Change keep_attrs default to True

doc/whats-new.rst

@@ -17,6 +17,90 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- **All xarray operations now preserve attributes by default** (:issue:`3891`, :issue:`2582`).
+  Previously, operations would drop attributes unless explicitly told to preserve them via ``keep_attrs=True``.
+  This aligns xarray with the common scientific workflow where metadata preservation is essential.
+
+  **What changed:**
+
+  .. code-block:: python
+
+      # Before (xarray <2025.09.1):
+      data = xr.DataArray([1, 2, 3], attrs={"units": "meters", "long_name": "height"})
+      result = data.mean()
+      result.attrs  # {}  - Attributes lost!
+
+      # After (xarray ≥2025.09.1):
+      data = xr.DataArray([1, 2, 3], attrs={"units": "meters", "long_name": "height"})
+      result = data.mean()
+      result.attrs  # {"units": "meters", "long_name": "height"}  - Attributes preserved!
+
+  **Affected operations include:**
+
+  *Computational operations:*
+
+  - Reductions: ``mean()``, ``sum()``, ``std()``, ``var()``, ``min()``, ``max()``, ``median()``, ``quantile()``, etc.
+  - Rolling windows: ``rolling().mean()``, ``rolling().sum()``, etc.
+  - Groupby: ``groupby().mean()``, ``groupby().sum()``, etc.
+  - Resampling: ``resample().mean()``, etc.
+  - Weighted: ``weighted().mean()``, ``weighted().sum()``, etc.
+  - ``apply_ufunc()`` and NumPy universal functions
+
+  *Binary operations:*
+
+  - Arithmetic: ``+``, ``-``, ``*``, ``/``, ``**``, ``//``, ``%`` (attributes from left operand)
+  - Comparisons: ``<``, ``>``, ``==``, ``!=``, ``<=``, ``>=`` (attributes from left operand)
+  - With scalars: ``data * 2``, ``10 - data`` (preserves data's attributes)
+
+  *Data manipulation:*
+
+  - Missing data: ``fillna()``, ``dropna()``, ``interpolate_na()``, ``ffill()``, ``bfill()``
+  - Indexing/selection: ``isel()``, ``sel()``, ``where()``, ``clip()``
+  - Alignment: ``interp()``, ``reindex()``, ``align()``
+  - Transformations: ``map()``, ``pipe()``, ``assign()``, ``assign_coords()``
+  - Shape operations: ``expand_dims()``, ``squeeze()``, ``transpose()``, ``stack()``, ``unstack()``
+
+  **Binary operations - attributes from left operand:**
+
+  .. code-block:: python
+
+      a = xr.DataArray([1, 2], attrs={"source": "sensor_a"})
+      b = xr.DataArray([3, 4], attrs={"source": "sensor_b"})
+      (a + b).attrs  # {"source": "sensor_a"}  - Left operand wins
+      (b + a).attrs  # {"source": "sensor_b"}  - Order matters!
+
+  **How to restore previous behavior:**
+
+  1. **Globally for your entire script:**
+
+     .. code-block:: python
+
+         import xarray as xr
+
+         xr.set_options(keep_attrs=False)  # Affects all subsequent operations
+
+  2. **For specific operations:**
+
+     .. code-block:: python
+
+         result = data.mean(dim="time", keep_attrs=False)
+
+  3. **For code blocks:**
+
+     .. code-block:: python
+
+         with xr.set_options(keep_attrs=False):
+             # All operations in this block drop attrs
+             result = data1 + data2
+
+  4. **Remove attributes after operations:**
+
+     .. code-block:: python
+
+         result = data.mean().drop_attrs()
+
+  By `Maximilian Roos <https://github.com/max-sixty>`_.
+
 - :py:meth:`Dataset.update` now returns ``None``, instead of the updated dataset. This
   completes the deprecation cycle started in version 0.17. The method still updates the
   dataset in-place. (:issue:`10167`)

xarray/computation/apply_ufunc.py

@@ -1214,7 +1214,7 @@ def apply_ufunc(
         func = functools.partial(func, **kwargs)
 
     if keep_attrs is None:
-        keep_attrs = _get_keep_attrs(default=False)
+        keep_attrs = _get_keep_attrs(default=True)
 
     if isinstance(keep_attrs, bool):
         keep_attrs = "override" if keep_attrs else "drop"

xarray/computation/computation.py

@@ -701,7 +701,7 @@ def where(cond, x, y, keep_attrs=None):
       * lon      (lon) int64 24B 10 11 12
 
     >>> xr.where(y.lat < 1, y, -1)
-    <xarray.DataArray (lat: 3, lon: 3)> Size: 72B
+    <xarray.DataArray 'lat' (lat: 3, lon: 3)> Size: 72B
     array([[ 0. ,  0.1,  0.2],
            [-1. , -1. , -1. ],
            [-1. , -1. , -1. ]])
@@ -726,7 +726,7 @@ def where(cond, x, y, keep_attrs=None):
     from xarray.core.dataset import Dataset
 
     if keep_attrs is None:
-        keep_attrs = _get_keep_attrs(default=False)
+        keep_attrs = _get_keep_attrs(default=True)
 
