gh-154719: Preserve trailing whitespace in t-string interpolation expressions

Lib/test/test_tstring.py

@@ -79,6 +79,28 @@ def upper(self):
         )
         self.assertEqual(fstring(t), "Name: Bob, Age: 30")
 
+    def test_interpolation_expression_whitespace(self):
+        x = 42
+        cases = [
+            (t"{x}", "x"),
+            (t"{x }", "x "),
+            (t"{ x}", " x"),
+            (t"{ x }", " x "),
+            (t"{  x  }", "  x  "),
+            (t"{x !r}", "x "),
+            (t"{x :}", "x "),
+            (t"{x = }", "x "),
+            (t"{x == 42 = }", "x == 42 "),
+            (t"""{
+  x
+}""", "\n  x\n"),
+        ]
+        for template, expected in cases:
+            with self.subTest(template=template):
+                self.assertEqual(
+                    template.interpolations[0].expression, expected
+                )
+
     def test_format_specifiers(self):
         # Test basic format specifiers
         value = 3.14159
@@ -136,7 +158,7 @@ def test_debug_specifier(self):
         # Test white space in debug specifier
         t = t"Value: {value = }"
         self.assertTStringEqual(
-            t, ("Value: value = ", ""), [(value, "value", "r")]
+            t, ("Value: value = ", ""), [(value, "value ", "r")]
         )
         self.assertEqual(fstring(t), "Value: value = 42")
 

Misc/NEWS.d/next/Core_and_Builtins/2026-07-26-17-20-49.gh-issue-154719.eI88Gs.rst

@@ -0,0 +1,3 @@
+Whitespace immediately before ``}``, ``!``, ``:``, or ``=`` is now included
+in :attr:`string.templatelib.Interpolation.expression` for interpolations
+created from t-string literals.

Parser/action_helpers.c

@@ -1540,21 +1540,21 @@ _get_interpolation_conversion(Parser *p, Token *debug, ResultTokenWithMetadata *
 }
 
 static PyObject *
-_strip_interpolation_expr(PyObject *exprstr)
+_strip_interpolation_debug_expr(PyObject *exprstr)
 {
     Py_ssize_t len = PyUnicode_GET_LENGTH(exprstr);
 
-    for (Py_ssize_t i = len - 1; i >= 0; i--) {
-        Py_UCS4 c = PyUnicode_READ_CHAR(exprstr, i);
-        if (_PyUnicode_IsWhitespace(c) || c == '=') {
-            len--;
-        }
-        else {
+    /* Discard whitespace after the debug "=" but preserve whitespace before it. */
+    while (len > 0) {
+        Py_UCS4 c = PyUnicode_READ_CHAR(exprstr, len - 1);
+        if (!_PyUnicode_IsWhitespace(c)) {
             break;
         }
+        len--;
     }
+    assert(len > 0 && PyUnicode_READ_CHAR(exprstr, len - 1) == '=');
 
-    return PyUnicode_Substring(exprstr, 0, len);
+    return PyUnicode_Substring(exprstr, 0, len - 1);
 }
 
 expr_ty _PyPegen_interpolation(Parser *p, expr_ty expression, Token *debug, ResultTokenWithMetadata *conversion,
@@ -1585,7 +1585,9 @@ expr_ty _PyPegen_interpolation(Parser *p, expr_ty expression, Token *debug, Resu
     }
 
     assert(exprstr != NULL);
-    PyObject *final_exprstr = _strip_interpolation_expr(exprstr);
+    PyObject *final_exprstr = debug
+        ? _strip_interpolation_debug_expr(exprstr)
+        : Py_NewRef(exprstr);
     if (!final_exprstr || _PyArena_AddPyObject(arena, final_exprstr) < 0) {
         Py_XDECREF(final_exprstr);
         return NULL;