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gh-132661: Disallow Template/str concatenation after PEP 750 spec update #135996

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gh-132661: Disallow Template/str concatenation after PEP 750 spec update #135996

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5d5e187
Disallow explicit concat between Template and str
davepeck Jun 26, 2025
1cc778f
First pass at removing implicit Template/str concat
davepeck Jun 26, 2025
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7 changes: 0 additions & 7 deletions Lib/test/test_ast/test_ast.py
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Original file line number Diff line number Diff line change
Expand Up @@ -999,13 +999,6 @@ def test_tstring(self):
self.assertIsInstance(tree.body[0].value.values[0], ast.Constant)
self.assertIsInstance(tree.body[0].value.values[1], ast.Interpolation)

# Test AST for implicit concat of t-string with f-string
tree = ast.parse('t"Hello {name}" f"{name}"')
self.assertIsInstance(tree.body[0].value, ast.TemplateStr)
self.assertIsInstance(tree.body[0].value.values[0], ast.Constant)
self.assertIsInstance(tree.body[0].value.values[1], ast.Interpolation)
self.assertIsInstance(tree.body[0].value.values[2], ast.FormattedValue)


class CopyTests(unittest.TestCase):
"""Test copying and pickling AST nodes."""
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61 changes: 17 additions & 44 deletions Lib/test/test_tstring.py
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Original file line number Diff line number Diff line change
Expand Up @@ -161,9 +161,8 @@ def test_template_concatenation(self):

# Test template + string
t1 = t"Hello"
combined = t1 + ", world"
self.assertTStringEqual(combined, ("Hello, world",), ())
self.assertEqual(fstring(combined), "Hello, world")
with self.assertRaises(TypeError):
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Will it make sense to check error message here?

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Good catch, will update.

_ = t1 + ", world"

# Test template + template with interpolation
name = "Python"
Expand All @@ -174,9 +173,8 @@ def test_template_concatenation(self):
self.assertEqual(fstring(combined), "Hello, Python")

# Test string + template
t = "Hello, " + t"{name}"
self.assertTStringEqual(t, ("Hello, ", ""), [(name, "name")])
self.assertEqual(fstring(t), "Hello, Python")
with self.assertRaises(TypeError):
_ = "Hello, " + t"{name}"

def test_nested_templates(self):
# Test a template inside another template expression
Expand Down Expand Up @@ -241,45 +239,20 @@ def test_literal_concatenation(self):
self.assertTStringEqual(t, ("Hello, ", ""), [(name, "name")])
self.assertEqual(fstring(t), "Hello, Python")

# Test concatenation with string literal
name = "Python"
t = t"Hello, {name}" "and welcome!"
self.assertTStringEqual(
t, ("Hello, ", "and welcome!"), [(name, "name")]
)
self.assertEqual(fstring(t), "Hello, Pythonand welcome!")

# Test concatenation with Unicode literal
name = "Python"
t = t"Hello, {name}" u"and welcome!"
self.assertTStringEqual(
t, ("Hello, ", "and welcome!"), [(name, "name")]
)
self.assertEqual(fstring(t), "Hello, Pythonand welcome!")

# Test concatenation with f-string literal
tab = '\t'
t = t"Tab: {tab}. " f"f-tab: {tab}."
self.assertTStringEqual(t, ("Tab: ", ". f-tab: \t."), [(tab, "tab")])
self.assertEqual(fstring(t), "Tab: \t. f-tab: \t.")

# Test concatenation with raw string literal
tab = '\t'
t = t"Tab: {tab}. " r"Raw tab: \t."
self.assertTStringEqual(
t, ("Tab: ", r". Raw tab: \t."), [(tab, "tab")]
)
self.assertEqual(fstring(t), "Tab: \t. Raw tab: \\t.")

# Test concatenation with raw f-string literal
tab = '\t'
t = t"Tab: {tab}. " rf"f-tab: {tab}. Raw tab: \t."
self.assertTStringEqual(
t, ("Tab: ", ". f-tab: \t. Raw tab: \\t."), [(tab, "tab")]
)
self.assertEqual(fstring(t), "Tab: \t. f-tab: \t. Raw tab: \\t.")

# Test disallowed mix of t-string and string
what = 't'
expected_msg = 'cannot mix str and Template literals'
for case in (
"t'{what}-string literal' 'str literal'",
"t'{what}-string literal' u'unicode literal'",
"t'{what}-string literal' f'f-string literal'",
"t'{what}-string literal' r'raw string literal'",
"t'{what}-string literal' rf'raw f-string literal'",
):
with self.assertRaisesRegex(SyntaxError, expected_msg):
eval(case)

# Test disallowed mix of t-string and bytes
expected_msg = 'cannot mix bytes and nonbytes literals'
for case in (
"t'{what}-string literal' b'bytes literal'",
Expand Down
4 changes: 0 additions & 4 deletions Lib/test/test_unparse.py
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Expand Up @@ -206,10 +206,6 @@ def test_tstrings(self):
self.check_ast_roundtrip("t'foo'")
self.check_ast_roundtrip("t'foo {bar}'")
self.check_ast_roundtrip("t'foo {bar!s:.2f}'")
self.check_ast_roundtrip("t'foo {bar}' f'{bar}'")
self.check_ast_roundtrip("f'{bar}' t'foo {bar}'")
self.check_ast_roundtrip("t'foo {bar}' fr'\\hello {bar}'")
self.check_ast_roundtrip("t'foo {bar}' u'bar'")

def test_strings(self):
self.check_ast_roundtrip("u'foo'")
Expand Down
88 changes: 5 additions & 83 deletions Objects/templateobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -245,54 +245,6 @@ template_iter(PyObject *op)
return (PyObject *)iter;
}

static PyObject *
template_strings_append_str(PyObject *strings, PyObject *str)
{
Py_ssize_t stringslen = PyTuple_GET_SIZE(strings);
PyObject *string = PyTuple_GET_ITEM(strings, stringslen - 1);
PyObject *concat = PyUnicode_Concat(string, str);
if (concat == NULL) {
return NULL;
}

