add Variable._replace (#3528)

* add Variable._replace

* assertions

* whatsew

* whatsnew

doc/whats-new.rst

@@ -138,6 +138,9 @@ Internal Changes
 - Enable type checking on default sentinel values (:pull:`3472`)
   By `Maximilian Roos <https://github.com/max-sixty>`_
 
+- Add :py:meth:`Variable._replace` for simpler replacing of a subset of attributes (:pull:`3472`)
+  By `Maximilian Roos <https://github.com/max-sixty>`_
+
 .. _whats-new.0.14.0:
 
 v0.14.0 (14 Oct 2019)

xarray/core/variable.py

@@ -1,3 +1,4 @@
+import copy
 import functools
 import itertools
 import warnings
@@ -24,10 +25,11 @@
 from .pycompat import dask_array_type, integer_types
 from .utils import (
     OrderedSet,
+    _default,
     decode_numpy_dict_values,
     either_dict_or_kwargs,
-    infix_dims,
     ensure_us_time_resolution,
+    infix_dims,
 )
 
 try:
@@ -887,7 +889,20 @@ def copy(self, deep=True, data=None):
         # note:
         # dims is already an immutable tuple
         # attributes and encoding will be copied when the new Array is created
-        return type(self)(self.dims, data, self._attrs, self._encoding, fastpath=True)
+        return self._replace(data=data)
+
+    def _replace(
+        self, dims=_default, data=_default, attrs=_default, encoding=_default
+    ) -> "Variable":
+        if dims is _default:
+            dims = copy.copy(self._dims)
+        if data is _default:
+            data = copy.copy(self.data)
+        if attrs is _default:
+            attrs = copy.copy(self._attrs)
+        if encoding is _default:
+            encoding = copy.copy(self._encoding)
+        return type(self)(dims, data, attrs, encoding, fastpath=True)
 
     def __copy__(self):
         return self.copy(deep=False)

xarray/tests/test_variable.py

@@ -542,6 +542,15 @@ def test_copy_index_with_data_errors(self):
         with raises_regex(ValueError, "must match shape of object"):
             orig.copy(data=new_data)
 
+    def test_replace(self):
+        var = Variable(("x", "y"), [[1.5, 2.0], [3.1, 4.3]], {"foo": "bar"})
+        result = var._replace()
+        assert_identical(result, var)
+
+        new_data = np.arange(4).reshape(2, 2)
+        result = var._replace(data=new_data)
+        assert_array_equal(result.data, new_data)
+
     def test_real_and_imag(self):
         v = self.cls("x", np.arange(3) - 1j * np.arange(3), {"foo": "bar"})
         expected_re = self.cls("x", np.arange(3), {"foo": "bar"})