fix nc4 shuffle option roundtrip (#1607)

* fix nc4 shuffle option roundtrip

* fix merge conflict

Author: Joe Hamman

doc/whats-new.rst

@@ -196,6 +196,7 @@ Bug fixes
 - ``:py:meth:`~xarray.Dataset.__init__` raises a ``MergeError`` if an
   coordinate shares a name with a dimension but is comprised of arbitrary
   dimensions(:issue:`1120`).
+
 - :py:func:`~xarray.open_rasterio` method now skips rasterio.crs -attribute if
   it is none.
   By `Leevi Annala <https://github.com/leevei>`_.
@@ -212,6 +213,10 @@ Bug fixes
   the first argument was a numpy variable (:issue:`1588`).
   By `Guido Imperiale <https://github.com/crusaderky>`_.
 
+- Fix ``netCDF4`` backend to properly roundtrip the ``shuffle`` encoding option
+  (:issue:`1606`).
+  By `Joe Hamman <https://github.com/jhamman>`_.
+
 - Fix bug when using ``pytest`` class decorators to skiping certain unittests.
   The previous behavior unintentionally causing additional tests to be skipped
   (:issue:`1531`). By `Joe Hamman <https://github.com/jhamman>`_.

xarray/backends/netCDF4_.py

@@ -153,7 +153,7 @@ def _extract_nc4_variable_encoding(variable, raise_on_invalid=False,
 
     safe_to_drop = set(['source', 'original_shape'])
     valid_encodings = set(['zlib', 'complevel', 'fletcher32', 'contiguous',
-                           'chunksizes'])
+                           'chunksizes', 'shuffle'])
     if lsd_okay:
         valid_encodings.add('least_significant_digit')
 

xarray/tests/test_backends.py

@@ -729,6 +729,7 @@ def test_compression_encoding(self):
         data['var2'].encoding.update({'zlib': True,
                                       'chunksizes': (5, 5),
                                       'fletcher32': True,
+                                      'shuffle': True,
                                       'original_shape': data.var2.shape})
         with self.roundtrip(data) as actual:
             for k, v in iteritems(data['var2'].encoding):
@@ -1846,6 +1847,11 @@ def test_extract_nc4_variable_encoding(self):
         encoding = _extract_nc4_variable_encoding(var)
         self.assertEqual({}, encoding)
 
+        # regression test
+        var = xr.Variable(('x',), [1, 2, 3], {}, {'shuffle': True})
+        encoding = _extract_nc4_variable_encoding(var, raise_on_invalid=True)
+        self.assertEqual({'shuffle': True}, encoding)
+
     def test_extract_h5nc_encoding(self):
         # not supported with h5netcdf (yet)
         var = xr.Variable(('x',), [1, 2, 3], {},