PyObject *newstrings = PyTuple_New(stringslen);
if (newstrings == NULL) {
Py_DECREF(concat);
return NULL;
}

for (Py_ssize_t i = 0; i < stringslen - 1; i++) {
PyTuple_SET_ITEM(newstrings, i, Py_NewRef(PyTuple_GET_ITEM(strings, i)));
}
PyTuple_SET_ITEM(newstrings, stringslen - 1, concat);

return newstrings;
}

static PyObject *
template_strings_prepend_str(PyObject *strings, PyObject *str)
{
Py_ssize_t stringslen = PyTuple_GET_SIZE(strings);
PyObject *string = PyTuple_GET_ITEM(strings, 0);
PyObject *concat = PyUnicode_Concat(str, string);
if (concat == NULL) {
return NULL;
}

PyObject *newstrings = PyTuple_New(stringslen);
if (newstrings == NULL) {
Py_DECREF(concat);
return NULL;
}

PyTuple_SET_ITEM(newstrings, 0, concat);
for (Py_ssize_t i = 1; i < stringslen; i++) {
PyTuple_SET_ITEM(newstrings, i, Py_NewRef(PyTuple_GET_ITEM(strings, i)));
}

return newstrings;
}

static PyObject *
template_strings_concat(PyObject *left, PyObject *right)
{
Expand Down Expand Up @@ -344,46 +296,16 @@ template_concat_templates(templateobject *self, templateobject *other)
return newtemplate;
}

static PyObject *
template_concat_template_str(templateobject *self, PyObject *other)
{
PyObject *newstrings = template_strings_append_str(self->strings, other);
if (newstrings == NULL) {
return NULL;
}

PyObject *newtemplate = _PyTemplate_Build(newstrings, self->interpolations);
Py_DECREF(newstrings);
return newtemplate;
}

static PyObject *
template_concat_str_template(templateobject *self, PyObject *other)
{
PyObject *newstrings = template_strings_prepend_str(self->strings, other);
if (newstrings == NULL) {
return NULL;
}

PyObject *newtemplate = _PyTemplate_Build(newstrings, self->interpolations);
Py_DECREF(newstrings);
return newtemplate;
}

PyObject *
_PyTemplate_Concat(PyObject *self, PyObject *other)
{
if (_PyTemplate_CheckExact(self) && _PyTemplate_CheckExact(other)) {
return template_concat_templates((templateobject *) self, (templateobject *) other);
}
else if ((_PyTemplate_CheckExact(self)) && PyUnicode_Check(other)) {
return template_concat_template_str((templateobject *) self, other);
}
else if (PyUnicode_Check(self) && (_PyTemplate_CheckExact(other))) {
return template_concat_str_template((templateobject *) other, self);
}
else {
Py_RETURN_NOTIMPLEMENTED;
} else {
PyErr_Format(PyExc_TypeError,
"can only concatenate Template (not \"%.200s\") to Template",
Py_TYPE(other)->tp_name);
return NULL;
}
}

Expand Down
15 changes: 4 additions & 11 deletions Objects/unicodeobject.c
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Expand Up @@ -56,7 +56,6 @@ OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
#include "pycore_pyhash.h" // _Py_HashSecret_t
#include "pycore_pylifecycle.h" // _Py_SetFileSystemEncoding()
#include "pycore_pystate.h" // _PyInterpreterState_GET()
#include "pycore_template.h" // _PyTemplate_Concat()
#include "pycore_tuple.h" // _PyTuple_FromArray()
#include "pycore_ucnhash.h" // _PyUnicode_Name_CAPI
#include "pycore_unicodeobject.h" // struct _Py_unicode_state
Expand Down Expand Up @@ -11610,16 +11609,10 @@ PyUnicode_Concat(PyObject *left, PyObject *right)
return NULL;

if (!PyUnicode_Check(right)) {
if (_PyTemplate_CheckExact(right)) {
// str + tstring is implemented in the tstring type
return _PyTemplate_Concat(left, right);
}
else {
PyErr_Format(PyExc_TypeError,
"can only concatenate str (not \"%.200s\") to str",
Py_TYPE(right)->tp_name);
return NULL;
}
PyErr_Format(PyExc_TypeError,
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You can use %T to format object's type.

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The diff here is just a dedent of existing code; I was hesitant to change anything else. But happy to update it to use %T if we'd prefer.

"can only concatenate str (not \"%.200s\") to str",
Py_TYPE(right)->tp_name);
return NULL;
}

/* Shortcuts */
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6 changes: 6 additions & 0 deletions Parser/action_helpers.c
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Expand Up @@ -1887,6 +1887,11 @@ _PyPegen_concatenate_strings(Parser *p, asdl_expr_seq *strings,
return NULL;
}

if ((unicode_string_found || f_string_found) && t_string_found) {
RAISE_SYNTAX_ERROR("cannot mix str and Template literals");
return NULL;
}

// If it's only bytes or only unicode string, do a simple concat
if (!f_string_found && !t_string_found) {
if (len == 1) {
Expand All @@ -1902,6 +1907,7 @@ _PyPegen_concatenate_strings(Parser *p, asdl_expr_seq *strings,
}
}


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unrelated change :)

if (t_string_found) {
return _build_concatenated_template_str(p, strings, lineno,
col_offset, end_lineno, end_col_offset, arena);
Expand Down
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