gh-143201: C warnings writes the full source line instead of the truncated one

Lib/test/test_warnings/__init__.py

@@ -1109,6 +1109,21 @@ def test_showwarnmsg_missing(self):
                 self.module._showwarnmsg = show
         self.assertIn(text, result)
 
+    @support.cpython_only
+    def test_showwarnmsg_missing_source_line(self):
+        # gh-143201: the C fallback must strip the source line like Python.
+        lineno = sys._getframe().f_lineno
+        with self.module.catch_warnings(), \
+             support.swap_attr(self.module, '_showwarnmsg', None), \
+             support.captured_stderr() as stream:
+            self.module.simplefilter("always")
+            del self.module._showwarnmsg  # use the C fallback
+            self.module.warn_explicit('eggs', UserWarning, __file__, lineno,
+                                      module_globals=globals())
+        self.assertEqual(stream.getvalue(), py_warnings.formatwarning(
+            'eggs', UserWarning, __file__, lineno,
+            linecache.getline(__file__, lineno)))
+
     def test_showwarning_not_callable(self):
         orig = self.module.showwarning
         try:

Misc/NEWS.d/next/Core_and_Builtins/2025-12-26-17-18-18.gh-issue-143201.UbXuX4.rst

@@ -0,0 +1,2 @@
+Fix the C implementation of :mod:`warnings` to strip the indentation of the
+source line like the Python implementation.

Python/_warnings.c

@@ -666,6 +666,7 @@ show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
 {
     PyObject *f_stderr = NULL;
     PyObject *name;
+    PyObject *truncated = NULL;
     char lineno_str[128];
 
     PyOS_snprintf(lineno_str, sizeof(lineno_str), ":%d: ", lineno);
@@ -701,7 +702,6 @@ show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
         const void *data;
         Py_ssize_t i, len;
         Py_UCS4 ch;
-        PyObject *truncated;
 
         kind = PyUnicode_KIND(sourceline);
         data = PyUnicode_DATA(sourceline);
@@ -716,9 +716,15 @@ show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
         if (truncated == NULL)
             goto error;
 
-        PyFile_WriteObject(sourceline, f_stderr, Py_PRINT_RAW);
-        Py_DECREF(truncated);
-        PyFile_WriteString("\n", f_stderr);
+        if (PyFile_WriteString("  ", f_stderr) < 0) {
+            goto error;
+        }
+        if (PyFile_WriteObject(truncated, f_stderr, Py_PRINT_RAW) < 0) {
+            goto error;
+        }
+        if (PyFile_WriteString("\n", f_stderr) < 0) {
+            goto error;
+        }
     }
     else {
         _Py_DisplaySourceLine(f_stderr, filename, lineno, 2, NULL, NULL);
@@ -727,6 +733,7 @@ show_warning(PyThreadState *tstate, PyObject *filename, int lineno,
 error:
     Py_XDECREF(f_stderr);
     Py_XDECREF(name);
+    Py_XDECREF(truncated);
     PyErr_Clear();
 }