Allow ellipsis to be used in stack (#3826)

* allow ellipsis to be used in stack

* doc fix

* support ellipsis only as part of an iterable

* docs

* whatsnew

* docstring, whatsnew

* docstring, whatsnew

* add passing a partial list of dims

* more wording changes

* improvement from @dcherian

doc/reshaping.rst

@@ -109,6 +109,13 @@ implemented :py:meth:`~xarray.DataArray.stack` and
     stacked
     stacked.unstack('z')
 
+As elsewhere in xarray, an ellipsis (`...`) can be used to represent all unlisted dimensions:
+
+.. ipython:: python
+
+    stacked = array.stack(z=[..., "x"])
+    stacked
+
 These methods are modeled on the :py:class:`pandas.DataFrame` methods of the
 same name, although in xarray they always create new dimensions rather than
 adding to the existing index or columns.
@@ -164,6 +171,7 @@ like this:
                       'b': ('x', [6, 7])},
             coords={'y': ['u', 'v', 'w']}
         )
+        data
         stacked = data.to_stacked_array("z", sample_dims=['x'])
         stacked
         unstacked = stacked.to_unstacked_dataset("z")

doc/whats-new.rst

@@ -39,13 +39,19 @@ New Features
   By `Justus Magin <https://github.com/keewis>`_.
 - :py:meth:`Dataset.groupby` and :py:meth:`DataArray.groupby` now raise a 
   `TypeError` on multiple string arguments. Receiving multiple string arguments
-  often means a user is attempting to pass multiple dimensions to group over
-  and should instead pass a list.
+  often means a user is attempting to pass multiple dimensions as separate
+  arguments and should instead pass a single list of dimensions.
+  (:pull:`3802`)
   By `Maximilian Roos <https://github.com/max-sixty>`_
 - The new ``Dataset._repr_html_`` and ``DataArray._repr_html_`` (introduced
   in 0.14.1) is now on by default. To disable, use
   ``xarray.set_options(display_style="text")``.
   By `Julia Signell <https://github.com/jsignell>`_.
+- An ellipsis (``...``) is now supported in the ``dims`` argument of
+  :py:meth:`Dataset.stack` and :py:meth:`DataArray.stack`, meaning all
+  unlisted dimensions, similar to its meaning in :py:meth:`DataArray.transpose`.
+  (:pull:`3826`)
+  By `Maximilian Roos <https://github.com/max-sixty>`_
 - :py:meth:`Dataset.where` and :py:meth:`DataArray.where` accept a lambda as a
   first argument, which is then called on the input; replicating pandas' behavior.
   By `Maximilian Roos <https://github.com/max-sixty>`_.

xarray/core/dataarray.py

@@ -1709,7 +1709,9 @@ def stack(
         ----------
         dimensions : Mapping of the form new_name=(dim1, dim2, ...)
             Names of new dimensions, and the existing dimensions that they
-            replace.
+            replace. An ellipsis (`...`) will be replaced by all unlisted dimensions.
+            Passing a list containing an ellipsis (`stacked_dim=[...]`) will stack over
+            all dimensions.
         **dimensions_kwargs:
             The keyword arguments form of ``dimensions``.
             One of dimensions or dimensions_kwargs must be provided.

xarray/core/dataset.py

@@ -87,6 +87,7 @@
     decode_numpy_dict_values,
     either_dict_or_kwargs,
     hashable,
+    infix_dims,
     is_dict_like,
     is_scalar,
     maybe_wrap_array,
@@ -3262,6 +3263,8 @@ def reorder_levels(
         return self._replace(variables, indexes=indexes)
 
     def _stack_once(self, dims, new_dim):
+        if ... in dims:
+            dims = list(infix_dims(dims, self.dims))
         variables = {}
         for name, var in self.variables.items():
             if name not in dims:
@@ -3304,7 +3307,9 @@ def stack(
         ----------
         dimensions : Mapping of the form new_name=(dim1, dim2, ...)
             Names of new dimensions, and the existing dimensions that they
-            replace.
+            replace. An ellipsis (`...`) will be replaced by all unlisted dimensions.
+            Passing a list containing an ellipsis (`stacked_dim=[...]`) will stack over
+            all dimensions.
         **dimensions_kwargs:
             The keyword arguments form of ``dimensions``.
             One of dimensions or dimensions_kwargs must be provided.

xarray/tests/test_dataarray.py

@@ -2040,6 +2040,9 @@ def test_stack_unstack(self):
         actual = orig.stack(z=["x", "y"]).unstack("z").drop_vars(["x", "y"])
         assert_identical(orig, actual)
 
+        actual = orig.stack(z=[...]).unstack("z").drop_vars(["x", "y"])
+        assert_identical(orig, actual)
+
         dims = ["a", "b", "c", "d", "e"]
         orig = xr.DataArray(np.random.rand(1, 2, 3, 2, 1), dims=dims)
         stacked = orig.stack(ab=["a", "b"], cd=["c", "d"])

xarray/tests/test_dataset.py

@@ -2879,6 +2879,17 @@ def test_stack(self):
         actual = ds.stack(z=["x", "y"])
         assert_identical(expected, actual)
 
+        actual = ds.stack(z=[...])
+        assert_identical(expected, actual)
+
+        # non list dims with ellipsis
+        actual = ds.stack(z=(...,))
+        assert_identical(expected, actual)
+
+        # ellipsis with given dim
+        actual = ds.stack(z=[..., "y"])
+        assert_identical(expected, actual)
+
         exp_index = pd.MultiIndex.from_product([["a", "b"], [0, 1]], names=["y", "x"])
         expected = Dataset(
             {"a": ("z", [0, 1, 0, 1]), "b": ("z", [0, 2, 1, 3]), "z": exp_index}