Fix: N5 keywords now emit UserWarning instead of raising a ValueError (#860)

* Changed n5 keywords to print a warning

Changed N5 and N5FS
Added test fixture to capture stdout
Updated test making sure a warning is printed

* flake8

* Change to UserWarning

Author: boazmohar

zarr/n5.py

@@ -128,7 +128,7 @@ def __setitem__(self, key, value):
 
             for k in n5_keywords:
                 if k in zarr_attrs.keys():
-                    raise ValueError("Can not set attribute %s, this is a reserved N5 keyword" % k)
+                    warnings.warn("attribute %s is a reserved N5 keyword" % k, UserWarning)
 
             # replace previous user attributes
             for k in list(n5_attrs.keys()):
@@ -424,9 +424,7 @@ def __setitem__(self, key, value):
 
             for k in n5_keywords:
                 if k in zarr_attrs.keys():
-                    raise ValueError(
-                        "Can not set attribute %s, this is a reserved N5 keyword" % k
-                    )
+                    warnings.warn("attribute %s is a reserved N5 keyword" % k, UserWarning)
 
             # replace previous user attributes
             for k in list(n5_attrs.keys()):

zarr/tests/test_core.py

@@ -1951,7 +1951,7 @@ def test_structured_array_contain_object(self):
     def test_attrs_n5_keywords(self):
         z = self.create_array(shape=(1050,), chunks=100, dtype='i4')
         for k in n5_keywords:
-            with pytest.raises(ValueError):
+            with pytest.warns(UserWarning):
                 z.attrs[k] = ""
 
     def test_compressors(self):