gh-129515: Clarify syntax error messages for conditional expressions (#129880)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Doc/whatsnew/3.14.rst

@@ -175,6 +175,31 @@ Improved error messages
      ValueError: too many values to unpack (expected 3, got 4)
 
 
+* If a statement (:keyword:`pass`, :keyword:`del`, :keyword:`return`,
+  :keyword:`yield`, :keyword:`raise`, :keyword:`break`, :keyword:`continue`,
+  :keyword:`assert`, :keyword:`import`, :keyword:`from`) is passed to the
+  :ref:`if_expr` after :keyword:`else`, or one of :keyword:`pass`,
+  :keyword:`break`, or :keyword:`continue` is passed before :keyword:`if`, then the
+  error message highlights where the :token:`~python-grammar:expression` is
+  required. (Contributed by Sergey Miryanov in :gh:`129515`.)
+
+  .. code-block:: pycon
+
+     >>> x = 1 if True else pass
+     Traceback (most recent call last):
+       File "<string>", line 1
+         x = 1 if True else pass
+                            ^^^^
+     SyntaxError: expected expression after 'else', but statement is given
+
+     >>> x = continue if True else break
+     Traceback (most recent call last):
+       File "<string>", line 1
+         x = continue if True else break
+             ^^^^^^^^
+     SyntaxError: expected expression before 'if', but statement is given
+
+
 * When incorrectly closed strings are detected, the error message suggests
   that the string may be intended to be part of the string. (Contributed by
   Pablo Galindo in :gh:`88535`.)

Grammar/python.gram

@@ -117,12 +117,12 @@ simple_stmt[stmt_ty] (memo):
     | &'return' return_stmt
     | &('import' | 'from') import_stmt
     | &'raise' raise_stmt
-    | 'pass' { _PyAST_Pass(EXTRA) }
+    | &'pass' pass_stmt
     | &'del' del_stmt
     | &'yield' yield_stmt
     | &'assert' assert_stmt
-    | 'break' { _PyAST_Break(EXTRA) }
-    | 'continue' { _PyAST_Continue(EXTRA) }
+    | &'break' break_stmt
+    | &'continue' continue_stmt
     | &'global' global_stmt
     | &'nonlocal' nonlocal_stmt
 
@@ -181,6 +181,15 @@ raise_stmt[stmt_ty]:
     | 'raise' a=expression b=['from' z=expression { z }] { _PyAST_Raise(a, b, EXTRA) }
     | 'raise' { _PyAST_Raise(NULL, NULL, EXTRA) }
 
+pass_stmt[stmt_ty]:
+    | 'pass' { _PyAST_Pass(EXTRA) }
+
+break_stmt[stmt_ty]:
+    | 'break' { _PyAST_Break(EXTRA) }
+
+continue_stmt[stmt_ty]:
+    | 'continue' { _PyAST_Continue(EXTRA) }
+
 global_stmt[stmt_ty]: 'global' a[asdl_expr_seq*]=','.NAME+ {
     _PyAST_Global(CHECK(asdl_identifier_seq*, _PyPegen_map_names_to_ids(p, a)), EXTRA) }
 
@@ -1187,6 +1196,10 @@ invalid_expression:
         _PyPegen_check_legacy_stmt(p, a) ? NULL : p->tokens[p->mark-1]->level == 0 ? NULL :
         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") }
+   | a=disjunction 'if' b=disjunction 'else' !expression {
+        RAISE_SYNTAX_ERROR_ON_NEXT_TOKEN("expected expression after 'else', but statement is given") }
+   | a[stmt_ty]=(pass_stmt|break_stmt|continue_stmt) 'if' b=disjunction 'else' c=simple_stmt {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION (a, "expected expression before 'if', but statement is given") }
    | a='lambda' [lambda_params] b=':' &FSTRING_MIDDLE  {
         RAISE_SYNTAX_ERROR_KNOWN_RANGE(a, b, "f-string: lambda expressions are not allowed without parentheses") }
 

Lib/test/test_syntax.py

@@ -168,6 +168,18 @@ SyntaxError: expected 'else' after 'if' expression
 Traceback (most recent call last):
 SyntaxError: expected 'else' after 'if' expression
 
+>>> x = 1 if 1 else pass
+Traceback (most recent call last):
+SyntaxError: expected expression after 'else', but statement is given
+
+>>> x = pass if 1 else 1
+Traceback (most recent call last):
+SyntaxError: expected expression before 'if', but statement is given
+
+>>> x = pass if 1 else pass
+Traceback (most recent call last):
+SyntaxError: expected expression before 'if', but statement is given
+
 >>> if True:
 ...     print("Hello"
 ...
@@ -2863,6 +2875,44 @@ while 1:
             end_offset=15 + len("obj.attr"),
         )
 
+    def test_ifexp_else_stmt(self):
+        msg = "expected expression after 'else', but statement is given"
+
+        for stmt in [
+            "pass",
+            "return",
+            "return 2",
+            "raise Exception('a')",
+            "del a",
+            "yield 2",
+            "assert False",
+            "break",
+            "continue",
+            "import",
+            "import ast",
+            "from",
+            "from ast import *"
+        ]:
+            self._check_error(f"x = 1 if 1 else {stmt}", msg)
+
+    def test_ifexp_body_stmt_else_expression(self):
+        msg = "expected expression before 'if', but statement is given"
+
+        for stmt in [
+            "pass",
+            "break",
+            "continue"
+        ]:
+            self._check_error(f"x = {stmt} if 1 else 1", msg)
+
+    def test_ifexp_body_stmt_else_stmt(self):
+        msg = "expected expression before 'if', but statement is given"
+        for lhs_stmt, rhs_stmt in [
+            ("pass", "pass"),
+            ("break", "pass"),
+            ("continue", "import ast")
+        ]:
+            self._check_error(f"x = {lhs_stmt} if 1 else {rhs_stmt}", msg)
 
 def load_tests(loader, tests, pattern):
     tests.addTest(doctest.DocTestSuite())

Misc/NEWS.d/next/Core_and_Builtins/2025-02-09-11-30-38.gh-issue-129515.3L3vmo.rst

@@ -0,0 +1,2 @@
+Clarify syntax error messages for conditional expressions when a statement
+is specified before an :keyword:`if` or after an :keyword:`else` keyword.