Fix reading dtype from zarr dataset (#72)

* Fix reading dtype from zarr dataset
---------

Co-authored-by: Oliver Ruebel <oruebel@users.noreply.github.com>

src/hdmf_zarr/backend.py

@@ -670,6 +670,7 @@ def __setup_chunked_dataset__(cls, parent, name, data, options=None):
                 io_settings['dtype'] = cls.__dtypes.get(io_settings['dtype'])
         try:
             dset = parent.create_dataset(name, **io_settings)
+            dset.attrs['zarr_dtype'] = np.dtype(io_settings['dtype']).str
         except Exception as exc:
             raise Exception("Could not create dataset %s in %s" % (name, parent.name)) from exc
         return dset
@@ -1099,11 +1100,18 @@ def __read_dataset(self, zarr_obj, name):
         if ret is not None:
             return ret
 
-        if 'zarr_dtype' not in zarr_obj.attrs:
+        if 'zarr_dtype' in zarr_obj.attrs:
+            zarr_dtype = zarr_obj.attrs['zarr_dtype']
+        elif hasattr(zarr_obj, 'dtype'):   # Fallback for invalid files that are mssing zarr_type
+            zarr_dtype = zarr_obj.dtype
+            warnings.warn(
+                "Inferred dtype from zarr type. Dataset missing zarr_dtype: " + str(name) + "   " + str(zarr_obj)
+            )
+        else:
             raise ValueError("Dataset missing zarr_dtype: " + str(name) + "   " + str(zarr_obj))
 
         kwargs = {"attributes": self.__read_attrs(zarr_obj),
-                  "dtype": zarr_obj.attrs['zarr_dtype'],
+                  "dtype": zarr_dtype,
                   "maxshape": zarr_obj.shape,
                   "chunks": not (zarr_obj.shape == zarr_obj.chunks),
                   "source": self.source}