Lazily create .zattrs

docs/release.rst

@@ -95,6 +95,13 @@ Enhancements
 * **Arrays with one or more zero-length dimensions** are now fully supported; by
   :user:`Prakhar Goel <newt0311>`, :issue:`150`, :issue:`154`, :issue:`160`.
 
+* **The .zattrs key is now optional** and will now only be created when the first
+  custom attribute is set; :issue:`121`, :issue:`200`.
+
+* **New Group.move() method** supports moving a sub-group or array to a different
+  location within the same hierarchy. By :user:`John Kirkham <jakirkham>`,
+  :issue:`191`, :issue:`193`, :issue:`196`.
+
 Bug fixes
 ~~~~~~~~~
 
@@ -120,13 +127,16 @@ Documentation
 ~~~~~~~~~~~~~
 
 * Some changes have been made to the :ref:`spec_v2` document to clarify
-  ambiguities and add some missing information. These changes do not modify any
-  of the material previously implemented, and so the changes have been made
+  ambiguities and add some missing information. These changes do not break compatibility
+  with any of the material as previously implemented, and so the changes have been made
   in-place in the document without incrementing the document version number. The
   specification now describes how bytes fill values should be encoded and
   decoded for arrays with a fixed-length byte string data type (:issue:`165`,
-  :issue:`176`). The specification now also clarifies that datetime64 and
-  timedelta64 data types are not supported in this version (:issue:`85`).
+  :issue:`176`). The specification now clarifies that datetime64 and
+  timedelta64 data types are not supported in this version (:issue:`85`). The
+  specification now clarifies that the '.zattrs' key does not have to be present for
+  either arrays or groups, and if absent then custom attributes should be treated as
+  empty.
 * A new :ref:`tutorial_indexing` section has been added to the tutorial.
 * A new :ref:`tutorial_strings` section has been added to the tutorial
   (:issue:`135`, :issue:`175`).

docs/spec/v2.rst

@@ -275,7 +275,8 @@ Attributes
 An array or group can be associated with custom attributes, which are simple
 key/value items with application-specific meaning. Custom attributes are
 encoded as a JSON object and stored under the ".zattrs" key within an array
-store.
+store. The ".zattrs" key does not have to be present, and if it is absent the
+attributes should be treated as empty.
 
 For example, the JSON object below encodes three attributes named
 "foo", "bar" and "baz"::
@@ -308,7 +309,7 @@ have been set in the store::
 
     >>> import os
     >>> sorted(os.listdir('data/example.zarr'))
-    ['.zarray', '.zattrs']
+    ['.zarray']
 
 Inspect the array metadata::
 
@@ -333,23 +334,18 @@ Inspect the array metadata::
         "zarr_format": 2
     }
 
-Inspect the array attributes::
-
-    >>> print(open('data/example.zarr/.zattrs').read())
-    {}
-
 Chunks are initialized on demand. E.g., set some data::
 
     >>> a[0:10, 0:10] = 1
     >>> sorted(os.listdir('data/example.zarr'))
-    ['.zarray', '.zattrs', '0.0']
+    ['.zarray', '0.0']
 
 Set some more data::
 
     >>> a[0:10, 10:20] = 2
     >>> a[10:20, :] = 3
     >>> sorted(os.listdir('data/example.zarr'))
-    ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
+    ['.zarray', '0.0', '0.1', '1.0', '1.1']
 
 Manually decompress a single chunk for illustration::
 
@@ -369,6 +365,8 @@ Modify the array attributes::
     >>> a.attrs['foo'] = 42
     >>> a.attrs['bar'] = 'apples'
     >>> a.attrs['baz'] = [1, 2, 3, 4]
+    >>> sorted(os.listdir('data/example.zarr'))
+    ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
     >>> print(open('data/example.zarr/.zattrs').read())
     {
         "bar": "apples",
@@ -398,12 +396,11 @@ Create the root group::
 
     >>> root_grp = zarr.group(store, overwrite=True)
 
