Avoid redundant metadata reads in ZarrArrayWrapper (#8297)

* Avoid redundant metadata reads in `ZarrArrayWrapper`

Modify ZarrArrayWrapper to avoid re-reading metadata each time data
is read from the array.

* Improve test documentation

* Update xarray/tests/test_backends.py

Co-authored-by: Joe Hamman <jhamman1@gmail.com>

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Add what's new entry

---------

Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>
Co-authored-by: Joe Hamman <jhamman1@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

doc/whats-new.rst

@@ -54,6 +54,7 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+
 - :py:meth:`DataArray.rename` & :py:meth:`Dataset.rename` would emit a warning
   when the operation was a no-op. (:issue:`8266`)
   By `Simon Hansen <https://github.com/hoxbro>`_.
@@ -64,6 +65,12 @@ Bug fixes
   (:issue:`8271`, :pull:`8272`). By `Spencer Clark
   <https://github.com/spencerkclark>`_.
 
+- Fix excess metadata requests when using a Zarr store. Prior to this, metadata
+  was re-read every time data was retrieved from the array, now metadata is retrieved only once
+  when they array is initialized.
+  (:issue:`8290`, :pull:`8297`).
+  By `Oliver McCormack <https://github.com/olimcc>`_.
+
 
 Documentation
 ~~~~~~~~~~~~~

xarray/backends/zarr.py

@@ -61,27 +61,29 @@ def encode_zarr_attr_value(value):
 
 
 class ZarrArrayWrapper(BackendArray):
-    __slots__ = ("datastore", "dtype", "shape", "variable_name")
+    __slots__ = ("datastore", "dtype", "shape", "variable_name", "_array")
 
     def __init__(self, variable_name, datastore):
         self.datastore = datastore
         self.variable_name = variable_name
 
-        array = self.get_array()
-        self.shape = array.shape
+        # some callers attempt to evaluate an array if an `array` property exists on the object.
+        # we prefix with _ to avoid this inference.
+        self._array = self.datastore.zarr_group[self.variable_name]
+        self.shape = self._array.shape
 
         # preserve vlen string object dtype (GH 7328)
-        if array.filters is not None and any(
-            [filt.codec_id == "vlen-utf8" for filt in array.filters]
+        if self._array.filters is not None and any(
+            [filt.codec_id == "vlen-utf8" for filt in self._array.filters]
         ):
             dtype = coding.strings.create_vlen_dtype(str)
         else:
-            dtype = array.dtype
+            dtype = self._array.dtype
 
         self.dtype = dtype
 
     def get_array(self):
-        return self.datastore.zarr_group[self.variable_name]
+        return self._array
 
     def _oindex(self, key):
         return self.get_array().oindex[key]

xarray/tests/test_backends.py

@@ -19,6 +19,7 @@
 from os import listdir
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Final, cast
+from unittest.mock import patch
 
 import numpy as np
 import pandas as pd
@@ -2862,6 +2863,47 @@ def create_zarr_target(self):
             yield tmp
 
 
+@requires_zarr
+class TestZarrArrayWrapperCalls(TestZarrKVStoreV3):
+    def test_avoid_excess_metadata_calls(self) -> None:
+        """Test that chunk requests do not trigger redundant metadata requests.
+
+        This test targets logic in backends.zarr.ZarrArrayWrapper, asserting that calls
+        to retrieve chunk data after initialization do not trigger additional
+        metadata requests.
+
+        https://github.com/pydata/xarray/issues/8290
+        """
+
+        import zarr
+
+        ds = xr.Dataset(data_vars={"test": (("Z",), np.array([123]).reshape(1))})
+
+        # The call to retrieve metadata performs a group lookup. We patch Group.__getitem__
+        # so that we can inspect calls to this method - specifically count of calls.
+        # Use of side_effect means that calls are passed through to the original method
+        # rather than a mocked method.
+        Group = zarr.hierarchy.Group
+        with (
+            self.create_zarr_target() as store,
+            patch.object(
+                Group, "__getitem__", side_effect=Group.__getitem__, autospec=True
+            ) as mock,
+        ):
+            ds.to_zarr(store, mode="w")
+
+            # We expect this to request array metadata information, so call_count should be >= 1,
+            # At time of writing, 2 calls are made
+            xrds = xr.open_zarr(store)
+            call_count = mock.call_count
+            assert call_count > 0
+
+            # compute() requests array data, which should not trigger additional metadata requests
+            # we assert that the number of calls has not increased after fetchhing the array
+            xrds.test.compute(scheduler="sync")
+            assert mock.call_count == call_count
+
+
 @requires_zarr
 @requires_fsspec
 def test_zarr_storage_options() -> None: