Alternative fix for #188 (#268)

* alternative fix for 188

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: TomNicholas

datatree/datatree.py

@@ -36,6 +36,7 @@
     HybridMappingProxy,
     _default,
     either_dict_or_kwargs,
+    maybe_wrap_array,
 )
 from xarray.core.variable import Variable
 
@@ -235,6 +236,65 @@ def _replace(
             inplace=inplace,
         )
 
+    def map(
+        self,
+        func: Callable,
+        keep_attrs: bool | None = None,
+        args: Iterable[Any] = (),
+        **kwargs: Any,
+    ) -> Dataset:
+        """Apply a function to each data variable in this dataset
+
+        Parameters
+        ----------
+        func : callable
+            Function which can be called in the form `func(x, *args, **kwargs)`
+            to transform each DataArray `x` in this dataset into another
+            DataArray.
+        keep_attrs : bool or None, optional
+            If True, both the dataset's and variables' attributes (`attrs`) will be
+            copied from the original objects to the new ones. If False, the new dataset
+            and variables will be returned without copying the attributes.
+        args : iterable, optional
+            Positional arguments passed on to `func`.
+        **kwargs : Any
+            Keyword arguments passed on to `func`.
+
+        Returns
+        -------
+        applied : Dataset
+            Resulting dataset from applying ``func`` to each data variable.
+
+        Examples
+        --------
+        >>> da = xr.DataArray(np.random.randn(2, 3))
+        >>> ds = xr.Dataset({"foo": da, "bar": ("x", [-1, 2])})
+        >>> ds
+        <xarray.Dataset>
+        Dimensions:  (dim_0: 2, dim_1: 3, x: 2)
+        Dimensions without coordinates: dim_0, dim_1, x
+        Data variables:
+            foo      (dim_0, dim_1) float64 1.764 0.4002 0.9787 2.241 1.868 -0.9773
+            bar      (x) int64 -1 2
+        >>> ds.map(np.fabs)
+        <xarray.Dataset>
+        Dimensions:  (dim_0: 2, dim_1: 3, x: 2)
+        Dimensions without coordinates: dim_0, dim_1, x
+        Data variables:
+            foo      (dim_0, dim_1) float64 1.764 0.4002 0.9787 2.241 1.868 0.9773
+            bar      (x) float64 1.0 2.0
+        """
+
+        # Copied from xarray.Dataset so as not to call type(self), which causes problems (see datatree GH188).
+        # TODO Refactor xarray upstream to avoid needing to overwrite this.
+        # TODO This copied version will drop all attrs - the keep_attrs stuff should be re-instated
+        variables = {
+            k: maybe_wrap_array(v, func(v, *args, **kwargs))
+            for k, v in self.data_vars.items()
+        }
+        # return type(self)(variables, attrs=attrs)
+        return Dataset(variables)
+
 
 class DataTree(
     NamedNode,