Fix importing xarray with python -OO

.travis.yml

@@ -89,6 +89,7 @@ install:
   - python xarray/util/print_versions.py
 
 script:
+  - python -OO -c "import xarray"
   - py.test xarray --cov=xarray --cov-config ci/.coveragerc --cov-report term-missing --verbose $EXTRA_FLAGS
   - git diff upstream/master **/*py | flake8 --diff --exit-zero || true
 

doc/whats-new.rst

@@ -43,6 +43,9 @@ Bug fixes
   ``NumpyIndexingAdapter``. (:issue:`1694`)
   By `Keisuke Fujii <https://github.com/fujiisoup>`_
 
+- Fix importing xarray when running Python with ``-OO`` (:issue:`1706`).
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
+
 - Fix two bugs that were preventing dask arrays from being specified as
   coordinates in the DataArray constructor (:issue:`1684`).
   By `Joe Hamman <https://github.com/jhamman>`_

xarray/plot/plot.py

@@ -402,7 +402,7 @@ def _plot2d(plotfunc):
     """
 
     # Build on the original docstring
-    plotfunc.__doc__ = '\n'.join((plotfunc.__doc__, commondoc))
+    plotfunc.__doc__ = '%s\n%s' % (plotfunc.__doc__, commondoc)
 
     @functools.wraps(plotfunc)
     def newplotfunc(darray, x=None, y=None, figsize=None, size=None,