-The metadata resource for the root group has been created, as well as a custom
-attributes resource::
+The metadata resource for the root group has been created::
 
     >>> import os
     >>> sorted(os.listdir('data/group.zarr'))
-    ['.zattrs', '.zgroup']
+    ['.zgroup']
 
 Inspect the group metadata::
 
@@ -412,33 +409,32 @@ Inspect the group metadata::
         "zarr_format": 2
     }
 
-Inspect the group attributes::
-
-    >>> print(open('data/group.zarr/.zattrs').read())
-    {}
-
 Create a sub-group::
 
     >>> sub_grp = root_grp.create_group('foo')
 
 What has been stored::
 
     >>> sorted(os.listdir('data/group.zarr'))
-    ['.zattrs', '.zgroup', 'foo']
+    ['.zgroup', 'foo']
     >>> sorted(os.listdir('data/group.zarr/foo'))
-    ['.zattrs', '.zgroup']
+    ['.zgroup']
 
 Create an array within the sub-group::
 
     >>> a = sub_grp.create_dataset('bar', shape=(20, 20), chunks=(10, 10))
     >>> a[:] = 42
 
+Set a custom attributes::
+
+    >>> a.attrs['comment'] = 'answer to life, the universe and everything'
+
 What has been stored::
 
     >>> sorted(os.listdir('data/group.zarr'))
-    ['.zattrs', '.zgroup', 'foo']
+    ['.zgroup', 'foo']
     >>> sorted(os.listdir('data/group.zarr/foo'))
-    ['.zattrs', '.zgroup', 'bar']
+    ['.zgroup', 'bar']
     >>> sorted(os.listdir('data/group.zarr/foo/bar'))
     ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
 
@@ -449,6 +445,7 @@ Here is the same example using a Zip file as storage::
     >>> sub_grp = root_grp.create_group('foo')
     >>> a = sub_grp.create_dataset('bar', shape=(20, 20), chunks=(10, 10))
     >>> a[:] = 42
+    >>> a.attrs['comment'] = 'answer to life, the universe and everything'
     >>> store.close()
 
 What has been stored::
@@ -457,9 +454,7 @@ What has been stored::
     >>> zf = zipfile.ZipFile('data/group.zip', mode='r')
     >>> for name in sorted(zf.namelist()):
     ...     print(name)
-    .zattrs
     .zgroup
-    foo/.zattrs
     foo/.zgroup
     foo/bar/.zarray
     foo/bar/.zattrs
@@ -471,9 +466,29 @@ What has been stored::
 Changes
 -------
 
