ENH: implement a base comparison operator (__eq__) for NDData

astropy/nddata/nddata.py

@@ -444,3 +444,26 @@
                 # setter).
                 value.parent_nddata = self
         self._uncertainty = value
+
+    def __eq__(self, other) -> bool:
+        def is_sequence(a) -> bool:
+            try:
+                len(a)
+            except TypeError:
+                return False
+            else:
+                return True
+
+        def seqeq(a, b) -> bool:
+            # return True iff a==b
+            if is_sequence(a) and is_sequence(b):
+                return bool(np.all(np.asanyarray(a) == np.asanyarray(b)))
+            else:
+                return a == b
+
+        # checks are ordered from cheapest to most expensive
+        return other is self or (
+            self.unit == other.unit
+            and seqeq(self.mask, other.mask)
+            and seqeq(self.data, other.data)
+        )

astropy/nddata/tests/test_nddata.py

@@ -707,3 +707,28 @@ def test_collapse(mask, unit, propagate_uncertainties, operation_ignores_mask):
             # as the data array, so we can just check for equality:
             if method in ext_methods and propagate_uncertainties:
                 assert np.ma.all(np.ma.equal(astropy_method, nddata_method))
+
+
+@pytest.mark.parametrize(
+    "nd1, nd2, expected",
+    [
+        (NDData([1]), None, True),
+        (NDData([1], mask=True), None, True),
+        (NDData([1], mask=False), None, True),
+        (NDData([1], unit=u.J), None, True),
+        (NDData([1]), NDData([1]), True),
+        (NDData([1]), NDData([1], mask=True), False),
+        (NDData([1]), NDData([1], unit=u.J), False),
+        (NDData([1], mask=True), NDData([1], mask=True, unit=u.J), False),
+        (NDData([1], mask=False, unit=u.J), NDData([1], mask=True, unit=u.J), False),
+        (NDData([1], mask=True, unit=u.K), NDData([1], mask=True, unit=u.J), False),
+        (NDData([1], mask=True, unit=u.J), NDData([1], mask=True, unit=u.J), True),
+    ],
+)
+def test_nddata_eq(nd1, nd2, expected):
+    if nd2 is None:
+        # special case to check that we don't break default comparison
+        # __eq__(self, other) = lambda self, other: self is other
+        nd2 = nd1
+    assert (nd1 == nd2) is expected
+    assert (nd1 != nd2) ^ expected

docs/changes/15903.other.rst

@@ -0,0 +1 @@
+``NDData`` objects now supports comparison operators ``==`` and ``!=``.