zarr-developers · Akash-t25 · Jun 21, 2026 · Jun 21, 2026 · Jun 21, 2026
diff --git a/changes/828.feature.md b/changes/828.feature.md
@@ -0,0 +1 @@
+`ZipStore` now supports deletes (`supports_deletes` is `True`). Because ZIP files cannot delete entries in place, `ZipStore.delete` and `ZipStore.delete_dir` remove keys by rewriting the archive without the deleted members; overwritten (duplicate) members are compacted as a side effect.
diff --git a/src/zarr/storage/_zip.py b/src/zarr/storage/_zip.py
@@ -2,6 +2,7 @@
 
 import os
 import shutil
+import tempfile
 import threading
 import time
 import zipfile
@@ -18,7 +19,7 @@
 from zarr.core.buffer import Buffer, BufferPrototype
 
 if TYPE_CHECKING:
-    from collections.abc import AsyncIterator, Iterable
+    from collections.abc import AsyncIterator, Callable, Iterable
 
 ZipStoreAccessModeLiteral = Literal["r", "w", "a"]
 
@@ -55,7 +56,7 @@ class ZipStore(Store):
     """
 
     supports_writes: bool = True
-    supports_deletes: bool = False
+    supports_deletes: bool = True
     supports_listing: bool = True
 
     path: Path
@@ -229,21 +230,65 @@ async def set_if_not_exists(self, key: str, value: Buffer) -> None:
             if key not in members:
                 self._set(key, value)
 
+    def _rewrite_without(self, should_delete: Callable[[str], bool]) -> None:
+        # Rewrite the archive, dropping every member for which ``should_delete``
+        # returns True. ZIP files do not support in-place deletion, so the only
+        # way to remove an entry is to copy the surviving entries into a fresh
+        # archive (see issue #828). Duplicate members (created when a chunk is
+        # overwritten via ``writestr``) are compacted to their most recent value
+        # as a side effect, since that is what reads already return.
+        #
+        # This must be called while holding ``self._lock``.
+        members: dict[str, zipfile.ZipInfo] = {}
+        for info in self._zf.infolist():
+            members[info.filename] = info  # keep the last entry for each name
+
+        to_delete = [name for name in members if should_delete(name)]
+        if not to_delete:
+            # nothing matched; leave the archive untouched
+            return
+
+        fd, tmp_path = tempfile.mkstemp(dir=self.path.parent)
+        os.close(fd)
+        try:
+            with zipfile.ZipFile(
+                tmp_path, mode="w", compression=self.compression, allowZip64=self.allowZip64
+            ) as new_zf:
+                for name, info in members.items():
+                    if should_delete(name):
+                        continue
+                    new_zf.writestr(info, self._zf.read(name))
+            self._zf.close()
+            os.replace(tmp_path, self.path)
+        except BaseException:
+            if os.path.exists(tmp_path):
+                os.remove(tmp_path)
+            raise
+
+        # Reopen in append mode so subsequent writes preserve the archive
+        # (the original mode may be "w"/"x", which would truncate or fail).
+        self._zf = zipfile.ZipFile(
+            self.path, mode="a", compression=self.compression, allowZip64=self.allowZip64
+        )
+
     async def delete_dir(self, prefix: str) -> None:
-        # only raise NotImplementedError if any keys are found
+        # docstring inherited
         self._check_writable()
+        if not self._is_open:
+            self._sync_open()
         if prefix != "" and not prefix.endswith("/"):
             prefix += "/"
-        async for _ in self.list_prefix(prefix):
-            raise NotImplementedError
+        with self._lock:
+            self._rewrite_without(lambda name: name.startswith(prefix))
 
