Add complex dtype support to FillValueCoder for Zarr backend

xarray/backends/zarr.py

@@ -122,7 +122,9 @@ class FillValueCoder:
     """
 
     @classmethod
-    def encode(cls, value: int | float | str | bytes, dtype: np.dtype[Any]) -> Any:
+    def encode(
+        cls, value: int | float | complex | str | bytes, dtype: np.dtype[Any]
+    ) -> Any:
         if dtype.kind in "S":
             # byte string, this implies that 'value' must also be `bytes` dtype.
             assert isinstance(value, bytes)
@@ -135,13 +137,28 @@ def encode(cls, value: int | float | str | bytes, dtype: np.dtype[Any]) -> Any:
             return int(value)
         elif dtype.kind in "f":
             return base64.standard_b64encode(struct.pack("<d", float(value))).decode()
+        elif dtype.kind in "c":
+            # complex - encode each component as base64, matching float encoding
+            assert isinstance(value, complex) or np.issubdtype(
+                type(value), np.complexfloating
+            )
+            return [
+                base64.standard_b64encode(
+                    struct.pack("<d", float(value.real))
+                ).decode(),
+                base64.standard_b64encode(
+                    struct.pack("<d", float(value.imag))
+                ).decode(),
+            ]
         elif dtype.kind in "U":
             return str(value)
         else:
             raise ValueError(f"Failed to encode fill_value. Unsupported dtype {dtype}")
 
     @classmethod
-    def decode(cls, value: int | float | str | bytes, dtype: str | np.dtype[Any]):
+    def decode(
+        cls, value: int | float | str | bytes | list, dtype: str | np.dtype[Any]
+    ):
         if dtype == "string":
             # zarr V3 string type
             return str(value)
@@ -153,6 +170,12 @@ def decode(cls, value: int | float | str | bytes, dtype: str | np.dtype[Any]):
         if np_dtype.kind in "f":
             assert isinstance(value, str | bytes)
             return struct.unpack("<d", base64.standard_b64decode(value))[0]
+        elif np_dtype.kind in "c":
+            # complex - decode each component from base64, matching float decoding
+            assert isinstance(value, list | tuple) and len(value) == 2
+            real = struct.unpack("<d", base64.standard_b64decode(value[0]))[0]
+            imag = struct.unpack("<d", base64.standard_b64decode(value[1]))[0]
+            return complex(real, imag)
         elif np_dtype.kind in "b":
             return bool(value)
         elif np_dtype.kind in "iu":

xarray/tests/test_backends.py

@@ -7099,6 +7099,18 @@ def test_encode_zarr_attr_value() -> None:
     assert actual3 == expected3
 
 
+@requires_zarr
+@pytest.mark.parametrize("dtype", [complex, np.complex64, np.complex128])
+def test_fill_value_coder_complex(dtype) -> None:
+    """Test that FillValueCoder round-trips complex fill values."""
+    from xarray.backends.zarr import FillValueCoder
+
+    for value in [dtype(1 + 2j), dtype(-3.5 + 4.5j), dtype(complex("nan+nanj"))]:
+        encoded = FillValueCoder.encode(value, np.dtype(dtype))
+        decoded = FillValueCoder.decode(encoded, np.dtype(dtype))
+        np.testing.assert_equal(np.array(decoded, dtype=dtype), np.array(value))
+
+
 @requires_zarr
 def test_extract_zarr_variable_encoding() -> None:
     var = xr.Variable("x", [1, 2])