Switch (some) coding/encoding in conventions.py to use xarray.coding. (#1803)

* Switch (some) coding/encoding in conventions.py to use xarray.coding.

The goal here is to eventually convert everything in xarray.conventions to
using the new coding module, which is more modular and supports dask arrays.
For now, I have switched over datetime, timedelta, unsigned integer, scaling
and mask coding to use new coders. Integrating these into xarray.conventions
lets us harness our existing test suite and delete a lot of redundant code.

Most of the code/tests is simply reorganized. There should be no changes to
public API (to keep this manageable for review). All of the original tests that
are still relevant should still be present, though I have reorganized many of
them into new locations to match the revised code.

* Fix zarr and cmds export

* add whats-new and small cleanup

* Move constant to top of module

* use _NS_PER_TIME_DELTA

Author: shoyer

doc/whats-new.rst

@@ -73,6 +73,8 @@ Bug fixes
   with size one in some dimension can now be plotted, which is good for
   exploring satellite imagery (:issue:`1780`).
   By `Zac Hatfield-Dodds <https://github.com/Zac-HD>`_.
+- Fixed ``UnboundLocalError`` when opening netCDF file `` (:issue:`1781`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 - The ``variables``, ``attrs``, and ``dimensions`` properties have been
   deprecated as part of a bug fix addressing an issue where backends were
   unintentionally loading the datastores data and attributes repeatedly during

xarray/backends/zarr.py

@@ -6,6 +6,7 @@
 
 import numpy as np
 
+from .. import coding
 from .. import Variable
 from ..core import indexing
 from ..core.utils import FrozenOrderedDict, HiddenKeyDict
@@ -259,13 +260,13 @@ def encode_zarr_variable(var, needs_copy=True, name=None):
         raise NotImplementedError("Variable `%s` is an object. Zarr "
                                   "store can't yet encode objects." % name)
 
-    var = conventions.maybe_encode_datetime(var, name=name)
-    var = conventions.maybe_encode_timedelta(var, name=name)
-    var, needs_copy = conventions.maybe_encode_offset_and_scale(var,
-                                                                needs_copy,
-                                                                name=name)
-    var, needs_copy = conventions.maybe_encode_fill_value(var, needs_copy,
-                                                          name=name)
+    for coder in [coding.times.CFDatetimeCoder(),
+                  coding.times.CFTimedeltaCoder(),
+                  coding.variables.CFScaleOffsetCoder(),
+                  coding.variables.CFMaskCoder(),
+                  coding.variables.UnsignedIntegerCoder()]:
+        var = coder.encode(var, name=name)
+
     var = conventions.maybe_encode_nonstring_dtype(var, name=name)
     var = conventions.maybe_default_fill_value(var)
     var = conventions.maybe_encode_bools(var)