Add set_index, reset_index and reorder_levels methods

doc/api.rst

@@ -106,6 +106,9 @@ Indexing
    Dataset.squeeze
    Dataset.reindex
    Dataset.reindex_like
+   Dataset.set_index
+   Dataset.reset_index
+   Dataset.reorder_levels
 
 Computation
 -----------
@@ -239,6 +242,9 @@ Indexing
    DataArray.squeeze
    DataArray.reindex
    DataArray.reindex_like
+   DataArray.set_index
+   DataArray.reset_index
+   DataArray.reorder_levels
 
 Comparisons
 -----------

doc/reshaping.rst

@@ -4,7 +4,7 @@
 Reshaping and reorganizing data
 ###############################
 
-These methods allow you to reorganize 
+These methods allow you to reorganize
 
 .. ipython:: python
    :suppress:
@@ -95,23 +95,79 @@ always succeeds, even if the multi-index being unstacked does not contain all
 possible levels. Missing levels are filled in with ``NaN`` in the resulting object:
 
 .. ipython:: python
-    
+
     stacked2 = stacked[::2]
-    stacked2    
+    stacked2
     stacked2.unstack('z')
 
 However, xarray's ``stack`` has an important difference from pandas: unlike
 pandas, it does not automatically drop missing values. Compare:
 
 .. ipython:: python
-    
+
     array = xr.DataArray([[np.nan, 1], [2, 3]], dims=['x', 'y'])
-    array.stack(z=('x', 'y'))    
+    array.stack(z=('x', 'y'))
     array.to_pandas().stack()
 
 We departed from pandas's behavior here because predictable shapes for new
 array dimensions is necessary for :ref:`dask`.
 
+.. _reshape.set_index:
+
+Set and reset index
+-------------------
+
+Complementary to stack / unstack, xarray's ``.set_index``, ``.reset_index`` and
+``.reorder_levels`` allow easy manipulation of ``DataArray`` or ``Dataset``
+multi-indexes without modifying the data and its dimensions.
+
+You can create a multi-index from several 1-dimensional variables and/or
+coordinates using :py:meth:`~xarray.DataArray.set_index`:
+
+.. ipython:: python
+
+     da = xr.DataArray(np.random.rand(4),
+                       coords={'band': ('x', ['a', 'a', 'b', 'b']),
+                               'wavenumber': ('x', np.linspace(200, 400, 4))},
+                       dims='x')
+     da
+     mda = da.set_index(x=['band', 'wavenumber'])
+     mda
+
+These coordinates can now be used for indexing, e.g.,
+
+.. ipython:: python
+
+     mda.sel(band='a')
+
+Conversely, you can use :py:meth:`~xarray.DataArray.reset_index`
+to extract multi-index levels as coordinates (this is mainly useful
+for serialization):
+
+.. ipython:: python
+
+     mda.reset_index('x')
+
+:py:meth:`~xarray.DataArray.reorder_levels` allows changing the order
+of multi-index levels:
+
+.. ipython:: python
+
+     mda.reorder_levels(x=['wavenumber', 'band'])
+
+As of xarray v0.9 coordinate labels for each dimension are optional.
+You can also  use ``.set_index`` / ``.reset_index`` to add / remove
+labels for one or several dimensions:
+
+.. ipython:: python
+
+    array = xr.DataArray([1, 2, 3], dims='x')
+    array
+    array['c'] = ('x', ['a', 'b', 'c'])
+    array.set_index(x='c')
+    array.set_index(x='c', inplace=True)
+    array.reset_index('x', drop=True)
+
 Shift and roll
 --------------
 

doc/whats-new.rst

@@ -104,6 +104,9 @@ Enhancements
   as keyword arguments, e.g., ``ds.sel(time='2000-01')``
   (see :ref:`multi-level indexing`).
   By `Benoit Bovy <https://github.com/benbovy>`_.
+- Added ``set_index``, ``reset_index`` and ``reorder_levels`` methods to
+  easily create and manipulate (multi-)indexes (see :ref:`reshape.set_index`).
+  By `Benoit Bovy <https://github.com/benbovy>`_.
 - Added the ``compat`` option ``'no_conflicts'`` to ``merge``, allowing the
   combination of xarray objects with disjoint (:issue:`742`) or
   overlapping (:issue:`835`) coordinates as long as all present data agrees.

