ENH: switch Dataset and DataArray to use explicit indexes (#2639)

* ENH: switch Dataset and DataArray to use explicit indexes

This change switches Dataset.indexes and DataArray.indexes to be backed by
explicit dictionaries of indexes, instead of being implicitly defined by
the set of coordinates with names matching dimensions.

There are no changes to the public interface yet: these will come later.

For now, indexes are recreated from coordinates every time a new DataArray
or Dataset is created. In follow-up PRs, I will refactor indexes to be
propagated explicitly in xarray operations. This will facilitate future API
changes, when indexes will no longer only be associated with dimensions.

* Add xarray.core.indexes

* Fixes per review

xarray/core/coordinates.py

@@ -196,6 +196,7 @@ def _update_coords(self, coords):
         self._data._variables = variables
         self._data._coord_names.update(new_coord_names)
         self._data._dims = dict(dims)
+        self._data._indexes = None
 
     def __delitem__(self, key):
         if key in self:
@@ -276,44 +277,6 @@ def __iter__(self):
         return iter(self._data._level_coords)
 
 
-class Indexes(Mapping, formatting.ReprMixin):
-    """Ordered Mapping[str, pandas.Index] for xarray objects.
-    """
-
-    def __init__(self, variables, sizes):
-        """Not for public consumption.
-
-        Parameters
-        ----------
-        variables : OrderedDict[Any, Variable]
-            Reference to OrderedDict holding variable objects. Should be the
-            same dictionary used by the source object.
-        sizes : OrderedDict[Any, int]
-            Map from dimension names to sizes.
-        """
-        self._variables = variables
-        self._sizes = sizes
-
-    def __iter__(self):
-        for key in self._sizes:
-            if key in self._variables:
-                yield key
-
-    def __len__(self):
-        return sum(key in self._variables for key in self._sizes)
-
-    def __contains__(self, key):
-        return key in self._sizes and key in self._variables
-
-    def __getitem__(self, key):
-        if key not in self._sizes:
-            raise KeyError(key)
-        return self._variables[key].to_index()
-
-    def __unicode__(self):
-        return formatting.indexes_repr(self)
-
-
 def assert_coordinate_consistent(obj, coords):
     """ Maeke sure the dimension coordinate of obj is
     consistent with coords.

xarray/core/dataarray.py

@@ -13,10 +13,11 @@
 from .alignment import align, reindex_like_indexers
 from .common import AbstractArray, DataWithCoords
 from .coordinates import (
-    DataArrayCoordinates, Indexes, LevelCoordinatesSource,
+    DataArrayCoordinates, LevelCoordinatesSource,
     assert_coordinate_consistent, remap_label_indexers)
 from .dataset import Dataset, merge_indexes, split_indexes
 from .formatting import format_item
+from .indexes import default_indexes, Indexes
 from .options import OPTIONS
 from .pycompat import OrderedDict, basestring, iteritems, range, zip
 from .utils import (
@@ -165,7 +166,7 @@ class DataArray(AbstractArray, DataWithCoords):
     dt = property(DatetimeAccessor)
 
     def __init__(self, data, coords=None, dims=None, name=None,
-                 attrs=None, encoding=None, fastpath=False):
+                 attrs=None, encoding=None, indexes=None, fastpath=False):
         """
         Parameters
         ----------
@@ -237,6 +238,10 @@ def __init__(self, data, coords=None, dims=None, name=None,
         self._coords = coords
         self._name = name
 
+        # TODO(shoyer): document this argument, once it becomes part of the
+        # public interface.
+        self._indexes = indexes
+
         self._file_obj = None
 
         self._initialized = True
@@ -534,9 +539,11 @@ def encoding(self, value):
 
     @property
     def indexes(self):
-        """OrderedDict of pandas.Index objects used for label based indexing
+        """Mapping of pandas.Index objects used for label based indexing
         """
-        return Indexes(self._coords, self.sizes)
+        if self._indexes is None:
+            self._indexes = default_indexes(self._coords, self.dims)
+        return Indexes(self._indexes)
 
