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gh-131798: Narrow the return type of _FORMAT_SIMPLE and _FORMAT_WITH_SPEC to str #146639

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gh-131798: Narrow the return type of _FORMAT_SIMPLE and _FORMAT_WITH_SPEC to str #146639

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38 changes: 38 additions & 0 deletions Lib/test/test_capi/test_opt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2263,6 +2263,44 @@ def testfunc(n):
self.assertNotIn("_GUARD_TOS_UNICODE", uops)
self.assertIn("_BINARY_OP_ADD_UNICODE", uops)

def test_format_simple_narrows_to_str(self):
def testfunc(n):
x = []
for _ in range(n):
v = 42
s = f"{v}"
t = "hello" + s
x.append(t)
return x

res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
self.assertEqual(res, ["hello42"] * TIER2_THRESHOLD)
self.assertIsNotNone(ex)
uops = get_opnames(ex)

self.assertIn("_FORMAT_SIMPLE", uops)
self.assertNotIn("_GUARD_TOS_UNICODE", uops)
self.assertIn("_BINARY_OP_ADD_UNICODE", uops)

def test_format_with_spec_narrows_to_str(self):
def testfunc(n):
x = []
for _ in range(n):
v = 3.14
s = f"{v:.2f}"
t = "pi=" + s
x.append(t)
return x

res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
self.assertEqual(res, ["pi=3.14"] * TIER2_THRESHOLD)
self.assertIsNotNone(ex)
uops = get_opnames(ex)

self.assertIn("_FORMAT_WITH_SPEC", uops)
self.assertNotIn("_GUARD_TOS_UNICODE", uops)
self.assertIn("_BINARY_OP_ADD_UNICODE", uops)

def test_binary_op_subscr_str_int(self):
def testfunc(n):
x = 0
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-03-30-17-01-34.gh-issue-131798.WSefcr.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Allow the JIT to remove unicode guards after ``_FORMAT_SIMPLE`` and
``_FORMAT_WITH_SPEC`` by setting the return type to string.
8 changes: 8 additions & 0 deletions Python/optimizer_bytecodes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1551,6 +1551,14 @@ dummy_func(void) {
set = sym_new_type(ctx, &PySet_Type);
}

op(_FORMAT_SIMPLE, (value -- res)) {
res = sym_new_type(ctx, &PyUnicode_Type);
}

op(_FORMAT_WITH_SPEC, (value, fmt_spec -- res)) {
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@eendebakpt eendebakpt Mar 30, 2026

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This can return a subclass, e.g.

class my(str):
    def __format__(self, spec):
        return self
m=my('aap')
type(f'{m}')

Does the sym_new_type(ctx, &PyUnicode_Type); not guarantee we have an exact type?

res = sym_new_type(ctx, &PyUnicode_Type);
}

op(_SET_UPDATE, (set, unused[oparg-1], iterable -- set, unused[oparg-1], i)) {
(void)set;
i = iterable;
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4 changes: 2 additions & 2 deletions Python/optimizer_cases.c.h
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