xarray/core/dataarray.py

@@ -18,7 +18,7 @@
 from .common import AbstractArray, BaseDataObject
 from .coordinates import (DataArrayCoordinates, LevelCoordinatesSource,
                           Indexes)
-from .dataset import Dataset
+from .dataset import Dataset, merge_indexes, split_indexes
 from .pycompat import iteritems, basestring, OrderedDict, zip, range
 from .variable import (as_variable, Variable, as_compatible_data,
                        IndexVariable,
@@ -846,6 +846,103 @@ def swap_dims(self, dims_dict):
         ds = self._to_temp_dataset().swap_dims(dims_dict)
         return self._from_temp_dataset(ds)
 
+    def set_index(self, append=False, inplace=False, **indexes):
+        """Set DataArray (multi-)indexes using one or more existing coordinates.
+
+        Parameters
+        ----------
+        append : bool, optional
+            If True, append the supplied index(es) to the existing index(es).
+            Otherwise replace the existing index(es) (default).
+        inplace : bool, optional
+            If True, set new index(es) in-place. Otherwise, return a new DataArray
+            object.
+        **indexes : {dim: index, ...}
+            Keyword arguments with names matching dimensions and values given
+            by (lists of) the names of existing coordinates or variables to set
+            as new (multi-)index.
+
+        Returns
+        -------
+        obj : DataArray
+            Another dataarray, with this dataarray's data but replaced coordinates.
+
+        See Also
+        --------
+        DataArray.reset_index
+        """
+        coords, _ = merge_indexes(indexes, self._coords, set(), append=append)
+        if inplace:
+            self._coords = coords
+        else:
+            return self._replace(coords=coords)
+
+    def reset_index(self, dims_or_levels, drop=False, inplace=False):
+        """Reset the specified index(es) or multi-index level(s).
+
+        Parameters
+        ----------
+        dims_or_levels : str or list
+            Name(s) of the dimension(s) and/or multi-index level(s) that will
+            be reset.
+        drop : bool, optional
+            If True, remove the specified indexes and/or multi-index levels
+            instead of extracting them as new coordinates (default: False).
+        inplace : bool, optional
+            If True, modify the dataarray in-place. Otherwise, return a new
+            DataArray object.
+
+        Returns
+        -------
+        obj : DataArray
+            Another dataarray, with this dataarray's data but replaced
+            coordinates.
+
+        See Also
+        --------
+        DataArray.set_index
+        """
+        coords, _ = split_indexes(dims_or_levels, self._coords, set(),
+                                  self._level_coords, drop=drop)
+        if inplace:
+            self._coords = coords
+        else:
+            return self._replace(coords=coords)
+
+    def reorder_levels(self, inplace=False, **dim_order):
+        """Rearrange index levels using input order.
+
+        Parameters
+        ----------
+        inplace : bool, optional
+            If True, modify the dataarray in-place. Otherwise, return a new
+            DataArray object.
+        **dim_order : optional
+            Keyword arguments with names matching dimensions and values given
+            by lists representing new level orders. Every given dimension
+            must have a multi-index.
+
+        Returns
+        -------
+        obj : DataArray
+            Another dataarray, with this dataarray's data but replaced
+            coordinates.
+        """
+        replace_coords = {}
+        for dim, order in dim_order.items():
+            coord = self._coords[dim]
+            index = coord.to_index()
+            if not isinstance(index, pd.MultiIndex):
+                raise ValueError("coordinate %r has no MultiIndex" % dim)
+            replace_coords[dim] = IndexVariable(coord.dims,
+                                                index.reorder_levels(order))
+        coords = self._coords.copy()
+        coords.update(replace_coords)
+        if inplace:
+            self._coords = coords
+        else:
+            return self._replace(coords=coords)
+
     def stack(self, **dimensions):
         """
         Stack any number of existing dimensions into a single new dimension.