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Keep str's concat error when __radd__ declines #8652

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Merged
youknowone merged 1 commit into from
Sep 5, 2026
Merged

Keep str's concat error when __radd__ declines #8652

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13 changes: 10 additions & 3 deletions crates/vm/src/builtins/str.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -653,10 +653,17 @@ impl PyStr {
Self::new_str_unchecked(bytes.into(), kind)
}
.to_pyobject(vm))
} else if let Some(radd) = vm.get_method(other.clone(), identifier!(vm, __radd__)) {
// hack to get around not distinguishing number add from seq concat
radd?.call((zelf,), vm)
} else {
// hack to get around not distinguishing number add from seq concat
if let Some(radd) = vm.get_method(other.clone(), identifier!(vm, __radd__)) {
let result = radd?.call((zelf,), vm)?;
// CPython reaches `str`'s sq_concat once the reflected call declines, so a
// `__radd__` returning NotImplemented must still report the concat error
// rather than the generic binary-op one.
if !result.is(&vm.ctx.not_implemented) {
return Ok(result);
}
}
Err(vm.new_type_error(format!(
r#"can only concatenate str (not "{}") to str"#,
other.class().slot_name()
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36 changes: 36 additions & 0 deletions extra_tests/snippets/builtin_str.py
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Expand Up @@ -979,3 +979,39 @@ def test_expandtabs_zero_tabsize():


test_expandtabs_zero_tabsize()


def test_concat_error_message():
# CPython falls back to str's sequence concatenation once a reflected
# __radd__ declines, so the message names the concatenation, not the
# generic binary operation. Types that define __radd__ (every number)
# used to take the generic path here.
def message(other):
try:
"a" + other
except TypeError as err:
return str(err)
raise AssertionError("TypeError was not raised")

assert message(1) == 'can only concatenate str (not "int") to str'
assert message(1.5) == 'can only concatenate str (not "float") to str'
assert message(True) == 'can only concatenate str (not "bool") to str'
assert message([]) == 'can only concatenate str (not "list") to str'
assert message(None) == 'can only concatenate str (not "NoneType") to str'
assert message(b"b") == 'can only concatenate str (not "bytes") to str'

class Declines:
def __radd__(self, other):
return NotImplemented

assert message(Declines()) == 'can only concatenate str (not "Declines") to str'

# A __radd__ that returns a value still wins.
class Accepts:
def __radd__(self, other):
return "from-radd"

assert "a" + Accepts() == "from-radd"


test_concat_error_message()
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