to_zarr(append_dim='dim0') doesn't need mode='a'

doc/io.rst

@@ -609,8 +609,9 @@ store is already present at that path, an error will be raised, preventing it
 from being overwritten. To override this behavior and overwrite an existing
 store, add ``mode='w'`` when invoking ``to_zarr``.
 
-It is also possible to append to an existing store. For that, add ``mode='a'``
-and set ``append_dim`` to the name of the dimension along which to append.
+It is also possible to append to an existing store. For that, set
+``append_dim`` to the name of the dimension along which to append. ``mode``
+can be omitted as it will internally be set to ``'a'``.
 
 .. ipython:: python
    :suppress:
@@ -628,7 +629,7 @@ and set ``append_dim`` to the name of the dimension along which to append.
                      coords={'x': [10, 20, 30, 40],
                              'y': [1,2,3,4,5],
                              't': pd.date_range('2001-01-03', periods=2)})
-    ds2.to_zarr('path/to/directory.zarr', mode='a', append_dim='t')
+    ds2.to_zarr('path/to/directory.zarr', append_dim='t')
 
 To store variable length strings use ``dtype=object``.
 

doc/whats-new.rst

@@ -27,6 +27,10 @@ New functions/methods
 Enhancements
 ~~~~~~~~~~~~
 
+- In :py:meth:`~xarray.Dataset.to_zarr`, passing ``mode`` is not mandatory if
+  ``append_dim`` is set, as it will automatically be set to ``'a'`` internally.
+  By `David Brochart <https://github.com/davidbrochart>`_.
+
 Bug fixes
 ~~~~~~~~~
 

xarray/backends/api.py

@@ -1102,7 +1102,7 @@ def _validate_append_dim_and_encoding(ds_to_append, store, append_dim,
                 )
 
 
-def to_zarr(dataset, store=None, mode='w-', synchronizer=None, group=None,
+def to_zarr(dataset, store=None, mode=None, synchronizer=None, group=None,
             encoding=None, compute=True, consolidated=False, append_dim=None):
     """This function creates an appropriate datastore for writing a dataset to
     a zarr ztore
@@ -1118,7 +1118,7 @@ def to_zarr(dataset, store=None, mode='w-', synchronizer=None, group=None,
     _validate_dataset_names(dataset)
     _validate_attrs(dataset)
 
-    if mode == "a":
+    if mode == 'a':
         _validate_datatypes_for_zarr_append(dataset)
         _validate_append_dim_and_encoding(dataset, store, append_dim,
                                           group=group,

xarray/core/dataset.py

@@ -1389,7 +1389,7 @@ def to_netcdf(
     def to_zarr(
         self,
         store: Union[MutableMapping, str, Path] = None,
-        mode: str = 'w-',
+        mode: str = None,
         synchronizer=None,
         group: str = None,
         encoding: Mapping = None,
@@ -1407,10 +1407,13 @@ def to_zarr(
         ----------
         store : MutableMapping, str or Path, optional
             Store or path to directory in file system.
-        mode : {'w', 'w-', 'a'}
+        mode : {'w', 'w-', 'a', None}
             Persistence mode: 'w' means create (overwrite if exists);
             'w-' means create (fail if exists);
             'a' means append (create if does not exist).
+            If ``append_dim`` is set, ``mode`` can be omitted as it is
+            internally set to ``'a'``. Otherwise, ``mode`` will default to
+            `w-` if not set.
         synchronizer : object, optional
             Array synchronizer
         group : str, optional
@@ -1426,14 +1429,24 @@ def to_zarr(
             If True, apply zarr's `consolidate_metadata` function to the store
             after writing.
         append_dim: hashable, optional
-            If mode='a', the dimension on which the data will be appended.
+            If set, the dimension on which the data will be appended.
 
