GH-145273: warn when we can't find the standard library

Lib/test/support/__init__.py

@@ -1716,9 +1716,10 @@ def _platform_specific(self):
                 ))
 
             self._env = {k.upper(): os.getenv(k) for k in os.environ}
-            self._env["PYTHONHOME"] = os.path.dirname(self.real)
+            home = os.path.dirname(self.real)
             if sysconfig.is_python_build():
-                self._env["PYTHONPATH"] = STDLIB_DIR
+                home = os.path.join(home, sysconfig.get_config_var('VPATH'))
+            self._env["PYTHONHOME"] = home
     else:
         def _platform_specific(self):
             pass

Lib/test/test_embed.py

@@ -1479,6 +1479,10 @@ def test_init_setpythonhome(self):
             stdlib = os.path.join(home, sys.platlibdir, f'python{version}{ABI_THREAD}')
             expected_paths = self.module_search_paths(prefix=home, exec_prefix=home)
 
+        # Create the expected paths to avoid the bad stdlib dir warning
+        for entry in expected_paths:
+            os.makedirs(entry, exist_ok=True)
+
         config = {
             'home': home,
             'module_search_paths': expected_paths,
@@ -1520,6 +1524,10 @@ def test_init_is_python_build_with_home(self):
             stdlib = os.path.join(home, sys.platlibdir, f'python{version}{ABI_THREAD}')
             expected_paths = self.module_search_paths(prefix=home, exec_prefix=home)
 
+        # Create the expected paths to avoid the bad stdlib dir warning
+        for entry in expected_paths:
+            os.makedirs(entry, exist_ok=True)
+
         config = {
             'home': home,
             'module_search_paths': expected_paths,
@@ -1575,6 +1583,8 @@ def test_init_pybuilddir(self):
             # The stdlib dir is dirname(executable) + VPATH + 'Lib'
             stdlibdir = os.path.normpath(os.path.join(tmpdir, vpath, 'Lib'))
             os.mkdir(libdir)
+            # Create the directory to avoid the bad stdlib dir warning
+            os.makedirs(stdlibdir)
 
             filename = os.path.join(tmpdir, 'pybuilddir.txt')
             with open(filename, "w", encoding="utf8") as fp:
@@ -1613,6 +1623,9 @@ def test_init_pybuilddir_win32(self):
             # The stdlib dir is dirname(executable) + VPATH + 'Lib'
             stdlibdir = os.path.normpath(os.path.join(tmpdir, vpath, 'Lib'))
 
+            # Create the directory to avoid the bad stdlib dir warning
+            os.makedirs(stdlibdir)
+
             filename = os.path.join(tmpdir, 'pybuilddir.txt')
             with open(filename, "w", encoding="utf8") as fp:
                 fp.write(tmpdir)

Misc/NEWS.d/next/Core_and_Builtins/2026-02-26-20-51-54.gh-issue-145273.B5QcUp.rst

@@ -0,0 +1,2 @@
+A warning is now shown during :ref:`sys-path-init` if it can't find a valid
+standard library.

Modules/getpath.py

@@ -236,6 +236,7 @@ def search_up(prefix, *landmarks, test=isfile):
 
 real_executable_dir = None
 platstdlib_dir = None
+stdlib_zip = None
 
 # ******************************************************************************
 # CALCULATE program_name
@@ -697,12 +698,13 @@ def search_up(prefix, *landmarks, test=isfile):
             library_dir = dirname(library)
         else:
             library_dir = executable_dir
-        pythonpath.append(joinpath(library_dir, ZIP_LANDMARK))
+        stdlib_zip = joinpath(library_dir, ZIP_LANDMARK)
     elif build_prefix:
         # QUIRK: POSIX uses the default prefix when in the build directory
-        pythonpath.append(joinpath(PREFIX, ZIP_LANDMARK))
+        stdlib_zip = joinpath(PREFIX, ZIP_LANDMARK)
     else:
-        pythonpath.append(joinpath(base_prefix, ZIP_LANDMARK))
+        stdlib_zip = joinpath(base_prefix, ZIP_LANDMARK)
+    pythonpath.append(stdlib_zip)
 
     if os_name == 'nt' and use_environment and winreg:
         # QUIRK: Windows also lists paths in the registry. Paths are stored
@@ -767,6 +769,21 @@ def search_up(prefix, *landmarks, test=isfile):
     config['module_search_paths_set'] = 1
 
 
+# ******************************************************************************
+# SANITY CHECKS
+# ******************************************************************************
+
+# Warn if the standard library is missing
+if not stdlib_zip or not isfile(stdlib_zip):
+    home_hint = f"The Python 'home' directory was set to {home!r}, is this correct?"
+    if not stdlib_dir or not isdir(stdlib_dir):
+        hint = home_hint if home else f'sys.prefix is set to {prefix}, is this correct?'
+        warn('WARN: Could not find the standard library directory! ' + hint)
+    elif (not platstdlib_dir and not build_prefix) or not isdir(platstdlib_dir):
+        hint = home_hint if home else f'sys.exec_prefix is set to {exec_prefix}, is this correct?'
+        warn('WARN: Could not find the platform standard library directory! ' + hint)
+
+
 # ******************************************************************************
 # POSIX prefix/exec_prefix QUIRKS
 # ******************************************************************************