Fix Windows Zarr directory detection in open_dataset

doc/whats-new.rst

@@ -174,6 +174,11 @@ Deprecations
 
 Bug Fixes
 ~~~~~~~~~
+- :py:func:`open_dataset` can detect local Zarr directories on Windows without
+  raising ``PermissionError`` while checking file-based backends. Permission
+  errors for ordinary files still propagate (:issue:`11799`, :pull:`11802`).
+  By `Gabriel Demetrios Lafis <https://github.com/galafis>`_.
+
 - :py:meth:`Dataset.query` now resolves coordinates in datasets with no data
   variables (:issue:`6449`).
   By `Liang, Jingyi <https://github.com/LJimmy007>`_.

xarray/core/utils.py

@@ -777,7 +777,7 @@ def read_magic_number_from_file(filename_or_obj, count=8) -> bytes:
     return magic_number
 
 
-def try_read_magic_number_from_path(pathlike, count=8) -> bytes | None:
+def try_read_magic_number_from_path(pathlike: Any, count: int = 8) -> bytes | None:
     if isinstance(pathlike, str) or hasattr(pathlike, "__fspath__"):
         path = os.fspath(pathlike)
         try:
@@ -788,6 +788,12 @@ def try_read_magic_number_from_path(pathlike, count=8) -> bytes | None:
                 return read_magic_number_from_file(f, count)
         except (FileNotFoundError, IsADirectoryError, TypeError):
             pass
+        except PermissionError:
+            # Windows reports PermissionError rather than IsADirectoryError
+            # when opening a directory. Let other backends inspect it, but
+            # preserve permission errors for ordinary files.
+            if not os.path.isdir(path):
+                raise
     return None
 
 

xarray/tests/backends/test_plugins.py

@@ -5,6 +5,7 @@
 import sys
 from importlib.metadata import EntryPoint, EntryPoints
 from itertools import starmap
+from pathlib import Path
 from unittest import mock
 
 import pytest
@@ -26,6 +27,23 @@
 importlib_metadata_mock = "importlib.metadata"
 
 
+@requires_netCDF4
+@requires_zarr
+@pytest.mark.parametrize("zarr_format", [2, 3])
+@pytest.mark.parametrize("path_as_string", [False, True])
+def test_guess_engine_local_zarr_directory(
+    tmp_path: Path, zarr_format: int, path_as_string: bool
+) -> None:
+    expected = xr.Dataset({"temperature": ("time", [1.0, 2.0, 3.0])})
+    path = tmp_path / "example.zarr"
+    expected.to_zarr(path, zarr_format=zarr_format, consolidated=False)
+    store = str(path) if path_as_string else path
+
+    assert plugins.guess_engine(store) == "zarr"
+    with xr.open_dataset(store, consolidated=False) as actual:
+        xr.testing.assert_identical(actual, expected)
+
+
 class DummyBackendEntrypointArgs(common.BackendEntrypoint):
     def open_dataset(filename_or_obj, *args):  # type: ignore[override]
         pass

xarray/tests/test_utils.py

@@ -448,6 +448,32 @@ def test_try_read_magic_number_from_path_missing(tmp_path: Path) -> None:
     assert try_read_magic_number_from_path(tmp_path / "missing.nc") is None
 
 
+def test_try_read_magic_number_from_directory(tmp_path: Path) -> None:
+    assert try_read_magic_number_from_path(tmp_path) is None
+    assert try_read_magic_number_from_path(str(tmp_path)) is None
+
+
+@pytest.mark.parametrize("directory", [False, True])
+def test_try_read_magic_number_permission_error(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch, directory: bool
+) -> None:
+    path = tmp_path / "store"
+    if directory:
+        path.mkdir()
+    else:
+        path.write_bytes(b"CDF")
+
+    def denied_open(*args: Any, **kwargs: Any) -> IO[Any]:
+        raise PermissionError("Access denied")
+
+    monkeypatch.setattr(utils, "open", denied_open, raising=False)
+    if directory:
+        assert try_read_magic_number_from_path(path) is None
+    else:
+        with pytest.raises(PermissionError, match="Access denied"):
+            try_read_magic_number_from_path(path)
+
+
 def test_try_read_magic_number_from_path_unbuffered(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: