soften error to warning, keep API compatibility

Author: alimanfoo

zarr/storage.py

@@ -343,7 +343,8 @@ def _init_array_metadata(store, shape, chunks=None, dtype=None, compressor='defa
     # deal with object encoding
     if dtype == object:
         if object_codec is None:
-            raise ValueError('an object_codec is required for object arrays')
+            warnings.warn('missing object_codec for object array; this will raise a '
+                          'ValueError in version 3.0', FutureWarning)
         else:
             filters_config.insert(0, object_codec.get_config())
     elif object_codec is not None:

zarr/tests/test_core.py

@@ -6,6 +6,7 @@
 import shutil
 import pickle
 import os
+import warnings
 
 
 import numpy as np
@@ -26,6 +27,12 @@
                        Categorize, JSON)
 
 
+# needed for PY2/PY3 consistent behaviour
+if PY2:  # pragma: py3 no cover
+    warnings.resetwarnings()
+    warnings.simplefilter('always')
+
+
 # noinspection PyMethodMayBeStatic
 class TestArray(unittest.TestCase):
 
@@ -869,7 +876,7 @@ def test_dtypes(self):
     def test_object_arrays(self):
 
         # an object_codec is required for object arrays
-        with assert_raises(ValueError):
+        with pytest.warns(FutureWarning):
             self.create_array(shape=10, chunks=3, dtype=object)
 
         # create an object array using msgpack