Riffle: insert a non-list separator only between elements

CHANGES.md

@@ -4,6 +4,10 @@
 
 Improve Monomial ordering.
 
+### Bugs Fixed
+
+1.  [\#2003](https://github.com/Mathics3/mathics-core/issues/2003) `Riffle[{a}, x]` gives `{a}`: a non-list separator is inserted only between elements
+
 ## 10.0.1
 
 April 18, 2026

mathics/builtin/import_export/importexport.py

@@ -778,15 +778,15 @@ class ExportString(Builtin):
 
     >> ExportString[{{1,2,3,4},{3},{2},{4}}, "CSV"]
      = 1,2,3,4
-     . 3,
-     . 2,
-     . 4,
+     . 3
+     . 2
+     . 4
 
     >> ExportString[{1,2,3,4}, "CSV"]
-     = 1,
-     . 2,
-     . 3,
-     . 4,
+     = 1
+     . 2
+     . 3
+     . 4
     >> ExportString[Integrate[f[x],{x,0,2}], "SVG"]//Head
      = String
     """

mathics/builtin/list/rearrange.py

@@ -1176,7 +1176,13 @@ def eval(self, list, sep, evaluation: Evaluation):
         if sep.has_form(SymbolList, None):
             result = riffle_lists(list.elements, sep.elements)
         else:
-            result = riffle_lists(list.elements, [sep])
+            # A non-list separator goes only between elements, never
+            # after the last one: Riffle[{a}, x] is {a}.
+            result = []
+            for i, element in enumerate(list.elements):
+                if i > 0:
+                    result.append(sep)
+                result.append(element)
 
         return list.restructure("List", result, evaluation, deps=(list, sep))
 

test/builtin/list/test_list.py

@@ -129,6 +129,17 @@
             "{{{1, 1, 1}, {1, 1, 2}, {1, 2, 1}, {1, 2, 2}}, {{2, 1, 1}, {2, 1, 2}, {2, 2, 1}, {2, 2, 2}}}",
             None,
         ),
+        # Riffle
+        (
+            "Riffle[{a}, x]",
+            None,
+            "{a}",
+            "non-list separator goes only between elements",
+        ),
+        ("Riffle[{a, b}, x]", None, "{a, x, b}", "Issue #2003"),
+        ("Riffle[{}, x]", None, "{}", "Degenerate Riffle on empty list"),
+        ("Riffle[{a}, {x}]", None, "{a, x}", "equal lengths: separator list appended"),
+        ("Riffle[{a, b}, {x, y}]", None, "{a, x, b, y}", None),
         # Union and Intersection
         (
             "Union[{1, -1, 2}, {-2, 3}, SameTest -> (Abs[#1] == Abs[#2] &)]",