     @property
     def coords(self):

xarray/core/dataset.py

@@ -13,16 +13,17 @@
 import xarray as xr
 
 from . import (
-    alignment, dtypes, duck_array_ops, formatting, groupby, indexing, ops,
-    pdcompat, resample, rolling, utils)
+    alignment, dtypes, duck_array_ops, formatting, groupby,
+    indexing, ops, pdcompat, resample, rolling, utils)
 from ..coding.cftimeindex import _parse_array_of_cftime_strings
 from .alignment import align
 from .common import (
     ALL_DIMS, DataWithCoords, ImplementsDatasetReduce,
     _contains_datetime_like_objects)
 from .coordinates import (
-    DatasetCoordinates, Indexes, LevelCoordinatesSource,
+    DatasetCoordinates, LevelCoordinatesSource,
     assert_coordinate_consistent, remap_label_indexers)
+from .indexes import Indexes, default_indexes
 from .merge import (
     dataset_merge_method, dataset_update_method, merge_data_and_coords,
     merge_variables)
@@ -364,6 +365,10 @@ def __init__(self, data_vars=None, coords=None, attrs=None,
             coords = {}
         if data_vars is not None or coords is not None:
             self._set_init_vars_and_dims(data_vars, coords, compat)
+
+        # TODO(shoyer): expose indexes as a public argument in __init__
+        self._indexes = None
+
         if attrs is not None:
             self.attrs = attrs
         self._encoding = None
@@ -642,14 +647,15 @@ def persist(self, **kwargs):
 
     @classmethod
     def _construct_direct(cls, variables, coord_names, dims=None, attrs=None,
-                          file_obj=None, encoding=None):
+                          indexes=None, file_obj=None, encoding=None):
         """Shortcut around __init__ for internal use when we want to skip
         costly validation
         """
         obj = object.__new__(cls)
         obj._variables = variables
         obj._coord_names = coord_names
         obj._dims = dims
+        obj._indexes = indexes
         obj._attrs = attrs
         obj._file_obj = file_obj
         obj._encoding = encoding
@@ -664,7 +670,8 @@ def _from_vars_and_coord_names(cls, variables, coord_names, attrs=None):
         return cls._construct_direct(variables, coord_names, dims, attrs)
 
     def _replace_vars_and_dims(self, variables, coord_names=None, dims=None,
-                               attrs=__default_attrs, inplace=False):
+                               attrs=__default_attrs, indexes=None,
+                               inplace=False):
         """Fastpath constructor for internal use.
 
         Preserves coord names and attributes. If not provided explicitly,
@@ -693,13 +700,15 @@ def _replace_vars_and_dims(self, variables, coord_names=None, dims=None,
                 self._coord_names = coord_names
             if attrs is not self.__default_attrs:
                 self._attrs = attrs
+            self._indexes = indexes
             obj = self
         else:
             if coord_names is None:
                 coord_names = self._coord_names.copy()
             if attrs is self.__default_attrs:
                 attrs = self._attrs_copy()
-            obj = self._construct_direct(variables, coord_names, dims, attrs)
+            obj = self._construct_direct(
+                variables, coord_names, dims, attrs, indexes)
         return obj
 
     def _replace_indexes(self, indexes):
@@ -1064,9 +1073,11 @@ def identical(self, other):
 
     @property
     def indexes(self):
-        """OrderedDict of pandas.Index objects used for label based indexing
+        """Mapping of pandas.Index objects used for label based indexing
         """
-        return Indexes(self._variables, self._dims)
+        if self._indexes is None:
+            self._indexes = default_indexes(self._variables, self._dims)
+        return Indexes(self._indexes)
 
     @property
     def coords(self):

xarray/core/indexes.py

@@ -0,0 +1,55 @@
+from __future__ import absolute_import, division, print_function
+try:
+    from collections.abc import Mapping
+except ImportError:
+    from collections import Mapping
+from collections import OrderedDict
+
+from . import formatting
+
+
+class Indexes(Mapping, formatting.ReprMixin):
+    """Immutable proxy for Dataset or DataArrary indexes."""
+    def __init__(self, indexes):
+        """Not for public consumption.
+
+        Parameters
+        ----------
+        indexes : Dict[Any, pandas.Index]
+           Indexes held by this object.
+        """
+        self._indexes = indexes
+
+    def __iter__(self):
+        return iter(self._indexes)
+
+    def __len__(self):
+        return len(self._indexes)
+
+    def __contains__(self, key):
+        return key in self._indexes
+
+    def __getitem__(self, key):
+        return self._indexes[key]
+
+    def __unicode__(self):
+        return formatting.indexes_repr(self)
+
+
+def default_indexes(coords, dims):
+    """Default indexes for a Dataset/DataArray.
+
+    Parameters
+    ----------
+    coords : Mapping[Any, xarray.Variable]
+       Coordinate variables from which to draw default indexes.
+    dims : iterable
+        Iterable of dimension names.
+
+    Returns
+    -------
+    Mapping[Any, pandas.Index] mapping indexing keys (levels/dimension names)
+    to indexes used for indexing along that dimension.
+    """
+    return OrderedDict((key, coords[key].to_index())
+                       for key in dims if key in coords)