feat(api): make deferred objects hashable by their identity

ibis/common/deferred.py

@@ -104,6 +104,9 @@ def __bool__(self):
             f"The truth value of {self.__class__.__name__} objects is not defined"
         )
 
+    def __hash__(self):
+        return id(self)
+
     def __getitem__(self, name):
         return Deferred(Item(self, name))
 

ibis/common/tests/test_deferred.py

@@ -202,10 +202,21 @@ def test_deferred_supports_string_arguments():
     assert b.resolve({}) == "3.14"
 
 
-def test_deferred_object_are_not_hashable():
-    # since __eq__ is overloaded, Deferred objects are not hashable
-    with pytest.raises(TypeError, match="unhashable type"):
-        hash(_.a)
+def test_deferred_objects_are_hashable():
+    a = _.a
+    assert hash(a) == hash(a)
+
+    # Works in dicts
+    d = {a: 1, _.b: 2}
+    assert d[a] == 1
+    assert _.c not in d
+    assert _.a not in d  # hash by identity only
+
+    # Works in sets
+    s = {a, a, _.b}
+    assert len(s) == 2
+    assert a in s
+    assert _.a not in s  # hash by identity only
 
 
 def test_deferred_const():
@@ -541,11 +552,6 @@ def myfunc(x):
     assert repr(myfunc(_.a)) == "<test>"
 
 
-def test_deferred_set_raises():
-    with pytest.raises(TypeError, match="unhashable type"):
-        {_.a, _.b}  # noqa: B018
-
-
 @pytest.mark.parametrize(
     "case",
     [