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.

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

@@ -11,6 +11,7 @@ Copyright (c) Corporation for National Research Initiatives.
 #include "Python.h"
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
 #include "pycore_codecs.h"        // export _PyCodec_LookupTextEncoding()
+#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()
@@ -1686,6 +1687,8 @@ _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);