Align DataArrays based on coords in Dataset constructor (pydata#1826)

* Align da based on explicit coords

* add tags to gitignore

* random formatting spot

* initial tests

* apply the test to the right degree of freedom - the coords of the variable added in

* couple more for gitignore

* @stickler-ci doesn't like `range`

* more tests

* more gitignores

* whats new

* raise

* message=

* Add all testmon files to gitignore

* cast single dim tuples to indexes

* test on different dataset coords types

* updated whatsnew

* version from Stephan's feedback; works but not clean

* I think much cleaner version

* formatting

.gitignore

@@ -38,6 +38,7 @@ nosetests.xml
 .ropeproject/
 .tags*
 .testmon*
+.tmontmp/
 .pytest_cache
 dask-worker-space/
 

doc/whats-new.rst

@@ -46,6 +46,11 @@ Enhancements
   to manage its version strings. (:issue:`1300`).
   By `Joe Hamman <https://github.com/jhamman>`_.
 
+- `:py:class:`Dataset`s align `:py:class:`DataArray`s to coords that are explicitly
+  passed into the constructor, where previously an error would be raised.
+  (:issue:`674`)
+  By `Maximilian Roos <https://github.com/maxim-lian`_.
+
 - :py:meth:`~DataArray.sel`, :py:meth:`~DataArray.isel` & :py:meth:`~DataArray.reindex`,
   (and their :py:class:`Dataset` counterparts) now support supplying a ``dict``
   as a first argument, as an alternative to the existing approach 

xarray/core/merge.py

@@ -360,7 +360,17 @@ def merge_data_and_coords(data, coords, compat='broadcast_equals',
     """Used in Dataset.__init__."""
     objs = [data, coords]
     explicit_coords = coords.keys()
-    return merge_core(objs, compat, join, explicit_coords=explicit_coords)
+    indexes = dict(extract_indexes(coords))
+    return merge_core(objs, compat, join, explicit_coords=explicit_coords,
+                      indexes=indexes)
+
+
+def extract_indexes(coords):
+    """Yields the name & index of valid indexes from a mapping of coords"""
+    for name, variable in coords.items():
+        variable = as_variable(variable, name=name)
+        if variable.dims == (name,):
+            yield name, variable.to_index()
 
 
 def assert_valid_explicit_coords(variables, dims, explicit_coords):

xarray/tests/test_dataset.py

@@ -4193,6 +4193,45 @@ def test_isin_dataset():
         ds.isin(ds)
 
 
+@pytest.mark.parametrize('unaligned_coords', (
+    {'x': [2, 1, 0]},
+    {'x': (['x'], np.asarray([2, 1, 0]))},
+    {'x': (['x'], np.asarray([1, 2, 0]))},
+    {'x': pd.Index([2, 1, 0])},
+    {'x': Variable(dims='x', data=[0, 2, 1])},
+    {'x': IndexVariable(dims='x', data=[0, 1, 2])},
+    {'y': 42},
+    {'y': ('x', [2, 1, 0])},
+    {'y': ('x', np.asarray([2, 1, 0]))},
+    {'y': (['x'], np.asarray([2, 1, 0]))},
+))
+@pytest.mark.parametrize('coords', (
+    {'x': ('x', [0, 1, 2])},
+    {'x': [0, 1, 2]},
+))
+def test_dataset_constructor_aligns_to_explicit_coords(
+        unaligned_coords, coords):
+
+    a = xr.DataArray([1, 2, 3], dims=['x'], coords=unaligned_coords)
+
+    expected = xr.Dataset(coords=coords)
+    expected['a'] = a
+
+    result = xr.Dataset({'a': a}, coords=coords)
+
+    assert_equal(expected, result)
+
+
+@pytest.mark.parametrize('unaligned_coords', (
+    {'y': ('b', np.asarray([2, 1, 0]))},
+))
+def test_constructor_raises_with_invalid_coords(unaligned_coords):
+
+    with pytest.raises(ValueError,
+                       message='not a subset of the DataArray dimensions'):
+        xr.DataArray([1, 2, 3], dims=['x'], coords=unaligned_coords)
+
+
 def test_dir_expected_attrs(data_set):
 
     some_expected_attrs = {'pipe', 'mean', 'isnull', 'var1',
@@ -4205,7 +4244,7 @@ def test_dir_non_string(data_set):
     # add a numbered key to ensure this doesn't break dir
     data_set[5] = 'foo'
     result = dir(data_set)
-    assert not (5 in result)
+    assert 5 not in result
 
     # GH2172
     sample_data = np.random.uniform(size=[2, 2000, 10000])