     # alignment for three arguments is complicated, so don't support it yet
     from xarray.computation.apply_ufunc import apply_ufunc

xarray/computation/weighted.py

@@ -448,7 +448,6 @@ def _weighted_quantile_1d(
 
         result = result.transpose("quantile", ...)
         result = result.assign_coords(quantile=q).squeeze()
-
         return result
 
     def _implementation(self, func, dim, **kwargs):
@@ -551,7 +550,6 @@ def _implementation(self, func, dim, **kwargs) -> DataArray:
 class DatasetWeighted(Weighted["Dataset"]):
     def _implementation(self, func, dim, **kwargs) -> Dataset:
         self._check_dim(dim)
-
         return self.obj.map(func, dim=dim, **kwargs)
 
 

xarray/core/common.py

@@ -1314,7 +1314,7 @@ def isnull(self, keep_attrs: bool | None = None) -> Self:
         from xarray.computation.apply_ufunc import apply_ufunc
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         return apply_ufunc(
             duck_array_ops.isnull,
@@ -1357,7 +1357,7 @@ def notnull(self, keep_attrs: bool | None = None) -> Self:
         from xarray.computation.apply_ufunc import apply_ufunc
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         return apply_ufunc(
             duck_array_ops.notnull,

xarray/core/dataarray.py

@@ -3889,8 +3889,8 @@ def reduce(
             supplied, then the reduction is calculated over the flattened array
             (by calling `f(x)` without an axis argument).
         keep_attrs : bool or None, optional
-            If True, the variable's attributes (`attrs`) will be copied from
-            the original object to the new one.  If False (default), the new
+            If True (default), the variable's attributes (`attrs`) will be copied from
+            the original object to the new one.  If False, the new
             object will be returned without attributes.
         keepdims : bool, default: False
             If True, the dimensions which are reduced are left in the result

xarray/core/dataset.py

@@ -6774,8 +6774,8 @@ def reduce(
             Dimension(s) over which to apply `func`. By default `func` is
             applied over all dimensions.
         keep_attrs : bool or None, optional
-            If True, the dataset's attributes (`attrs`) will be copied from
-            the original object to the new one.  If False (default), the new
+            If True (default), the dataset's attributes (`attrs`) will be copied from
+            the original object to the new one.  If False, the new
             object will be returned without attributes.
         keepdims : bool, default: False
             If True, the dimensions which are reduced are left in the result
@@ -6833,7 +6833,7 @@ def reduce(
         dims = parse_dims_as_set(dim, set(self._dims.keys()))
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         variables: dict[Hashable, Variable] = {}
         for name, var in self._variables.items():
@@ -6924,7 +6924,7 @@ def map(
             bar      (x) float64 16B 1.0 2.0
         """
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
         variables = {
             k: maybe_wrap_array(v, func(v, *args, **kwargs))
             for k, v in self.data_vars.items()
@@ -6937,11 +6937,14 @@ def map(
         if keep_attrs:
             for k, v in variables.items():
                 v._copy_attrs_from(self.data_vars[k])
-
             for k, v in coords.items():
-                if k not in self.coords:
-                    continue
-                v._copy_attrs_from(self.coords[k])
+                if k in self.coords:
+                    v._copy_attrs_from(self.coords[k])
+        else:
+            for v in variables.values():
+                v.attrs = {}
+            for v in coords.values():
+                v.attrs = {}
 
         attrs = self.attrs if keep_attrs else None
         return type(self)(variables, coords=coords, attrs=attrs)
@@ -7669,7 +7672,7 @@ def _binary_op(self, other, f, reflexive=False, join=None) -> Dataset:
             self, other = align(self, other, join=align_type, copy=False)
         g = f if not reflexive else lambda x, y: f(y, x)
         ds = self._calculate_binary_op(g, other, join=align_type)
-        keep_attrs = _get_keep_attrs(default=False)
+        keep_attrs = _get_keep_attrs(default=True)
         if keep_attrs:
             ds.attrs = self.attrs
         return ds
@@ -8265,7 +8268,7 @@ def quantile(
         coord_names = {k for k in self.coords if k in variables}
         indexes = {k: v for k, v in self._indexes.items() if k in variables}
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
         attrs = self.attrs if keep_attrs else None
         new = self._replace_with_new_dims(
             variables, coord_names=coord_names, attrs=attrs, indexes=indexes
@@ -8327,7 +8330,7 @@ def rank(
 
         coord_names = set(self.coords)
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
         attrs = self.attrs if keep_attrs else None
         return self._replace(variables, coord_names, attrs=attrs)
 

xarray/core/datatree.py

@@ -428,7 +428,7 @@ def map(  # type: ignore[override]
         # Copied from xarray.Dataset so as not to call type(self), which causes problems (see https://github.com/xarray-contrib/datatree/issues/188).
         # TODO Refactor xarray upstream to avoid needing to overwrite this.
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
         variables = {
             k: maybe_wrap_array(v, func(v, *args, **kwargs))
             for k, v in self.data_vars.items()

