[3.15] gh-152708: Fix AST generation asymmetry for unary positive in match statements (GH-152712) (#158079)

gh-152708: Fix AST generation asymmetry for unary positive in match statements (GH-152712)
(cherry picked from commit 7884f48)

Co-authored-by: Rishit Agnihotri <67520802+Twix1288@users.noreply.github.com>
Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: johnslavik <bartosz@ilikepython.com>

Author: 4 people

Grammar/python.gram

@@ -555,12 +555,12 @@ complex_number[expr_ty]:
 
 signed_number[expr_ty]:
     | NUMBER
-    | '+' number=NUMBER { number }
+    | '+' number=NUMBER { _PyAST_UnaryOp(UAdd, number, EXTRA) }
     | '-' number=NUMBER { _PyAST_UnaryOp(USub, number, EXTRA) }
 
 signed_real_number[expr_ty]:
     | real_number
-    | '+' real=real_number { real }
+    | '+' real=real_number { _PyAST_UnaryOp(UAdd, real, EXTRA) }
     | '-' real=real_number { _PyAST_UnaryOp(USub, real, EXTRA) }
 
 real_number[expr_ty]:

Lib/test/test_unparse.py

@@ -693,6 +693,24 @@ def test_unparse_interactive_integrity_3(self):
 
             self.check_src_roundtrip(src, out, mode='single')
 
+    @test.support.subTests('case', [
+        "case 'a string':",
+        "case True:",
+        "case _:",
+        "case [*_]:",
+        "case tuple():",
+        "case cls(arg, more=arg2):",
+        "case tuple() | list():",
+        "case [tuple() as obj, _, {'a': a, **more}]:",
+        "case -2:",
+        "case +2:",
+        "case 2 + 3j:",
+        "case +2 - 3j:",
+    ])
+    def test_unparse_match(self, case):
+        src = 'match x:\n    ' + case + '\n        pass'
+        self.check_src_roundtrip(src)
+
 
 class CosmeticTestCase(ASTTestCase):
     """Test if there are cosmetic issues caused by unnecessary additions"""

Misc/NEWS.d/next/Core_and_Builtins/2026-06-30-14-50-00.gh-issue-152708.aBcDeF.rst

@@ -0,0 +1 @@
+Fix AST generation asymmetry where unary positive ``+`` was implicitly dropped in pattern match expressions, restoring parsing parity with unary negative ``-``.

Parser/parser.c

@@ -426,11 +426,11 @@ ensure_literal_number(expr_ty exp, bool allow_real, bool allow_imaginary)
 }
 
 static int
-ensure_literal_negative(expr_ty exp, bool allow_real, bool allow_imaginary)
+ensure_literal_signed(expr_ty exp, bool allow_real, bool allow_imaginary)
 {
     assert(exp->kind == UnaryOp_kind);
-    // Must be negation ...
-    if (exp->v.UnaryOp.op != USub) {
+    // Must be negation or positive ...
+    if (exp->v.UnaryOp.op != USub && exp->v.UnaryOp.op != UAdd) {
         return 0;
     }
     // ... of a constant ...
@@ -461,7 +461,7 @@ ensure_literal_complex(expr_ty exp)
             }
             break;
         case UnaryOp_kind:
-            if (!ensure_literal_negative(left, /*real=*/true, /*imaginary=*/false)) {
+            if (!ensure_literal_signed(left, /*real=*/true, /*imaginary=*/false)) {
                 return 0;
             }
             break;
@@ -512,9 +512,9 @@ validate_pattern_match_value(expr_ty exp)
             // Constants and attribute lookups are always permitted
             return 1;
         case UnaryOp_kind:
-            // Negated numbers are permitted (whether real or imaginary)
+            // Signed numbers are permitted (whether real or imaginary)
             // Compiler will complain if AST folding doesn't create a constant
-            if (ensure_literal_negative(exp, /*real=*/true, /*imaginary=*/true)) {
+            if (ensure_literal_signed(exp, /*real=*/true, /*imaginary=*/true)) {
                 return 1;
             }
             break;

Python/ast.c

@@ -867,11 +867,11 @@ fold_const_match_patterns(expr_ty node, PyArena *ctx_, _PyASTPreprocessState *st
     {
         case UnaryOp_kind:
         {
-            if (node->v.UnaryOp.op == USub &&
+            if ((node->v.UnaryOp.op == USub || node->v.UnaryOp.op == UAdd) &&
                 node->v.UnaryOp.operand->kind == Constant_kind)
             {
                 PyObject *operand = node->v.UnaryOp.operand->v.Constant.value;
-                PyObject *folded = PyNumber_Negative(operand);
+                PyObject *folded = node->v.UnaryOp.op == USub ? PyNumber_Negative(operand) : PyNumber_Positive(operand);
                 return make_const(node, folded, ctx_);
             }
             break;