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gh-149498: Fix dis.FOR_ITER reporting wrong exhausted-path jump target #149503

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gh-149498: Fix dis.FOR_ITER reporting wrong exhausted-path jump target #149503

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gh-149498: Fix dis.FOR_ITER reporting wrong exhausted-path jump target
P403n1x87 May 7, 2026
f11a6f0
Merge branch 'main' into gh-149498-fix-for-iter-jump-target
StanFromIreland May 7, 2026
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6 changes: 6 additions & 0 deletions Lib/dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -352,6 +352,10 @@ def _get_jump_target(op, arg, offset):
arg = -arg
target = offset + 2 + arg*2
target += 2 * caches
# FOR_ITER uses JUMPBY(oparg + 1) at runtime to land on POP_ITER,
# skipping END_FOR. Add the extra 2 bytes to match the actual target.
if deop == FOR_ITER:
target += 2
elif deop in hasjabs:
target = arg*2
else:
Expand Down Expand Up @@ -569,6 +573,8 @@ def offset_from_jump_arg(self, op, arg, offset):
argval = offset + 2 + signed_arg*2
caches = _get_cache_size(_all_opname[deop])
argval += 2 * caches
if deop == FOR_ITER:
argval += 2
return argval
return None

Expand Down
29 changes: 18 additions & 11 deletions Lib/test/test_dis.py
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Original file line number Diff line number Diff line change
Expand Up @@ -180,8 +180,8 @@ def bug708901():

%3d JUMP_BACKWARD 5 (to L1)

%3d L2: END_FOR
POP_ITER
%3d END_FOR
L2: POP_ITER
LOAD_COMMON_CONSTANT 7 (None)
RETURN_VALUE
""" % (bug708901.__code__.co_firstlineno,
Expand Down Expand Up @@ -849,8 +849,8 @@ def foo(x):
L1: FOR_ITER 3 (to L2)
LIST_APPEND 3
JUMP_BACKWARD 5 (to L1)
L2: END_FOR
POP_ITER
END_FOR
L2: POP_ITER
RETURN_VALUE
L3: PUSH_NULL
LOAD_FAST_BORROW 0 (x)
Expand Down Expand Up @@ -893,8 +893,8 @@ def foo(x):
RESUME 9
POP_TOP
JUMP_BACKWARD 17 (to L2)
L3: END_FOR
POP_ITER
END_FOR
L3: POP_ITER
LOAD_COMMON_CONSTANT 7 (None)
RETURN_VALUE

Expand Down Expand Up @@ -947,8 +947,8 @@ def loop_test():
POP_TOP
JUMP_BACKWARD_{: <6} 16 (to L1)

%3d L2: END_FOR
POP_ITER
%3d END_FOR
L2: POP_ITER
LOAD_COMMON_CONSTANT 7 (None)
RETURN_VALUE
""" % (loop_test.__code__.co_firstlineno,
Expand Down Expand Up @@ -1858,7 +1858,7 @@ def _prepare_test_cases():
make_inst(opname='LOAD_SMALL_INT', arg=10, argval=10, argrepr='', offset=14, start_offset=14, starts_line=False, line_number=3),
make_inst(opname='CALL', arg=1, argval=1, argrepr='', offset=16, start_offset=16, starts_line=False, line_number=3, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
make_inst(opname='GET_ITER', arg=0, argval=0, argrepr='', offset=24, start_offset=24, starts_line=False, line_number=3, cache_info=[('counter', 1, b'\x00\x00')]),
make_inst(opname='FOR_ITER', arg=33, argval=98, argrepr='to L4', offset=28, start_offset=28, starts_line=False, line_number=3, label=1, cache_info=[('counter', 1, b'\x00\x00')]),
make_inst(opname='FOR_ITER', arg=33, argval=100, argrepr='to L4', offset=28, start_offset=28, starts_line=False, line_number=3, label=1, cache_info=[('counter', 1, b'\x00\x00')]),
make_inst(opname='STORE_FAST', arg=0, argval='i', argrepr='i', offset=32, start_offset=32, starts_line=False, line_number=3),
make_inst(opname='LOAD_GLOBAL', arg=3, argval='print', argrepr='print + NULL', offset=34, start_offset=34, starts_line=True, line_number=4, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_FAST_BORROW', arg=0, argval='i', argrepr='i', offset=44, start_offset=44, starts_line=False, line_number=4),
Expand All @@ -1879,8 +1879,8 @@ def _prepare_test_cases():
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=92, start_offset=92, starts_line=True, line_number=8, label=3),
make_inst(opname='POP_TOP', arg=None, argval=None, argrepr='', offset=94, start_offset=94, starts_line=False, line_number=8),
make_inst(opname='JUMP_FORWARD', arg=13, argval=124, argrepr='to L5', offset=96, start_offset=96, starts_line=False, line_number=8),
make_inst(opname='END_FOR', arg=None, argval=None, argrepr='', offset=98, start_offset=98, starts_line=True, line_number=3, label=4),
make_inst(opname='POP_ITER', arg=None, argval=None, argrepr='', offset=100, start_offset=100, starts_line=False, line_number=3),
make_inst(opname='END_FOR', arg=None, argval=None, argrepr='', offset=98, start_offset=98, starts_line=True, line_number=3),
make_inst(opname='POP_ITER', arg=None, argval=None, argrepr='', offset=100, start_offset=100, starts_line=False, line_number=3, label=4),
make_inst(opname='LOAD_GLOBAL', arg=3, argval='print', argrepr='print + NULL', offset=102, start_offset=102, starts_line=True, line_number=10, cache_info=[('counter', 1, b'\x00\x00'), ('index', 1, b'\x00\x00'), ('module_keys_version', 1, b'\x00\x00'), ('builtin_keys_version', 1, b'\x00\x00')]),
make_inst(opname='LOAD_CONST', arg=1, argval='I can haz else clause?', argrepr="'I can haz else clause?'", offset=112, start_offset=112, starts_line=False, line_number=10),
make_inst(opname='CALL', arg=1, argval=1, argrepr='', offset=114, start_offset=114, starts_line=False, line_number=10, cache_info=[('counter', 1, b'\x00\x00'), ('func_version', 2, b'\x00\x00\x00\x00')]),
Expand Down Expand Up @@ -2200,6 +2200,13 @@ def test_jump_target(self):
positions=None)
self.assertEqual(10 + 2 + 1*2 + 100*2, instruction.jump_target)

# FOR_ITER uses JUMPBY(oparg + 1) at runtime, skipping END_FOR to land
# on POP_ITER; the reported target must reflect the extra +2 bytes.
instruction = make_inst(opname="FOR_ITER", arg=delta, argval=delta,
argrepr='', offset=10, start_offset=10, starts_line=True, line_number=1, label=None,
positions=None)
self.assertEqual(10 + 2 + 1*2 + 100*2 + 2, instruction.jump_target)

def test_argval_argrepr(self):
def f(opcode, oparg, offset, *init_args):
arg_resolver = dis.ArgResolver(*init_args)
Expand Down
4 changes: 4 additions & 0 deletions ...d/next/Library/2026-05-07-17-38-44.gh-issue-149498.fix-for-iter-jump-target.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Fix :func:`dis.get_instructions` reporting the wrong ``jump_target`` for
:opcode:`FOR_ITER`. The exhausted-iterator path now correctly points to
:opcode:`POP_ITER` rather than :opcode:`END_FOR`, matching the runtime
behaviour of ``JUMPBY(oparg + 1)``.
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