Don't overwrite codec config values when set.

A codec doesn't know where it is in the codec pipeline, so it
cannot know whether `array_spec.dtype` should match `codec.dtype`
or not.

Author: dcherian

numcodecs/zarr3.py

@@ -271,7 +271,7 @@ def __init__(self, **codec_config: JSON) -> None:
         super().__init__(**codec_config)
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Shuffle:
-        if array_spec.dtype.itemsize != self.codec_config.get("elementsize"):
+        if self.codec_config.get("elementsize", None) is None:
             return Shuffle(**{**self.codec_config, "elementsize": array_spec.dtype.itemsize})
         return self  # pragma: no cover
 
@@ -308,7 +308,7 @@ def resolve_metadata(self, chunk_spec: ArraySpec) -> ArraySpec:
         return chunk_spec
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> FixedScaleOffset:
-        if str(array_spec.dtype) != self.codec_config.get("dtype"):
+        if self.codec_config.get("dtype") is None:
             return FixedScaleOffset(**{**self.codec_config, "dtype": str(array_spec.dtype)})
         return self
 
@@ -321,7 +321,7 @@ def __init__(self, **codec_config: JSON) -> None:
         super().__init__(**codec_config)
 
     def evolve_from_array_spec(self, array_spec: ArraySpec) -> Quantize:
-        if str(array_spec.dtype) != self.codec_config.get("dtype"):
+        if self.codec_config.get("dtype") is None:
             return Quantize(**{**self.codec_config, "dtype": str(array_spec.dtype)})
         return self