Allow calling Dataset.map with a str method name

Author: toddrjen

doc/whats-new.rst

@@ -59,6 +59,12 @@ New Features
   :py:meth:`core.groupby.DatasetGroupBy.quantile`, :py:meth:`core.groupby.DataArrayGroupBy.quantile`
   (:issue:`3843`, :pull:`3844`) 
   By `Aaron Spring <https://github.com/aaronspring>`_.
+- Allow calling :py:meth:`Dataset.map` with a str for the function, which
+  will call the method with that name on each variable.
+- Implement :py:meth:`Dataset.idxmax`, :py:meth:`Dataset.idxmin`,
+  :py:meth:`DataArray.idxmax`, and :py:meth:`DataArray.idxmin`.
+  (:issue:`60`, :pull:`xxxx`)
+  By `Todd Jennings <https://github.com/toddrjen>`_.
 
 
 Bug fixes

xarray/core/dataset.py

@@ -4284,7 +4284,7 @@ def reduce(
 
     def map(
         self,
-        func: Callable,
+        func: Union[Callable, str],
         keep_attrs: bool = None,
         args: Iterable[Any] = (),
         **kwargs: Any,
@@ -4293,10 +4293,11 @@ def map(
 
         Parameters
         ----------
-        func : callable
+        func : callable or str
             Function which can be called in the form `func(x, *args, **kwargs)`
             to transform each DataArray `x` in this dataset into another
-            DataArray.
+            DataArray.  Alternatively the string name of a method can be used,
+            which will be called individually on each data variable.
         keep_attrs : bool, optional
             If True, the dataset's attributes (`attrs`) will be copied from
             the original object to the new one. If False, the new object will
@@ -4330,10 +4331,16 @@ def map(
             foo      (dim_0, dim_1) float64 0.3751 1.951 1.945 0.2948 0.711 0.3948
             bar      (x) float64 1.0 2.0
         """
-        variables = {
-            k: maybe_wrap_array(v, func(v, *args, **kwargs))
-            for k, v in self.data_vars.items()
-        }
+        if isinstance(func, str):
+            variables = {
+                k: maybe_wrap_array(v, getattr(v, func)(*args, **kwargs))
+                for k, v in self.data_vars.items()
+            }
+        else:
+            variables = {
+                k: maybe_wrap_array(v, func(v, *args, **kwargs))
+                for k, v in self.data_vars.items()
+            }
         if keep_attrs is None:
             keep_attrs = _get_keep_attrs(default=False)
         attrs = self.attrs if keep_attrs else None

xarray/tests/test_dataset.py

@@ -4782,6 +4782,7 @@ def test_map(self):
         data.attrs["foo"] = "bar"
 
         assert_identical(data.map(np.mean), data.mean())
+        assert_identical(data.map("mean"), data.mean())
 
         expected = data.mean(keep_attrs=True)
         actual = data.map(lambda x: x.mean(keep_attrs=True), keep_attrs=True)