feat: Implement Array.__len__ (fixes zarr-developers#3740)

Add __len__ method to both AsyncArray and Array classes to restore numpy compatibility.

- AsyncArray.__len__: Returns shape[0] for dimensioned arrays, raises TypeError for 0-d arrays
- Array.__len__: Delegates to async_array.__len__()
- Matches numpy behavior exactly with error message 'len() of unsized object'
- Added comprehensive tests covering:
  - 1-D, 2-D, 3-D, 4-D arrays returning shape[0]
  - 0-dimensional arrays raising TypeError
  - Both synchronous and asynchronous versions

This restores the zarr v2 behavior that was removed in the v3 rewrite, essential
for ecosystem compatibility with code that uses hasattr(obj, '__len__') to
distinguish arrays from scalars.

Author: Cipher

src/zarr/core/array.py

@@ -1039,6 +1039,38 @@ def shape(self) -> tuple[int, ...]:
         """
         return self.metadata.shape
 
+    def __len__(self) -> int:
+        """Return the length of the first dimension.
+
+        Matches numpy behavior: returns shape[0] for dimensioned arrays,
+        raises TypeError for 0-dimensional arrays.
+
+        Returns
+        -------
+        int
+            The size of the first dimension.
+
+        Raises
+        ------
+        TypeError
+            If the array is 0-dimensional (empty shape).
+
+        Examples
+        --------
+        >>> import zarr
+        >>> a = zarr.zeros((5, 10))
+        >>> len(a)
+        5
+        >>> b = zarr.zeros(())
+        >>> len(b)  # doctest: +SKIP
+        Traceback (most recent call last):
+            ...
+        TypeError: len() of unsized object
+        """
+        if self.ndim == 0:
+            raise TypeError("len() of unsized object")
+        return self.shape[0]
+
     @property
     def chunks(self) -> tuple[int, ...]:
         """Returns the chunk shape of the Array.
@@ -2263,6 +2295,36 @@ def shape(self, value: tuple[int, ...]) -> None:
         """Sets the shape of the array by calling resize."""
         self.resize(value)
 
+    def __len__(self) -> int:
+        """Return the length of the first dimension.
+
+        Matches numpy behavior: returns shape[0] for dimensioned arrays,
+        raises TypeError for 0-dimensional arrays.
+
+        Returns
+        -------
+        int
+            The size of the first dimension.
+
+        Raises
+        ------
+        TypeError
+            If the array is 0-dimensional (empty shape).
+
+        Examples
+        --------
+        >>> import zarr
+        >>> a = zarr.zeros((5, 10))
+        >>> len(a)
+        5
+        >>> b = zarr.zeros(())
+        >>> len(b)  # doctest: +SKIP
+        Traceback (most recent call last):
+            ...
+        TypeError: len() of unsized object
+        """
+        return self.async_array.__len__()
+
     @property
     def chunks(self) -> tuple[int, ...]:
         """Returns a tuple of integers describing the length of each dimension of a chunk of the array.

tests/test_array.py

@@ -2299,3 +2299,31 @@ def test_with_config_polymorphism() -> None:
     arr_source_config_dict = arr.with_config(source_config_dict)
 
     assert arr_source_config.config == arr_source_config_dict.config
+
+
+@pytest.mark.parametrize("shape", [(10,), (5, 10), (3, 4, 5), (2, 3, 4, 5)])
+def test_array_len_dimensioned(shape: tuple[int, ...]) -> None:
+    """Test __len__ for dimensioned arrays returns shape[0]."""
+    arr = zarr.create_array({}, shape=shape, dtype="uint8")
+    assert len(arr) == shape[0]
+
+
+@pytest.mark.parametrize("shape", [(10,), (5, 10), (3, 4, 5)])
+async def test_array_len_dimensioned_async(shape: tuple[int, ...]) -> None:
+    """Test __len__ for async dimensioned arrays returns shape[0]."""
+    arr = await AsyncArray.create({}, shape=shape, dtype="uint8")
+    assert len(arr) == shape[0]
+
+
+def test_array_len_0d_raises() -> None:
+    """Test __len__ raises TypeError for 0-dimensional arrays."""
+    arr = zarr.create_array({}, shape=(), dtype="uint8")
+    with pytest.raises(TypeError, match="len\\(\\) of unsized object"):
+        len(arr)
+
+
+async def test_array_len_0d_raises_async() -> None:
+    """Test __len__ raises TypeError for async 0-dimensional arrays."""
+    arr = await AsyncArray.create({}, shape=(), dtype="uint8")
+    with pytest.raises(TypeError, match="len\\(\\) of unsized object"):
+        len(arr)