+Version 2 clarifications
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following changes have been made to the version 2 specification since it was
+initially published to clarify ambiguities and add some missing information.
+
+* The specification now describes how bytes fill values should be encoded and
+  decoded for arrays with a fixed-length byte string data type (:issue:`165`,
+  :issue:`176`).
+
+* The specification now clarifies that datetime64 and timedelta64 data types are not
+  supported in this version (:issue:`85`).
+
+* The specification now clarifies that the '.zattrs' key does not have to be present for
+  either arrays or groups, and if absent then custom attributes should be treated as
+  empty.
+
+
 Changes in version 2
 ~~~~~~~~~~~~~~~~~~~~
 
+The following changes were made between version 1 and version 2 of this specification:
+
 * Added support for storing multiple arrays in the same store and organising
   arrays into hierarchies using groups.
 * Array metadata is now stored under the ".zarray" key instead of the "meta"

docs/tutorial.rst

@@ -178,7 +178,7 @@ print some diagnostics, e.g.::
                        : blocksize=0)
     Store type         : builtins.dict
     No. bytes          : 400000000 (381.5M)
-    No. bytes stored   : 4565055 (4.4M)
+    No. bytes stored   : 4565053 (4.4M)
     Storage ratio      : 87.6
     Chunks initialized : 100/100
 
@@ -270,7 +270,7 @@ Here is an example using a delta filter with the Blosc compressor::
     Compressor         : Blosc(cname='zstd', clevel=1, shuffle=SHUFFLE, blocksize=0)
     Store type         : builtins.dict
     No. bytes          : 400000000 (381.5M)
-    No. bytes stored   : 648607 (633.4K)
+    No. bytes stored   : 648605 (633.4K)
     Storage ratio      : 616.7
     Chunks initialized : 100/100
 
@@ -394,7 +394,7 @@ property. E.g.::
     Compressor         : Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0)
     Store type         : zarr.storage.DictStore
     No. bytes          : 8000000 (7.6M)
-    No. bytes stored   : 37482 (36.6K)
+    No. bytes stored   : 37480 (36.6K)
     Storage ratio      : 213.4
     Chunks initialized : 10/10
 
@@ -409,7 +409,7 @@ property. E.g.::
     Compressor         : Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0)
     Store type         : zarr.storage.DictStore
     No. bytes          : 4000000 (3.8M)
-    No. bytes stored   : 23245 (22.7K)
+    No. bytes stored   : 23243 (22.7K)
     Storage ratio      : 172.1
     Chunks initialized : 100/100
 
@@ -898,7 +898,7 @@ ratios, depending on the correlation structure within the data. E.g.::
     Compressor         : Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0)
     Store type         : builtins.dict
     No. bytes          : 400000000 (381.5M)
-    No. bytes stored   : 26805737 (25.6M)
+    No. bytes stored   : 26805735 (25.6M)
     Storage ratio      : 14.9
     Chunks initialized : 100/100
     >>> f = zarr.array(a, chunks=(1000, 1000), order='F')
@@ -912,7 +912,7 @@ ratios, depending on the correlation structure within the data. E.g.::
     Compressor         : Blosc(cname='lz4', clevel=5, shuffle=SHUFFLE, blocksize=0)
     Store type         : builtins.dict
     No. bytes          : 400000000 (381.5M)
-    No. bytes stored   : 9633603 (9.2M)
+    No. bytes stored   : 9633601 (9.2M)
     Storage ratio      : 41.5
     Chunks initialized : 100/100
 
@@ -1099,3 +1099,7 @@ behaviour, set the value of the ``blosc.use_threads`` variable to
 ``True`` (Blosc always uses multiple internal threads) or ``False``
 (Blosc always runs in single-threaded contextual mode). To re-enable
 automatic switching, set ``blosc.use_threads`` to ``None``.
+
+Please note that if Zarr is being used within a multi-process program, Blosc may not
+be safe to use in multi-threaded mode and may cause the program to hang. If using Blosc
+in a multi-process program then it is recommended to set ``blosc.use_threads = False``.

zarr/storage.py

@@ -10,7 +10,6 @@
 from collections import MutableMapping
 import os
 import tempfile
-import json
 import zipfile
 import shutil
 import atexit
@@ -212,7 +211,7 @@ def init_array(store, shape, chunks=True, dtype=None, compressor='default',
         >>> store = dict()
         >>> init_array(store, shape=(10000, 10000), chunks=(1000, 1000))
