concat would load dask variables if first argument was numpy-based (#…

…1589)

doc/whats-new.rst

@@ -208,6 +208,10 @@ Bug fixes
   when objects other than  ``Dataset`` are provided (:issue:`1555`).
   By `Joe Hamman <https://github.com/jhamman>`_.
 
+- :py:func:`xarray.concat` would eagerly load dask variables into memory if
+  the first argument was a numpy variable (:issue:`1588`).
+  By `Guido Imperiale <https://github.com/crusaderky>`_.
+
 .. _whats-new.0.9.6:
 
 v0.9.6 (8 June 2017)

xarray/core/duck_array_ops.py

@@ -40,9 +40,11 @@ def _dask_or_eager_func(name, eager_module=np, list_of_args=False,
     """Create a function that dispatches to dask for dask array inputs."""
     if has_dask:
         def f(*args, **kwargs):
-            dispatch_args = args[0] if list_of_args else args
-            if any(isinstance(a, da.Array)
-                   for a in dispatch_args[:n_array_args]):
+            if list_of_args:
+                dispatch_args = args[0]
+            else:
+                dispatch_args = args[:n_array_args]
+            if any(isinstance(a, da.Array) for a in dispatch_args):
                 module = da
             else:
                 module = eager_module

xarray/tests/test_dask.py

@@ -178,6 +178,8 @@ def test_concat(self):
         v = self.lazy_var
         self.assertLazyAndIdentical(u, Variable.concat([v[:2], v[2:]], 'x'))
         self.assertLazyAndIdentical(u[:2], Variable.concat([v[0], v[1]], 'x'))
+        self.assertLazyAndIdentical(u[:2], Variable.concat([u[0], v[1]], 'x'))
+        self.assertLazyAndIdentical(u[:2], Variable.concat([v[0], u[1]], 'x'))
         self.assertLazyAndIdentical(
             u[:3], Variable.concat([v[[0, 2]], v[[1]]], 'x', positions=[[0, 2], [1]]))