gh-93691: fix too broad source locations of for statement iterators (#120330)

Author: iritkatriel

Lib/test/test_compiler_codegen.py

@@ -49,6 +49,7 @@ def test_for_loop(self):
             ('GET_ITER', None, 1),
             loop_lbl := self.Label(),
             ('FOR_ITER', exit_lbl := self.Label(), 1),
+            ('NOP', None, 1, 1),
             ('STORE_NAME', 1, 1),
             ('LOAD_NAME', 2, 2),
             ('PUSH_NULL', None, 2),

Lib/test/test_iter.py

@@ -10,6 +10,7 @@
 import functools
 import contextlib
 import builtins
+import traceback
 
 # Test result of triple loop (too big to inline)
 TRIPLETS = [(0, 0, 0), (0, 0, 1), (0, 0, 2),
@@ -1143,6 +1144,51 @@ def test_error_iter(self):
             self.assertRaises(TypeError, iter, typ())
         self.assertRaises(ZeroDivisionError, iter, BadIterableClass())
 
+    def test_exception_locations(self):
+        # The location of an exception raised from __init__ or
+        # __next__ should should be the iterator expression
+
+        class Iter:
+            def __init__(self, init_raises=False, next_raises=False):
+                if init_raises:
+                    1/0
+                self.next_raises = next_raises
+
+            def __next__(self):
+                if self.next_raises:
+                    1/0
+
+            def __iter__(self):
+                return self
+
+        def init_raises():
+            try:
+                for x in Iter(init_raises=True):
+                    pass
+            except Exception as e:
+                return e
+
+        def next_raises():
+            try:
+                for x in Iter(next_raises=True):
+                    pass
+            except Exception as e:
+                return e
+
+        for func, expected in [(init_raises, "Iter(init_raises=True)"),
+                               (next_raises, "Iter(next_raises=True)"),
+                              ]:
+            with self.subTest(func):
+                exc = func()
+                f = traceback.extract_tb(exc.__traceback__)[0]
+                indent = 16
+                co = func.__code__
+                self.assertEqual(f.lineno, co.co_firstlineno + 2)
+                self.assertEqual(f.end_lineno, co.co_firstlineno + 2)
+                self.assertEqual(f.line[f.colno - indent : f.end_colno - indent],
+                                 expected)
+
+
 
 if __name__ == "__main__":
     unittest.main()

Lib/test/test_sys_settrace.py

@@ -1650,15 +1650,15 @@ def func():
         EXPECTED_EVENTS = [
             (0, 'call'),
             (2, 'line'),
-            (1, 'line'),
             (-3, 'call'),
             (-2, 'line'),
             (-2, 'return'),
-            (4, 'line'),
             (1, 'line'),
+            (4, 'line'),
+            (2, 'line'),
             (-2, 'call'),
             (-2, 'return'),
-            (1, 'return'),
+            (2, 'return'),
         ]
 
         # C level events should be the same as expected and the same as Python level.

Misc/NEWS.d/next/Core and Builtins/2024-06-10-22-30-26.gh-issue-93691.68WOTS.rst

@@ -0,0 +1,2 @@
+Fix source locations of instructions generated for the iterator of a for
+statement.

Programs/test_frozenmain.h

@@ -3025,11 +3025,18 @@ compiler_for(struct compiler *c, stmt_ty s)
     RETURN_IF_ERROR(compiler_push_fblock(c, loc, FOR_LOOP, start, end, NULL));
 
     VISIT(c, expr, s->v.For.iter);
+
+    loc = LOC(s->v.For.iter);
     ADDOP(c, loc, GET_ITER);
 
     USE_LABEL(c, start);
     ADDOP_JUMP(c, loc, FOR_ITER, cleanup);
 
+    /* Add NOP to ensure correct line tracing of multiline for statements.
+     * It will be removed later if redundant.
+     */
+    ADDOP(c, LOC(s->v.For.target), NOP);
+
     USE_LABEL(c, body);
     VISIT(c, expr, s->v.For.target);
     VISIT_SEQ(c, stmt, s->v.For.body);