gh-151253: Dump the Python path configuration on _PyCodec_InitRegistry() failure (#151250)

If "import encodings" fails at Python startup, dump the Python path
configuration to help users debugging their configuration. The
encodings module is the first module imported during Python startup.

(cherry picked from commit 7b6e989)

Author: vstinner

Lib/test/test_cmd_line.py

@@ -1314,6 +1314,17 @@ def test_presite(self):
         proc = assert_python_ok("-X", f"presite={entrypoint}", "-c", "pass")
         self.assertEqual(proc.out.rstrip(), b"presite func")
 
+    def test_dump_path_config(self):
+        # gh-151253: At the first import (import encodings) during Python
+        # startup, if the import fails, dump the Python path configuration.
+        nonexistent = '/nonexistent-python-path'
+        # Use -X frozen_modules=off to disable frozen encodings module
+        # on release build.
+        cmd = ["-X", "frozen_modules=off", "-c", "pass"]
+        proc = assert_python_failure(*cmd, PYTHONHOME=nonexistent)
+        self.assertIn(b'Python path configuration:', proc.err)
+        self.assertIn(f"PYTHONHOME = '{nonexistent}'".encode(), proc.err)
+
 
 @unittest.skipIf(interpreter_requires_environment(),
                  'Cannot run -I tests when PYTHON env vars are required.')

Misc/NEWS.d/next/Core_and_Builtins/2026-06-10-15-42-46.gh-issue-151253.7MMQ8P.rst

@@ -0,0 +1,3 @@
+If ``import encodings`` (first import) fails at Python startup, dump the
+Python path configuration to help users debugging their configuration. Patch
+by Victor Stinner.

Python/codecs.c

@@ -10,6 +10,7 @@ Copyright (c) Corporation for National Research Initiatives.
 
 #include "Python.h"
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
+#include "pycore_initconfig.h"    // _Py_DumpPathConfig()
 #include "pycore_interp.h"        // PyInterpreterState.codec_search_path
 #include "pycore_pyerrors.h"      // _PyErr_FormatNote()
 #include "pycore_pystate.h"       // _PyInterpreterState_GET()
@@ -1685,6 +1686,8 @@ _PyCodec_InitRegistry(PyInterpreterState *interp)
     // search functions, so this is done after everything else is initialized.
     PyObject *mod = PyImport_ImportModule("encodings");
     if (mod == NULL) {
+        PyThreadState *tstate = _PyThreadState_GET();
+        _Py_DumpPathConfig(tstate);
         return PyStatus_Error("Failed to import encodings module");
     }
     Py_DECREF(mod);