %r with mask %r",
+                data_array.name, mask_name)
             new_data_array = cast(xr.DataArray, data_array.where(mask_data_array, other=fill_value))
             new_data_array.attrs = data_array.attrs
             new_data_array.encoding = data_array.encoding
             return new_data_array
 
     # Fallback, no appropriate mask was found, so don't apply any.
     # This generally happens for data arrays such as time, record, x_grid, etc.
+    logger.debug(
+        "Data array %r had no relevant mask, leaving as is",
+        data_array.name)
     return data_array
 
 
@@ -182,24 +191,16 @@ def find_fill_value(data_array: xr.DataArray) -> Any:
         # constructed a dataset using one...
         return np.ma.masked
 
-    if '_FillValue' in data_array.encoding:
-        # The dataset was opened with mask_and_scale=True and a mask has been
-        # applied. Masked values are now represented as np.nan, not _FillValue.
-        return np.nan
-
-    if '_FillValue' in data_array.attrs:
-        # The dataset was opened with mask_and_scale=False and a mask has not
-        # been applied. Masked values should be represented using _FillValue.
-        return data_array.attrs['_FillValue']
-
-    if issubclass(data_array.dtype.type, np.floating):
-        # NaN is a useful fallback for a _FillValue, but only if the dtype
-        # is some sort of float. We won't actually _set_ a _FillValue
-        # attribute though, as that can play havok when trying to save
-        # existing datasets. xarray gets real grumpy when you have
-        # a _FillValue and a missing_value, and some existing datasets play
-        # fast and loose with mixing the two.
-        return np.nan
+    attrs = ['_FillValue', 'missing_value']
+    for attr in attrs:
+        if attr in data_array.attrs:
+            # The dataset was opened with mask_and_scale=False and a mask has not
+            # been applied. Masked values should be represented using _FillValue/missing_value.
+            return data_array.attrs[attr]
+
+    promoted_dtype, fill_value = maybe_promote(data_array.dtype)
+    if promoted_dtype == data_array.dtype:
+        return fill_value
 
     raise ValueError("No appropriate fill value found")
 

src/emsarray/utils.py

@@ -31,6 +31,7 @@
 from packaging.version import Version
 from xarray.coding import times
 from xarray.core.common import contains_cftime_datetimes
+from xarray.core.dtypes import maybe_promote
 
 from emsarray.types import Pathish
 
@@ -233,8 +234,10 @@ def disable_default_fill_value(dataset_or_array: Union[xr.Dataset, xr.DataArray]
         The :class:`xarray.Dataset` or :class:`xarray.DataArray` to update
     """
     for variable in _get_variables(dataset_or_array):
+        current_dtype = variable.dtype
+        promoted_dtype, fill_value = maybe_promote(current_dtype)
         if (
-            issubclass(variable.dtype.type, np.floating)
+            current_dtype == promoted_dtype
             and "_FillValue" not in variable.encoding
             and "_FillValue" not in variable.attrs
         ):

tests/datasets/masking/find_fill_value/float_with_fill_value.nc

@@ -1,5 +1,19 @@
 #!/usr/bin/env python3
 
+"""
+Make some datasets for testing fill values.
+Because of how xarray preprocesses variables to apply masks,
+it is easier to construct these datasets using the plain netCDF4 library,
+save the datasets to disk, and then load them using xarray.
+This guarantees that the behaviour in the tests will replicate real-world use.
+
+Running this script will overwrite any datasets already constructed in this directory.
+This operation should result in byte-for-byte identical datasets each time it is run.
+However each netCDF4 dataset will encode the versions of the
+netCDF4, hdf5 and other relevant libraries used to construct the dataset.
+If the versions have changed, the script will create new files that git thinks have changed.
+"""
+
 import pathlib
 
 import netCDF4
@@ -8,8 +22,10 @@
 here = pathlib.Path(__file__).parent
 
 
-def make_float_with_fill_value() -> None:
-    ds = netCDF4.Dataset(here / "float_with_fill_value.nc", "w", "NETCDF4")
+def make_float_with_fill_value(
+    output_path: pathlib.Path = here / "float_with_fill_value.nc"
+) -> None:
+    ds = netCDF4.Dataset(output_path, "w", "NETCDF4")
     ds.createDimension("x", 2)
     ds.createDimension("y", 2)
 
@@ -20,8 +36,10 @@ def make_float_with_fill_value() -> None:
     ds.close()
 