xarray/core/variable.py

@@ -1741,8 +1741,8 @@ def reduce(  # type: ignore[override]
             the reduction is calculated over the flattened array (by calling
             `func(x)` without an axis argument).
         keep_attrs : bool, optional
-            If True, the variable's attributes (`attrs`) will be copied from
-            the original object to the new one.  If False (default), the new
+            If True (default), the variable's attributes (`attrs`) will be copied from
+            the original object to the new one.  If False, the new
             object will be returned without attributes.
         keepdims : bool, default: False
             If True, the dimensions which are reduced are left in the result
@@ -1757,7 +1757,7 @@ def reduce(  # type: ignore[override]
             removed.
         """
         keep_attrs_ = (
-            _get_keep_attrs(default=False) if keep_attrs is None else keep_attrs
+            _get_keep_attrs(default=True) if keep_attrs is None else keep_attrs
         )
 
         # Note that the call order for Variable.mean is
@@ -2009,7 +2009,7 @@ def quantile(
             _quantile_func = duck_array_ops.quantile
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         scalar = utils.is_scalar(q)
         q = np.atleast_1d(np.asarray(q, dtype=np.float64))
@@ -2350,7 +2350,7 @@ def isnull(self, keep_attrs: bool | None = None):
         from xarray.computation.apply_ufunc import apply_ufunc
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         return apply_ufunc(
             duck_array_ops.isnull,
@@ -2384,7 +2384,7 @@ def notnull(self, keep_attrs: bool | None = None):
         from xarray.computation.apply_ufunc import apply_ufunc
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
 
         return apply_ufunc(
             duck_array_ops.notnull,
@@ -2435,7 +2435,7 @@ def _binary_op(self, other, f, reflexive=False):
             other_data, self_data, dims = _broadcast_compat_data(other, self)
         else:
             self_data, other_data, dims = _broadcast_compat_data(self, other)
-        keep_attrs = _get_keep_attrs(default=False)
+        keep_attrs = _get_keep_attrs(default=True)
         attrs = self._attrs if keep_attrs else None
         with np.errstate(all="ignore"):
             new_data = (
@@ -2526,7 +2526,7 @@ def _unravel_argminmax(
         }
 
         if keep_attrs is None:
-            keep_attrs = _get_keep_attrs(default=False)
+            keep_attrs = _get_keep_attrs(default=True)
         if keep_attrs:
             for v in result.values():
                 v.attrs = self.attrs

xarray/tests/test_coarsen.py

@@ -100,7 +100,7 @@ def test_coarsen_keep_attrs(funcname, argument) -> None:
         attrs=global_attrs,
     )
 
-    # attrs are now kept per default
+    # attrs are kept by default
     func = getattr(ds.coarsen(dim={"coord": 5}), funcname)
     result = func(*argument)
     assert result.attrs == global_attrs
@@ -199,7 +199,7 @@ def test_coarsen_da_keep_attrs(funcname, argument) -> None:
         name="name",
     )
 
-    # attrs are now kept per default
+    # attrs are kept by default
     func = getattr(da.coarsen(dim={"coord": 5}), funcname)
     result = func(*argument)
     assert result.attrs == attrs_da

xarray/tests/test_computation.py

@@ -694,10 +694,7 @@ def test_broadcast_compat_data_2d() -> None:
 
 def test_keep_attrs() -> None:
     def add(a, b, keep_attrs):
-        if keep_attrs:
-            return apply_ufunc(operator.add, a, b, keep_attrs=keep_attrs)
-        else:
-            return apply_ufunc(operator.add, a, b)
+        return apply_ufunc(operator.add, a, b, keep_attrs=keep_attrs)
 
     a = xr.DataArray([0, 1], [("x", [0, 1])])
     a.attrs["attr"] = "da"
@@ -733,7 +730,7 @@ def add(a, b, keep_attrs):
         pytest.param(
             None,
             [{"a": 1}, {"a": 2}, {"a": 3}],
-            {},
+            {"a": 1},
             False,
             id="default",
         ),
@@ -802,7 +799,7 @@ def test_keep_attrs_strategies_variable(strategy, attrs, expected, error) -> Non
         pytest.param(
             None,
             [{"a": 1}, {"a": 2}, {"a": 3}],
-            {},
+            {"a": 1},
             False,
             id="default",
         ),
@@ -872,7 +869,7 @@ def test_keep_attrs_strategies_dataarray(strategy, attrs, expected, error) -> No
         pytest.param(
             None,
             [{"a": 1}, {"a": 2}, {"a": 3}],
-            {},
+            {"a": 1},
             False,
             id="default",
         ),
@@ -967,7 +964,7 @@ def test_keep_attrs_strategies_dataarray_variables(
         pytest.param(
             None,
             [{"a": 1}, {"a": 2}, {"a": 3}],
-            {},
+            {"a": 1},
             False,
             id="default",
         ),
@@ -1037,7 +1034,7 @@ def test_keep_attrs_strategies_dataset(strategy, attrs, expected, error) -> None
         pytest.param(
             None,
             [{"a": 1}, {"a": 2}, {"a": 3}],
-            {},
+            {"a": 1},
             False,
             id="default",
         ),