Issue #14605: Stop having implicit entries for sys.meta_path.

ImportWarning is raised if sys.meta_path is found to be empty.

2 files changed, 22 insertions, 4 deletions

diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 2f42906..b88acc5 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -956,8 +956,9 @@ def _resolve_name(name, package, level):
 
 def _find_module(name, path):
     """Find a module's loader."""
-    meta_path = sys.meta_path + _IMPLICIT_META_PATH
-    for finder in meta_path:
+    if not sys.meta_path:
+        _warnings.warn('sys.meta_path is empty', ImportWarning)
+    for finder in sys.meta_path:
         loader = finder.find_module(name, path)
         if loader is not None:
             # The parent import may have already imported this module.
@@ -986,8 +987,6 @@ def _sanity_check(name, package, level):
         raise ValueError("Empty module name")
 
 
-_IMPLICIT_META_PATH = [BuiltinImporter, FrozenImporter, PathFinder]
-
 _ERR_MSG = 'No module named {!r}'
 
 def _find_and_load(name, import_):
@@ -1195,3 +1194,4 @@ def _install(sys_module, _imp_module):
                          (SourcelessFileLoader, _suffix_list(2), True)]
     sys.path_hooks.extend([FileFinder.path_hook(*supported_loaders),
                            _imp.NullImporter])
+    sys.meta_path.extend([BuiltinImporter, FrozenImporter, PathFinder])
diff --git a/Lib/importlib/test/import_/test_meta_path.py b/Lib/importlib/test/import_/test_meta_path.py
index 3b130c9..2f65af9 100644
--- a/Lib/importlib/test/import_/test_meta_path.py
+++ b/Lib/importlib/test/import_/test_meta_path.py
@@ -1,7 +1,10 @@
 from .. import util
 from . import util as import_util
+import importlib._bootstrap
+import sys
 from types import MethodType
 import unittest
+import warnings
 
 
 class CallingOrder(unittest.TestCase):
@@ -33,6 +36,21 @@ class CallingOrder(unittest.TestCase):
             with util.import_state(meta_path=[first, second]):
                 self.assertEqual(import_util.import_(mod_name), 42)
 
+    def test_empty(self):
+        # Raise an ImportWarning if sys.meta_path is empty.
+        module_name = 'nothing'
+        try:
+            del sys.modules[module_name]
+        except KeyError:
+            pass
+        with util.import_state(meta_path=[]):
+            with warnings.catch_warnings(record=True) as w:
+                warnings.simplefilter('always')
+                self.assertIsNone(importlib._bootstrap._find_module('nothing',
+                                                                    None))
+                self.assertEqual(len(w), 1)
+                self.assertTrue(issubclass(w[-1].category, ImportWarning))
+
 
 class CallSignature(unittest.TestCase):