Added Store.getsize

Closes #2420

Author: TomAugspurger

src/zarr/abc/store.py

@@ -5,6 +5,8 @@
 from itertools import starmap
 from typing import TYPE_CHECKING, NamedTuple, Protocol, runtime_checkable
 
+from zarr.core.buffer.core import default_buffer_prototype
+
 if TYPE_CHECKING:
     from collections.abc import AsyncGenerator, Iterable
     from types import TracebackType
@@ -386,6 +388,32 @@ async def _get_many(
         for req in requests:
             yield (req[0], await self.get(*req))
 
+    async def getsize(self, key: str) -> int:
+        """
+        Return the size, in bytes, of a value in a Store.
+
+        Parameters
+        ----------
+        key : str
+
+        Returns
+        -------
+        nbytes: int
+            The size of the value in bytes.
+
+        Raises
+        ------
+        FileNotFoundError
+            When the given key does not exist in the store.
+        """
+        # Note to implementers: this default implementation is very inefficient since
+        # it requires reading the entire object. Many systems will have ways to get the
+        # size of an object without reading it.
+        value = await self.get(key, prototype=default_buffer_prototype())
+        if value is None:
+            raise FileNotFoundError(key)
+        return len(value)
+
 
 @runtime_checkable
 class ByteGetter(Protocol):

src/zarr/storage/local.py

@@ -242,3 +242,6 @@ async def list_dir(self, prefix: str) -> AsyncGenerator[str, None]:
                 yield str(key).replace(to_strip, "")
         except (FileNotFoundError, NotADirectoryError):
             pass
+
+    async def getsize(self, key: str) -> int:
+        return os.path.getsize(self.root / key)

src/zarr/storage/remote.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Any, Self
+from typing import TYPE_CHECKING, Any, Self, cast
 
 import fsspec
 
@@ -301,3 +301,16 @@ async def list_prefix(self, prefix: str) -> AsyncGenerator[str, None]:
         find_str = f"{self.path}/{prefix}"
         for onefile in await self.fs._find(find_str, detail=False, maxdepth=None, withdirs=False):
             yield onefile.removeprefix(find_str)
+
+    async def getsize(self, key: str) -> int:
+        path = _dereference_path(self.path, key)
+        info = await self.fs._info(path)
+
+        size = info.get("size")
+
+        if size is None:
+            # Not all filesystems support size. Fall back to reading the entire object
+            return await super().getsize(key)
+        else:
+            # fsspec doesn't have typing. We'll need to assume this is correct.
+            return cast(int, size)

src/zarr/testing/store.py

@@ -338,3 +338,15 @@ async def test_set_if_not_exists(self, store: S) -> None:
 
         result = await store.get("k2", default_buffer_prototype())
         assert result == new
+
+    async def test_getsize(self, store: S) -> None:
+        key = "k"
+        data = self.buffer_cls.from_bytes(b"0" * 10)
+        await self.set(store, key, data)
+
+        result = await store.getsize(key)
+        assert result == 10
+
+    async def test_getsize_raises(self, store: S) -> None:
+        with pytest.raises(FileNotFoundError):
+            await store.getsize("not-a-real-key")