safer N5 dataset metadata handling

pyn5/attributes.py

@@ -3,6 +3,8 @@
 import json
 from contextlib import contextmanager
 from copy import deepcopy
+from functools import wraps
+
 from pathlib import Path
 
 from typing import Iterator, Any, Dict
@@ -22,6 +24,16 @@ def default(self, obj):
         return super().default(obj)
 
 
+def restrict_metadata(fn):
+    """Decorator for AttributeManager methods which prevents mutation of N5 metadata"""
+    @wraps(fn)
+    def wrapped(obj: AttributeManager, key, *args, **kwargs):
+        if obj._is_dataset() and key in obj._dataset_keys:
+            raise RuntimeError(f"N5 metadata (key '{key}') cannot be mutated")
+        return fn(obj, key, *args, **kwargs)
+    return mutation(wrapped)
+
+
 class AttributeManager(AttributeManagerBase):
     """Object which reads and writes group attributes as JSON.
 
@@ -44,6 +56,7 @@ def __init__(self, dpath: Path, mode=Mode.default()):
         """
         self._path = Path(dpath) / "attributes.json"
         self._dump_kwargs = deepcopy(self._dump_kwargs)
+        self._has_dataset_keys_ = None
         super().__init__(mode)
 
     @classmethod
@@ -56,12 +69,12 @@ def from_parent(cls, parent: H5ObjectLike) -> AttributeManager:
         """
         return cls(parent._path, parent.mode)
 
-    @mutation
+    @restrict_metadata
     def __setitem__(self, k, v) -> None:
         with self._open_attributes(True) as attrs:
             attrs[k] = v
 
-    @mutation
+    @restrict_metadata
     def __delitem__(self, v) -> None:
         with self._open_attributes(True) as attrs:
             del attrs[v]
@@ -95,21 +108,40 @@ def __contains__(self, item):
         with self._open_attributes() as attrs:
             return item in attrs
 
-    def _is_dataset(self):
-        with self._open_attributes() as attrs:
-            return self._dataset_keys.issubset(attrs)
+    def _is_dataset(self) -> bool:
+        if self._has_dataset_keys_ is None:
+            try:
+                with open(self._path, "r") as f:
+                    self._has_dataset_keys_ = self._dataset_keys.issubset(json.load(f))
+            except ValueError:
+                self._has_dataset_keys_ = False
+            except IOError as e:
+                if e.errno == errno.ENOENT:
+                    self._has_dataset_keys_ = False
+                else:
+                    raise
+        return self._has_dataset_keys_
 
     @contextmanager
     def _open_attributes(self, write: bool = False) -> Dict[str, Any]:
         """Return attributes as a context manager.
 
+        N5 metadata keys are stripped from the dict.
+
         :param write: Whether to write changes to the attributes dict.
-        :return: attributes as a dict (including N5 metadata)
+        :return: attributes as a dict
         """
         attributes = self._read_attributes()
+
+        if self._is_dataset():
+            hidden_attrs = {k: attributes.pop(k) for k in self._dataset_keys}
+        else:
+            hidden_attrs = dict()
+
         yield attributes
         if write:
-            self._write_attributes(attributes)
+            hidden_attrs.update(attributes)
+            self._write_attributes(hidden_attrs)
 
     def _read_attributes(self):
         """Return attributes or an empty dict if they do not exist"""

pyn5/dataset.py

@@ -45,10 +45,16 @@ def __init__(self, name: str, parent: "Group"):  # noqa would need circular impo
         self._path = self.parent._path / name
         self._attrs = AttributeManager.from_parent(self)
 
-        with self._attrs._open_attributes() as attrs:
+        attrs = self._attrs._read_attributes()
+
+        try:
             self._shape = tuple(attrs["dimensions"][::-1])
-            self._dtype = np.dtype(self.attrs["dataType"].lower())
-            self._chunks = tuple(self.attrs["blockSize"][::-1])
+            self._dtype = np.dtype(attrs["dataType"].lower())
+            self._chunks = tuple(attrs["blockSize"][::-1])
+        except KeyError:
+            raise ValueError(
+                f"Not a dataset (missing metadata key): " + str(self._path)
+            )
 
         self._impl = dataset_types[self.dtype](
             str(self.file._path),

tests/test_h5_like.py

@@ -1,3 +1,6 @@
+import json
+
+import pytest
 import shutil
 import tempfile
 from copy import deepcopy
@@ -23,7 +26,30 @@ class TestGroup(GroupTestBase):
 
 class TestDataset(DatasetTestBase):
     dataset_kwargs = ds_kwargs
-    pass
+
+    def test_has_metadata(self, file_):
+        ds = self.dataset(file_)
+        with open(ds.attrs._path) as f:
+            attrs = json.load(f)
+        for key in ds.attrs._dataset_keys:
+            assert key in attrs
+
+    def test_no_return_metadata(self, file_):
+        ds = self.dataset(file_)
+
+        for key in ds.attrs._dataset_keys:
+            assert key not in ds.attrs
+            assert key not in dict(ds.attrs)
+
+    def test_no_mutate_metadata(self, file_):
+        ds = self.dataset(file_)
+
+        for key in ds.attrs._dataset_keys:
+            with pytest.raises(RuntimeError):
+                ds.attrs[key] = "not a datatype"
+
+            with pytest.raises(RuntimeError):
+                del ds.attrs[key]
 
 
 class TestMode(ModeTestBase):