     async def delete(self, key: str) -> None:
         # docstring inherited
-        # we choose to only raise NotImplementedError here if the key exists
-        # this allows the array/group APIs to avoid the overhead of existence checks
+        # deleting a missing key is a no-op, matching the other stores
         self._check_writable()
-        if await self.exists(key):
-            raise NotImplementedError
+        if not self._is_open:
+            self._sync_open()
+        with self._lock:
+            self._rewrite_without(lambda name: name == key)
 
     async def exists(self, key: str) -> bool:
         # docstring inherited

diff --git a/tests/test_codecs/test_sharding.py b/tests/test_codecs/test_sharding.py
@@ -645,6 +645,9 @@ def test_write_partial_sharded_chunks(store: Store) -> None:
     assert np.array_equal(a[0:16, 0:16], data)
 
 
+# ZipStore overwrites shards by appending duplicate archive members (reads return
+# the most recent), which zipfile reports via a "Duplicate name" UserWarning.
+@pytest.mark.filterwarnings("ignore:Duplicate name:UserWarning")
 @pytest.mark.parametrize("store", ["local", "memory", "zip"], indirect=["store"])
 async def test_delete_empty_shards(store: Store) -> None:
     if not store.supports_deletes:

diff --git a/tests/test_store/test_zip.py b/tests/test_store/test_zip.py
@@ -84,6 +84,114 @@ def test_store_supports_writes(self, store: ZipStore) -> None:
     def test_store_supports_listing(self, store: ZipStore) -> None:
         assert store.supports_listing
 
