fix(substitute): allow mappings with None keys

We don't need to sort this dictionary anymore.

ibis/backends/tests/test_generic.py

@@ -1608,3 +1608,19 @@ def test_sample_with_seed(backend):
     df1 = expr.to_pandas()
     df2 = expr.to_pandas()
     backend.assert_frame_equal(df1, df2)
+
+
+@pytest.mark.broken(
+    ["dask"], reason="implementation somehow differs from pandas", raises=ValueError
+)
+def test_substitute(backend):
+    val = "400"
+    t = backend.functional_alltypes
+    expr = (
+        t.string_col.nullif("1")
+        .substitute({None: val})
+        .name("subs")
+        .value_counts()
+        .filter(lambda t: t.subs == val)
+    )
+    assert expr["subs_count"].execute()[0] == t.count().execute() // 10

ibis/expr/types/generic.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import contextlib
 from typing import TYPE_CHECKING, Any, Iterable, Literal, Sequence
 
 from public import public
@@ -683,12 +684,18 @@ def substitute(
         │ torg   │           52 │
         └────────┴──────────────┘
         """
-        expr = self.case()
         if isinstance(value, dict):
-            for k, v in sorted(value.items()):
-                expr = expr.when(k, v)
+            expr = ibis.case()
+            try:
+                null_replacement = value.pop(None)
+            except KeyError:
+                pass
+            else:
+                expr = expr.when(self.isnull(), null_replacement)
+            for k, v in value.items():
+                expr = expr.when(self == k, v)
         else:
-            expr = expr.when(value, replacement)
+            expr = self.case().when(value, replacement)
 
         return expr.else_(else_ if else_ is not None else self).end()
 

ibis/tests/expr/test_value_exprs.py

@@ -792,13 +792,21 @@ def test_substitute_dict():
 
     result = table.foo.substitute(subs)
     expected = (
-        table.foo.case().when("a", "one").when("b", table.bar).else_(table.foo).end()
+        ibis.case()
+        .when(table.foo == "a", "one")
+        .when(table.foo == "b", table.bar)
+        .else_(table.foo)
+        .end()
     )
     assert_equal(result, expected)
 
     result = table.foo.substitute(subs, else_=ibis.NA)
     expected = (
-        table.foo.case().when("a", "one").when("b", table.bar).else_(ibis.NA).end()
+        ibis.case()
+        .when(table.foo == "a", "one")
+        .when(table.foo == "b", table.bar)
+        .else_(ibis.NA)
+        .end()
     )
     assert_equal(result, expected)