Defaults autoclose=False for open_mfdataset

This choice of default is to select standard xarray performance over
general removal of the OSError associated with opening too many files as
encountered using open_mfdataset

doc/whats-new.rst

@@ -22,6 +22,12 @@ v0.9.2 (unreleased)
 Enhancements
 ~~~~~~~~~~~~
 
+- It is now possible to set the ``autoclose=True`` argument to
+  :py:func:`~xarray.open_mfdataset` to explicitly close opened files when not
+  in use to prevent occurrence of an OS Error related to too many open files.
+  Note, the default is ``autoclose=False``, which is consistent with previous
+  xarray behavior.  By `Phillip J. Wolfram <https://github.com/pwolfram>`_.
+
 Bug fixes
 ~~~~~~~~~
 

xarray/backends/api.py

@@ -132,7 +132,7 @@ def _protect_dataset_variables_inplace(dataset, cache):
 
 
 def open_dataset(filename_or_obj, group=None, decode_cf=True,
-                 mask_and_scale=True, decode_times=True, autoclose=True,
+                 mask_and_scale=True, decode_times=True, autoclose=False,
                  concat_characters=True, decode_coords=True, engine=None,
                  chunks=None, lock=None, cache=None, drop_variables=None):
     """Load and decode a dataset from a file or file-like object.

xarray/tests/test_backends.py

@@ -1077,17 +1077,19 @@ def test_open_mfdataset(self):
             with create_tmp_file() as tmp2:
                 original.isel(x=slice(5)).to_netcdf(tmp1)
                 original.isel(x=slice(5, 10)).to_netcdf(tmp2)
-                with open_mfdataset([tmp1, tmp2]) as actual:
-                    self.assertIsInstance(actual.foo.variable.data, da.Array)
-                    self.assertEqual(actual.foo.variable.data.chunks,
-                                     ((5, 5),))
-                    self.assertDatasetAllClose(original, actual)
-                with open_mfdataset([tmp1, tmp2], chunks={'x': 3}) as actual:
-                    self.assertEqual(actual.foo.variable.data.chunks,
-                                     ((3, 2, 3, 2),))
+                for close in [True, False]:
+                    with open_mfdataset([tmp1, tmp2], autoclose=close) as actual:
+                        self.assertIsInstance(actual.foo.variable.data, da.Array)
+                        self.assertEqual(actual.foo.variable.data.chunks,
+                                         ((5, 5),))
+                        self.assertDatasetAllClose(original, actual)
+                    with open_mfdataset([tmp1, tmp2], chunks={'x': 3}, autoclose=close) as actual:
+                        self.assertEqual(actual.foo.variable.data.chunks,
+                                         ((3, 2, 3, 2),))
 
         with self.assertRaisesRegexp(IOError, 'no files to open'):
-            open_mfdataset('foo-bar-baz-*.nc')
+            for close in [True, False]:
+                open_mfdataset('foo-bar-baz-*.nc', autoclose=close)
 
     def test_preprocess_mfdataset(self):
         original = Dataset({'foo': ('x', np.random.randn(10))})
@@ -1098,8 +1100,9 @@ def preprocess(ds):
                 return ds.assign_coords(z=0)
 
             expected = preprocess(original)
-            with open_mfdataset(tmp, preprocess=preprocess) as actual:
-                self.assertDatasetIdentical(expected, actual)
+            for close in [True, False]:
+                with open_mfdataset(tmp, preprocess=preprocess, autoclose=close) as actual:
+                    self.assertDatasetIdentical(expected, actual)
 
     def test_save_mfdataset_roundtrip(self):
         original = Dataset({'foo': ('x', np.random.randn(10))})
@@ -1108,8 +1111,9 @@ def test_save_mfdataset_roundtrip(self):
         with create_tmp_file() as tmp1:
             with create_tmp_file() as tmp2:
                 save_mfdataset(datasets, [tmp1, tmp2])
-                with open_mfdataset([tmp1, tmp2]) as actual:
-                    self.assertDatasetIdentical(actual, original)
+                for close in [True, False]:
+                    with open_mfdataset([tmp1, tmp2], autoclose=close) as actual:
+                        self.assertDatasetIdentical(actual, original)
 
     def test_save_mfdataset_invalid(self):
         ds = Dataset()
@@ -1122,18 +1126,21 @@ def test_open_and_do_math(self):
         original = Dataset({'foo': ('x', np.random.randn(10))})
         with create_tmp_file() as tmp:
             original.to_netcdf(tmp)
-            with open_mfdataset(tmp) as ds:
-                actual = 1.0 * ds
-                self.assertDatasetAllClose(original, actual)
+            for close in [True, False]:
+                with open_mfdataset(tmp, autoclose=close) as ds:
+                    actual = 1.0 * ds
+                    self.assertDatasetAllClose(original, actual)
 
     def test_open_mfdataset_concat_dim_none(self):
         with create_tmp_file() as tmp1:
             with create_tmp_file() as tmp2:
                 data = Dataset({'x': 0})
                 data.to_netcdf(tmp1)
                 Dataset({'x': np.nan}).to_netcdf(tmp2)
-                with open_mfdataset([tmp1, tmp2], concat_dim=None) as actual:
-                    self.assertDatasetIdentical(data, actual)
+                for close in [True, False]:
+                    with open_mfdataset([tmp1, tmp2],
+                                        concat_dim=None, autoclose=close) as actual:
+                        self.assertDatasetIdentical(data, actual)
 
     def test_open_dataset(self):
         original = Dataset({'foo': ('x', np.random.randn(10))})
@@ -1165,16 +1172,17 @@ def test_deterministic_names(self):
         with create_tmp_file() as tmp:
             data = create_test_data()
             data.to_netcdf(tmp)
-            with open_mfdataset(tmp) as ds:
-                original_names = dict((k, v.data.name)
-                                      for k, v in ds.data_vars.items())
-            with open_mfdataset(tmp) as ds:
-                repeat_names = dict((k, v.data.name)
-                                    for k, v in ds.data_vars.items())
-            for var_name, dask_name in original_names.items():
-                self.assertIn(var_name, dask_name)
-                self.assertIn(tmp, dask_name)
-            self.assertEqual(original_names, repeat_names)
+            for close in [True, False]:
+                with open_mfdataset(tmp, autoclose=close) as ds:
+                    original_names = dict((k, v.data.name)
+                                          for k, v in ds.data_vars.items())
+                with open_mfdataset(tmp, autoclose=close) as ds:
+                    repeat_names = dict((k, v.data.name)
+                                        for k, v in ds.data_vars.items())
+                for var_name, dask_name in original_names.items():
+                    self.assertIn(var_name, dask_name)
+                    self.assertIn(tmp, dask_name)
+                self.assertEqual(original_names, repeat_names)
 
     def test_dataarray_compute(self):
         # Test DataArray.compute() on dask backend.