fix: initialize _lock attribute in ZipStore (#3588)

src/zarr/storage/_zip.py

@@ -87,13 +87,12 @@ def __init__(
         self._zmode = mode
         self.compression = compression
         self.allowZip64 = allowZip64
+        self._lock = threading.RLock()
 
     def _sync_open(self) -> None:
         if self._is_open:
             raise ValueError("store is already open")
 
-        self._lock = threading.RLock()
-
         self._zf = zipfile.ZipFile(
             self.path,
             mode=self._zmode,

tests/test_store/test_zip.py

@@ -152,3 +152,15 @@ async def test_move(self, tmp_path: Path) -> None:
         assert destination.exists()
         assert not origin.exists()
         assert np.array_equal(array[...], np.arange(10))
+
+    def test_lock_initialized_before_open(self, store_kwargs: dict[str, Any]) -> None:
+        """Regression test for https://github.com/zarr-developers/zarr-python/issues/3588.
+
+        ZipStore._lock must be available immediately after __init__, before _open() is called,
+        so that operations like get/set/exists can use it even on non-opened stores.
+        """
+        import threading
+
+        store = self.store_cls(**store_kwargs)
+        assert hasattr(store, "_lock"), "ZipStore._lock must be initialized in __init__"
+        assert isinstance(store._lock, type(threading.RLock())), "_lock must be an RLock"