idxmax with multiple dimensions

xarray/computation/computation.py

@@ -9,10 +9,7 @@
 
 import functools
 from collections import Counter
-from collections.abc import (
-    Callable,
-    Hashable,
-)
+from collections.abc import Callable, Hashable
 from typing import TYPE_CHECKING, Any, Literal, cast, overload
 
 import numpy as np
@@ -23,10 +20,7 @@
 from xarray.core.duck_array_ops import datetime_to_numeric
 from xarray.core.options import OPTIONS, _get_keep_attrs
 from xarray.core.types import Dims, T_DataArray
-from xarray.core.utils import (
-    is_scalar,
-    parse_dims_as_set,
-)
+from xarray.core.utils import is_scalar, parse_dims_as_set
 from xarray.core.variable import Variable
 from xarray.namedarray.parallelcompat import get_chunked_array_type
 from xarray.namedarray.pycompat import is_chunked_array
@@ -903,7 +897,9 @@ def _calc_idxminmax(
     if not array.ndim:
         raise ValueError("This function does not apply for scalars")
 
-    if dim is not None:
+    if dim is Ellipsis:
+        dim = array.dims
+    elif dim is not None:
         pass  # Use the dim if available
     elif array.ndim == 1:
         # it is okay to guess the dim if there is only 1
@@ -912,14 +908,19 @@ def _calc_idxminmax(
         # The dim is not specified and ambiguous.  Don't guess.
         raise ValueError("Must supply 'dim' argument for multidimensional arrays")
 
-    if dim not in array.dims:
-        raise KeyError(
-            f"Dimension {dim!r} not found in array dimensions {array.dims!r}"
-        )
-    if dim not in array.coords:
-        raise KeyError(
-            f"Dimension {dim!r} is not one of the coordinates {tuple(array.coords.keys())}"
-        )
+    dim_is_str = isinstance(dim, str)
+    # Standardize to an iterable format
+    dims = [dim] if dim_is_str else dim
+
+    for _dim in dims:
+        if _dim not in array.dims:
+            raise KeyError(
+                f"Dimension {_dim!r} not found in array dimensions {array.dims!r}"
+            )
+        if _dim not in array.coords:
+            raise KeyError(
+                f"Dimension {_dim!r} is not one of the coordinates {tuple(array.coords.keys())}"
+            )
 
     # These are dtypes with NaN values argmin and argmax can handle
     na_dtypes = "cfO"
@@ -931,25 +932,30 @@ def _calc_idxminmax(
 
     # This will run argmin or argmax.
     index = func(array, dim=dim, axis=None, keep_attrs=keep_attrs, skipna=skipna)
+    # Force dictionary format in case of single dim so that we can iterate over it in for loop below
+    if dim_is_str:
+        index = {dim: index}
+
+    res = {}
+    for _dim, _da_idx in zip(dims, index.values(), strict=False):
+        # Handle chunked arrays (e.g. dask).
+        coord = array[_dim]._variable.to_base_variable()
+        if is_chunked_array(array.data):
+            chunkmanager = get_chunked_array_type(array.data)
+            coord_array = chunkmanager.from_array(
+                array[_dim].data, chunks=((array.sizes[_dim],),)
+            )
+            coord = coord.copy(data=coord_array)
+        else:
+            coord = coord.copy(data=to_like_array(array[_dim].data, array.data))
 
-    # Handle chunked arrays (e.g. dask).
-    coord = array[dim]._variable.to_base_variable()
-    if is_chunked_array(array.data):
-        chunkmanager = get_chunked_array_type(array.data)
-        coord_array = chunkmanager.from_array(
-            array[dim].data, chunks=((array.sizes[dim],),)
-        )
-        coord = coord.copy(data=coord_array)
-    else:
-        coord = coord.copy(data=to_like_array(array[dim].data, array.data))
-
-    res = index._replace(coord[(index.variable,)]).rename(dim)
-
-    if skipna or (skipna is None and array.dtype.kind in na_dtypes):
-        # Put the NaN values back in after removing them
-        res = res.where(~allna, fill_value)
-
-    # Copy attributes from argmin/argmax, if any
-    res.attrs = index.attrs
+        _res = _da_idx._replace(coord[(_da_idx.variable,)]).rename(_dim)
+        if skipna or (skipna is None and array.dtype.kind in na_dtypes):
+            # Put the NaN values back in after removing them
+            _res = _res.where(~allna, fill_value)
+        _res.attrs = _da_idx.attrs
+        res[_dim] = _res
 
+    if dim_is_str:
+        return res[dim]
     return res