[3.10] bpo-44792: Improve syntax errors for if expressions (GH-27506)

Grammar/python.gram

@@ -859,6 +859,7 @@ invalid_expression:
     # Soft keywords need to also be ignored because they can be parsed as NAME NAME
    | !(NAME STRING | SOFT_KEYWORD) a=disjunction b=expression_without_invalid {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "invalid syntax. Perhaps you forgot a comma?") }
+   | a=disjunction 'if' b=disjunction !'else' { RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "expected 'else' after 'if' expression") }
 
 invalid_named_expression:
     | a=expression ':=' expression {

Lib/test/test_syntax.py

@@ -140,6 +140,18 @@
 Traceback (most recent call last):
 SyntaxError: cannot assign to conditional expression
 
+>>> a = 42 if True
+Traceback (most recent call last):
+SyntaxError: expected 'else' after 'if' expression
+
+>>> a = (42 if True)
+Traceback (most recent call last):
+SyntaxError: expected 'else' after 'if' expression
+
+>>> a = [1, 42 if True, 4]
+Traceback (most recent call last):
+SyntaxError: expected 'else' after 'if' expression
+
 >>> True = True = 3
 Traceback (most recent call last):
 SyntaxError: cannot assign to True

Misc/ACKS

@@ -227,6 +227,7 @@ Tom Bridgman
 Anthony Briggs
 Keith Briggs
 Tobias Brink
+Miguel Brito
 Dillon Brock
 Richard Brodie
 Michael Broghton

Misc/NEWS.d/next/Core and Builtins/2021-07-31-12-12-57.bpo-44792.mOReTW.rst

@@ -0,0 +1 @@
+Improve syntax errors for if expressions. Patch by Miguel Brito

Parser/parser.c

@@ -18229,6 +18229,7 @@ invalid_legacy_expression_rule(Parser *p)
 // invalid_expression:
 //     | invalid_legacy_expression
 //     | !(NAME STRING | SOFT_KEYWORD) disjunction expression_without_invalid
+//     | disjunction 'if' disjunction !'else'
 static void *
 invalid_expression_rule(Parser *p)
 {
@@ -18287,6 +18288,38 @@ invalid_expression_rule(Parser *p)
         D(fprintf(stderr, "%*c%s invalid_expression[%d-%d]: %s failed!\n", p->level, ' ',
                   p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "!(NAME STRING | SOFT_KEYWORD) disjunction expression_without_invalid"));
     }
+    { // disjunction 'if' disjunction !'else'
+        if (p->error_indicator) {
+            D(p->level--);
+            return NULL;
+        }
+        D(fprintf(stderr, "%*c> invalid_expression[%d-%d]: %s\n", p->level, ' ', _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+        Token * _keyword;
+        expr_ty a;
+        expr_ty b;
+        if (
+            (a = disjunction_rule(p))  // disjunction
+            &&
+            (_keyword = _PyPegen_expect_token(p, 510))  // token='if'
+            &&
+            (b = disjunction_rule(p))  // disjunction
+            &&
+            _PyPegen_lookahead_with_int(0, _PyPegen_expect_token, p, 516)  // token='else'
+        )
+        {
+            D(fprintf(stderr, "%*c+ invalid_expression[%d-%d]: %s succeeded!\n", p->level, ' ', _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+            _res = RAISE_SYNTAX_ERROR_KNOWN_RANGE ( a , b , "expected 'else' after 'if' expression" );
+            if (_res == NULL && PyErr_Occurred()) {
+                p->error_indicator = 1;
+                D(p->level--);
+                return NULL;
+            }
+            goto done;
+        }
+        p->mark = _mark;
+        D(fprintf(stderr, "%*c%s invalid_expression[%d-%d]: %s failed!\n", p->level, ' ',
+                  p->error_indicator ? "ERROR!" : "-", _mark, p->mark, "disjunction 'if' disjunction !'else'"));
+    }
     _res = NULL;
   done:
     D(p->level--);