         >>> sorted(store.keys())
-        ['.zarray', '.zattrs']
+        ['.zarray']
 
     Array metadata is stored as JSON::
 
@@ -240,17 +239,12 @@ def init_array(store, shape, chunks=True, dtype=None, compressor='default',
             "zarr_format": 2
         }
 
-    User-defined attributes are also stored as JSON, initially empty::
-
-        >>> print(store['.zattrs'].decode())
-        {}
-
     Initialize an array using a storage path::
 
         >>> store = dict()
         >>> init_array(store, shape=100000000, chunks=1000000, dtype='i1', path='foo')
         >>> sorted(store.keys())
-        ['.zattrs', '.zgroup', 'foo/.zarray', 'foo/.zattrs']
+        ['.zgroup', 'foo/.zarray']
         >>> print(store['foo/.zarray'].decode())
         {
             "chunks": [
@@ -276,8 +270,7 @@ def init_array(store, shape, chunks=True, dtype=None, compressor='default',
     Notes
     -----
     The initialisation process involves normalising all array metadata, encoding
-    as JSON and storing under the '.zarray' key. User attributes are also
-    initialized and stored as JSON under the '.zattrs' key.
+    as JSON and storing under the '.zarray' key.
 
     """
 
@@ -349,10 +342,6 @@ def _init_array_metadata(store, shape, chunks=None, dtype=None, compressor='defa
     key = _path_to_prefix(path) + array_meta_key
     store[key] = encode_array_metadata(meta)
 
-    # initialize attributes
-    key = _path_to_prefix(path) + attrs_key
-    store[key] = json.dumps(dict()).encode('ascii')
-
 
 # backwards compatibility
 init_store = init_array
@@ -408,10 +397,6 @@ def _init_group_metadata(store, overwrite=False, path=None, chunk_store=None):
     key = _path_to_prefix(path) + group_meta_key
     store[key] = encode_group_metadata(meta)
 
-    # initialize attributes
-    key = _path_to_prefix(path) + attrs_key
-    store[key] = json.dumps(dict()).encode('ascii')
-
 
 def ensure_bytes(s):
     if isinstance(s, binary_type):
@@ -654,7 +639,7 @@ class DirectoryStore(MutableMapping):
 
         >>> import os
         >>> sorted(os.listdir('data/array.zarr'))
-        ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
+        ['.zarray', '0.0', '0.1', '1.0', '1.1']
 
     Store a group::
 
@@ -668,11 +653,11 @@ class DirectoryStore(MutableMapping):
     directories on the file system, i.e.::
 
         >>> sorted(os.listdir('data/group.zarr'))
-        ['.zattrs', '.zgroup', 'foo']
+        ['.zgroup', 'foo']
         >>> sorted(os.listdir('data/group.zarr/foo'))
-        ['.zattrs', '.zgroup', 'bar']
+        ['.zgroup', 'bar']
         >>> sorted(os.listdir('data/group.zarr/foo/bar'))
-        ['.zarray', '.zattrs', '0.0', '0.1', '1.0', '1.1']
+        ['.zarray', '0.0', '0.1', '1.0', '1.1']
 
     Notes
     -----
@@ -909,7 +894,7 @@ class NestedDirectoryStore(DirectoryStore):
 
         >>> import os
         >>> sorted(os.listdir('data/array.zarr'))
-        ['.zarray', '.zattrs', '0', '1']
+        ['.zarray', '0', '1']
         >>> sorted(os.listdir('data/array.zarr/0'))
         ['0', '1']
         >>> sorted(os.listdir('data/array.zarr/1'))
@@ -927,11 +912,11 @@ class NestedDirectoryStore(DirectoryStore):
     directories on the file system, i.e.::
 
         >>> sorted(os.listdir('data/group.zarr'))
-        ['.zattrs', '.zgroup', 'foo']
+        ['.zgroup', 'foo']
         >>> sorted(os.listdir('data/group.zarr/foo'))
-        ['.zattrs', '.zgroup', 'bar']
+        ['.zgroup', 'bar']
         >>> sorted(os.listdir('data/group.zarr/foo/bar'))
-        ['.zarray', '.zattrs', '0', '1']
+        ['.zarray', '0', '1']
         >>> sorted(os.listdir('data/group.zarr/foo/bar/0'))
         ['0', '1']
         >>> sorted(os.listdir('data/group.zarr/foo/bar/1'))
@@ -1487,7 +1472,9 @@ def _lmdb_decode_key_bytes(key):
 
 
 class LMDBStore(MutableMapping):
-    """Storage class using LMDB.
+    """Storage class using LMDB. Requires the `lmdb <http://lmdb.readthedocs.io/>`_
+    package to be installed.
+
 
     Parameters
     ----------
@@ -1499,10 +1486,6 @@ class LMDBStore(MutableMapping):
     **kwargs
         Keyword arguments passed through to the `lmdb.open` function.
 
-    Notes
-    -----
-    Requires the `lmdb <http://lmdb.readthedocs.io/>`_ package to be installed.
-
     Examples
     --------
     Store a single array::