         References
         ----------
         https://zarr.readthedocs.io/
         """
         if encoding is None:
             encoding = {}
+        if (mode == 'a') or (append_dim is not None):
+            if mode is None:
+                mode = 'a'
+            elif mode != 'a':
+                raise ValueError(
+                    "append_dim was set along with mode='{}', either set "
+                    "mode='a' or don't set it.".format(mode)
+                )
+        elif mode is None:
+            mode = 'w-'
         if mode not in ['w', 'w-', 'a']:
             # TODO: figure out how to handle 'r+'
             raise ValueError("The only supported options for mode are 'w',"

xarray/tests/test_backends.py

@@ -1654,7 +1654,7 @@ def test_write_persistence_modes(self):
         # check append mode for append write
         with self.create_zarr_target() as store_target:
             ds.to_zarr(store_target, mode='w')
-            ds_to_append.to_zarr(store_target, mode='a', append_dim='time')
+            ds_to_append.to_zarr(store_target, append_dim='time')
             original = xr.concat([ds, ds_to_append], dim='time')
             assert_identical(original, xr.open_zarr(store_target))
 
@@ -1690,7 +1690,7 @@ def test_append_write(self):
         ds, ds_to_append, _ = create_append_test_data()
         with self.create_zarr_target() as store_target:
             ds.to_zarr(store_target, mode='w')
-            ds_to_append.to_zarr(store_target, mode='a', append_dim='time')
+            ds_to_append.to_zarr(store_target, append_dim='time')
             original = xr.concat([ds, ds_to_append], dim='time')
             assert_identical(original, xr.open_zarr(store_target))
 
@@ -1706,8 +1706,7 @@ def test_append_with_invalid_dim_raises(self):
         with pytest.raises(ValueError):
             with self.create_zarr_target() as store_target:
                 ds.to_zarr(store_target, mode='w')
-                ds_to_append.to_zarr(store_target, mode='a',
-                                     append_dim='notvalid')
+                ds_to_append.to_zarr(store_target, append_dim='notvalid')
 
     def test_append_with_append_dim_not_set_raises(self):
 
@@ -1719,6 +1718,17 @@ def test_append_with_append_dim_not_set_raises(self):
                 ds.to_zarr(store_target, mode='w')
                 ds_to_append.to_zarr(store_target, mode='a')
 
+    def test_append_with_mode_not_a_raises(self):
+
+        ds, ds_to_append, _ = create_append_test_data()
+
+        # check failure when append_dim is set and mode != 'a'
+        with pytest.raises(ValueError):
+            with self.create_zarr_target() as store_target:
+                ds.to_zarr(store_target, mode='w')
+                ds_to_append.to_zarr(store_target, mode='w',
+                                     append_dim='time')
+
     def test_append_with_existing_encoding_raises(self):
 
         ds, ds_to_append, _ = create_append_test_data()
@@ -1727,8 +1737,7 @@ def test_append_with_existing_encoding_raises(self):
         with pytest.raises(ValueError):
             with self.create_zarr_target() as store_target:
                 ds.to_zarr(store_target, mode='w')
-                ds_to_append.to_zarr(store_target, mode='a',
-                                     append_dim='time',
+                ds_to_append.to_zarr(store_target, append_dim='time',
                                      encoding={'da': {'compressor': None}})
 
     def test_check_encoding_is_consistent_after_append(self):
@@ -1741,7 +1750,7 @@ def test_check_encoding_is_consistent_after_append(self):
             compressor = zarr.Blosc()
             encoding = {'da': {'compressor': compressor}}
             ds.to_zarr(store_target, mode='w', encoding=encoding)
-            ds_to_append.to_zarr(store_target, mode='a', append_dim='time')
+            ds_to_append.to_zarr(store_target, append_dim='time')
             actual_ds = xr.open_zarr(store_target)
             actual_encoding = actual_ds['da'].encoding['compressor']
             assert actual_encoding.get_config() == compressor.get_config()
@@ -1802,7 +1811,7 @@ def test_to_zarr_append_compute_false_roundtrip(self):
                 assert_identical(ds, actual)
 
             delayed_obj = self.save(ds_to_append, store, compute=False,
-                                    mode='a', append_dim='time')
+                                    append_dim='time')
             assert isinstance(delayed_obj, Delayed)
 
             with pytest.raises(AssertionError):