 
-def make_float_with_fill_value_and_offset() -> None:
-    ds = netCDF4.Dataset(here / "float_with_fill_value_and_offset.nc", "w", "NETCDF4")
+def make_float_with_fill_value_and_offset(
+    output_path: pathlib.Path = here / "float_with_fill_value_and_offset.nc",
+) -> None:
+    ds = netCDF4.Dataset(output_path, "w", "NETCDF4")
     ds.createDimension("x", 2)
     ds.createDimension("y", 2)
 
@@ -34,8 +52,27 @@ def make_float_with_fill_value_and_offset() -> None:
     ds.close()
 
 
-def make_int_with_fill_value_and_offset() -> None:
-    ds = netCDF4.Dataset(here / "int_with_fill_value_and_offset.nc", "w", "NETCDF4")
+def make_timedelta_with_missing_value(
+    output_path: pathlib.Path = here / "timedelta_with_missing_value.nc",
+) -> None:
+    ds = netCDF4.Dataset(output_path, "w", "NETCDF4")
+    ds.createDimension("x", 2)
+    ds.createDimension("y", 2)
+
+    missing_value = np.float32(1.e+35)
+    var = ds.createVariable("var", "f4", ["y", "x"], fill_value=False)
+    var.missing_value = missing_value
+    var.units = "days"
+    var[:] = np.arange(4).reshape((2, 2))
+    var[1, 1] = missing_value
+
+    ds.close()
+
+
+def make_int_with_fill_value_and_offset(
+    output_path: pathlib.Path = here / "int_with_fill_value_and_offset.nc",
+) -> None:
+    ds = netCDF4.Dataset(output_path, "w", "NETCDF4")
     ds.createDimension("x", 2)
     ds.createDimension("y", 2)
 
@@ -51,4 +88,5 @@ def make_int_with_fill_value_and_offset() -> None:
 if __name__ == '__main__':
     make_float_with_fill_value()
     make_float_with_fill_value_and_offset()
+    make_timedelta_with_missing_value()
     make_int_with_fill_value_and_offset()

tests/datasets/masking/find_fill_value/float_with_fill_value_and_offset.nc

@@ -155,10 +155,10 @@ def test_mask_dataset(tmp_path: pathlib.Path):
         data=np.random.normal(0, 0.2, (records, j_size, i_size)),
         dims=["record", "j_centre", "i_centre"],
         attrs={
-                "units": "metre",
-                "long_name": "Surface elevation",
-                "standard_name": "sea_surface_height_above_geoid",
-            }
+            "units": "metre",
+            "long_name": "Surface elevation",
+            "standard_name": "sea_surface_height_above_geoid",
+        }
     )
     temp = xr.DataArray(
         data=np.random.normal(12, 0.5, (records, k_size, j_size, i_size)),
@@ -262,7 +262,7 @@ def test_mask_dataset(tmp_path: pathlib.Path):
         assert nc_flag2.shape == (k_size, 4, 3)
         flag2_mask = np.stack([np.array([
             [0, 0, 0], [0, 0, 0], [0, 0, 1], [0, 1, 1]
-        ])]*k_size).astype(bool)
+        ])] * k_size).astype(bool)
         expected: np.ndarray = np.ma.masked_array(
             flag2.values[:, 1:5, 1:4].copy(),
             mask=flag2_mask,

tests/datasets/masking/find_fill_value/int_with_fill_value_and_offset.nc

@@ -9,6 +9,7 @@
 from numpy.testing import assert_equal
 
 from emsarray import masking
+from emsarray.utils import to_netcdf_with_fixes
 from tests.utils import mask_from_strings
 
 
@@ -102,6 +103,26 @@ def test_find_fill_value_masked_and_scaled_int(datasets):
         assert_dtype_equal(masking.find_fill_value(data_array), np.int8(-1))
 
 
+def test_find_fill_value_timedelta_with_missing_value(
+    datasets: pathlib.Path,
+    tmp_path: pathlib.Path,
+) -> None:
+    dataset_path = datasets / 'masking/find_fill_value/timedelta_with_missing_value.nc'
+
+    missing_value = np.float32(1.e35)
+    assert_raw_values(
+        dataset_path, 'var',
+        np.array([[0, 1], [2, missing_value]], dtype=np.float32))
+
+    with xr.open_dataset(dataset_path) as dataset:
+        data_array = dataset['var']
+        assert dataset['var'].dtype == np.dtype('timedelta64[ns]')
+        fill_value = masking.find_fill_value(data_array)
+        assert np.isnat(fill_value)
+
+        to_netcdf_with_fixes(dataset, tmp_path / 'dataset.nc')
+
+
 def test_calculate_mask_bounds():
     mask = xr.Dataset(
         data_vars={