+    def test_store_supports_deletes(self, store: ZipStore) -> None:
+        assert store.supports_deletes
+
+    async def test_delete_compacts_duplicates(self, store: ZipStore) -> None:
+        # Overwriting a key leaves a duplicate member in the archive; deleting
+        # another key rewrites the archive and should compact the duplicates so
+        # the surviving key has a single, most-recent entry (issue #828).
+        await store.set("foo", cpu.Buffer.from_bytes(b"v1"))
+        with pytest.warns(UserWarning, match="Duplicate name: 'foo'"):
+            await store.set("foo", cpu.Buffer.from_bytes(b"v2"))
+        await store.set("bar", cpu.Buffer.from_bytes(b"bar"))
+
+        await store.delete("bar")
+
+        assert not await store.exists("bar")
+        assert store._zf.namelist().count("foo") == 1
+        buf = await self.get(store, "foo")
+        assert buf.to_bytes() == b"v2"
+
+    async def test_delete_then_set(self, store: ZipStore) -> None:
+        # after a delete (which reopens the archive) writes must still work
+        await store.set("foo", cpu.Buffer.from_bytes(b"foo"))
+        await store.delete("foo")
+        assert not await store.exists("foo")
+        await store.set("baz", cpu.Buffer.from_bytes(b"baz"))
+        buf = await self.get(store, "baz")
+        assert buf.to_bytes() == b"baz"
+
+    async def test_delete_and_delete_dir_auto_open(self, tmp_path: Path) -> None:
+        # delete() and delete_dir() should auto-open the archive like _get/_set,
+        # rather than assuming the caller opened it first.
+        store = ZipStore(tmp_path / "del.zip", mode="w", read_only=False)
+        assert not store._is_open
+        await store.delete("missing")  # exercises the auto-open branch in delete()
+        assert store._is_open
+
+        store2 = ZipStore(tmp_path / "deldir.zip", mode="w", read_only=False)
+        assert not store2._is_open
+        await store2.delete_dir("missing")  # auto-open branch in delete_dir()
+        assert store2._is_open
+
+    async def test_delete_dir_prefix_already_normalized(self, store: ZipStore) -> None:
+        # a prefix that already ends in "/" must skip the slash-appending branch
+        await store.set("foo/zarr.json", cpu.Buffer.from_bytes(b"a"))
+        await store.set("foo/c/0", cpu.Buffer.from_bytes(b"b"))
+        await store.set("bar/zarr.json", cpu.Buffer.from_bytes(b"c"))
+
+        await store.delete_dir("foo/")
+
+        assert not await store.exists("foo/zarr.json")
+        assert not await store.exists("foo/c/0")
+        assert await store.exists("bar/zarr.json")
+
+    async def test_delete_dir_empty_prefix_removes_all(self, store: ZipStore) -> None:
+        # an empty prefix also skips normalization and should remove everything
+        await store.set("a", cpu.Buffer.from_bytes(b"a"))
+        await store.set("b/c", cpu.Buffer.from_bytes(b"b"))
+
+        await store.delete_dir("")
+
+        assert not await store.exists("a")
+        assert not await store.exists("b/c")
+        assert store._zf.namelist() == []
+
+    async def test_delete_cleans_up_temp_on_failure(
+        self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # if the rewrite fails (e.g. os.replace raises), the temporary archive
+        # must be removed and the original left untouched.
+        import zarr.storage._zip as zip_module
+
+        store = ZipStore(tmp_path / "fail.zip", mode="w", read_only=False)
+        await store.set("foo", cpu.Buffer.from_bytes(b"v"))
+
+        def boom(*args: Any, **kwargs: Any) -> None:
+            raise OSError("replace failed")
+
+        monkeypatch.setattr(zip_module.os, "replace", boom)
+
+        with pytest.raises(OSError, match="replace failed"):
+            await store.delete("foo")
+
+        # no leftover temp file: only the original archive remains
+        assert set(os.listdir(tmp_path)) == {"fail.zip"}
+
+    async def test_delete_failure_when_temp_already_removed(
+        self, tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        # defensive cleanup branch: if the temp archive is already gone when the
+        # rewrite fails, cleanup is skipped and the original error still propagates.
+        import zarr.storage._zip as zip_module
+
+        store = ZipStore(tmp_path / "fail2.zip", mode="w", read_only=False)
+        await store.set("foo", cpu.Buffer.from_bytes(b"v"))
+
+        real_remove = zip_module.os.remove
+
+        def replace_then_vanish(src: str, dst: str) -> None:
+            real_remove(src)  # temp disappears before the except block runs
+            raise OSError("replace failed")
+
+        monkeypatch.setattr(zip_module.os, "replace", replace_then_vanish)
+
+        with pytest.raises(OSError, match="replace failed"):
+            await store.delete("foo")
+
+        assert set(os.listdir(tmp_path)) == {"fail2.zip"}
+
     # TODO: fix this warning
     @pytest.mark.filterwarnings("ignore:Unclosed client session:ResourceWarning")
     def test_api_integration(self, store: ZipStore) -> None:
@@ -101,17 +209,19 @@ def test_api_integration(self, store: ZipStore) -> None:
         with pytest.warns(UserWarning, match="Duplicate name: 'foo/c/0/0'"):
             z[0, 0] = 100
 
-        # TODO: assigning an entire chunk to fill value ends up deleting the chunk which is not supported
-        # a work around will be needed here.
-        with pytest.raises(NotImplementedError):
-            z[0:10, 0:10] = 99
+        # assigning an entire chunk to the fill value deletes the chunk;
+        # ZipStore now supports deletes by rewriting the archive (issue #828)
+        z[0:10, 0:10] = 99
+        expected = data.copy()
+        expected[0:10, 0:10] = 99
+        assert np.array_equal(expected, z[:])
 
         bar = root.create_group("bar", attributes={"hello": "world"})
         assert "hello" in dict(bar.attrs)
 
-        # keys cannot be deleted
-        with pytest.raises(NotImplementedError):
-            del root["bar"]
+        # keys can now be deleted
+        del root["bar"]
+        assert "bar" not in root
 
         store.close()
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		`ZipStore` now supports deletes (`supports_deletes` is `True`). Because ZIP files cannot delete entries in place, `ZipStore.delete` and `ZipStore.delete_dir` remove keys by rewriting the archive without the deleted members; overwritten (duplicate) members are compacted as a side effect.