Actually copy dask arrays in Variable.copy() (#1180)

* Actually copy dask arrays in Variable.copy()

Fixes GH1166

* Add documentation / whats new

* Remove Python 3.3 from install page

Author: shoyer

doc/installing.rst

@@ -6,7 +6,7 @@ Installation
 Required dependencies
 ---------------------
 
-- Python 2.7, 3.3, 3.4 or 3.5
+- Python 2.7, 3.4 or 3.5
 - `numpy <http://www.numpy.org/>`__ (1.7 or later)
 - `pandas <http://pandas.pydata.org/>`__ (0.15.0 or later)
 
@@ -36,7 +36,8 @@ For accelerating xarray
 For parallel computing
 ~~~~~~~~~~~~~~~~~~~~~~
 
-- `dask.array <http://dask.pydata.org>`__: required for :ref:`dask`.
+- `dask.array <http://dask.pydata.org>`__ (0.9.0 or later): required for
+  :ref:`dask`.
 
 For plotting
 ~~~~~~~~~~~~

doc/whats-new.rst

@@ -70,7 +70,7 @@ Breaking changes
 - Coordinates used to index a dimension are now loaded eagerly into
   :py:class:`pandas.Index` objects, instead of loading the values lazily.
   By `Guido Imperiale <https://github.com/crusaderky>`_.
-- xarray no longer supports python 3.3
+- xarray no longer supports python 3.3 or versions of dask prior to v0.9.0.
 
 Deprecations
 ~~~~~~~~~~~~
@@ -223,6 +223,9 @@ Bug fixes
 - Fixed a bug with facetgrid (the ``norm`` keyword was ignored, :issue:`1159`).
   By `Fabien Maussion <https://github.com/fmaussion>`_.
 
+- Fix to make ``.copy()`` actually copy dask arrays, which will be relevant for
+  future releases of dask in which dask arrays will be mutable (:issue:`1180`).
+
 .. _whats-new.0.8.2:
 
 v0.8.2 (18 August 2016)

xarray/core/variable.py

@@ -437,10 +437,12 @@ def copy(self, deep=True):
             # don't share caching between copies
             data = indexing.MemoryCachedArray(data.array)
 
-        if deep and not isinstance(
-                data, (dask_array_type, PandasIndexAdapter)):
-            # pandas.Index and dask.array objects are immutable
-            data = np.array(data)
+        if deep:
+            if isinstance(data, dask_array_type):
+                data = data.copy()
+            elif not isinstance(data, PandasIndexAdapter):
+                # pandas.Index is immutable
+                data = np.array(data)
 
         # note:
         # dims is already an immutable tuple