Avoid two importlib copies

Lib/importlib/test/util.py

 from contextlib import contextmanager
 import imp
 import os.path
 from test import support
 import unittest
 import sys
 
 
 CASE_INSENSITIVE_FS = True
 # Windows is the only OS that is *always* case-insensitive
 # (OS X *can* be case-sensitive).
 if sys.platform not in ('win32', 'cygwin'):
     changed_name = __file__.upper()
     if changed_name == __file__:
         changed_name = __file__.lower()
     if not os.path.exists(changed_name):
         CASE_INSENSITIVE_FS = False
 
 
 def case_insensitive_tests(test):
     """Class decorator that nullifies tests requiring a case-insensitive
     file system."""
     return unittest.skipIf(not CASE_INSENSITIVE_FS,
                             "requires a case-insensitive filesystem")(test)
+
+
+def set_bootstrap(importlib, bootstrap):
+    """Set importlib to use the passed bootstrap module."""
+    importlib._w_long = bootstrap._w_long
+    importlib._r_long = bootstrap._r_long
+    importlib._bootstrap = bootstrap
+    importlib.__import__ = bootstrap.__import__
+    sys.modules['importlib._bootstrap'] = bootstrap
+
+
+@contextmanager
+def bootstrap_context(importlib, bootstrap):
+    """Temporarily use an alternate bootstrap module."""
+    old_bootstrap = importlib._bootstrap
+    try:
+        set_bootstrap(importlib, bootstrap)
+        yield
+    finally:
+        set_bootstrap(importlib, old_bootstrap)
+
+
+def pure_bootstrap_context(importlib):
+    """Temporarily use importlib._bootstrap."""
+    # importlib._bootstrap will normally be the same as _frozen_importlib
+    if not hasattr(importlib._pure_bootstrap, 'sys'):
+        importlib._pure_bootstrap._setup(sys, imp)
+    return bootstrap_context(importlib, importlib._pure_bootstrap)
 
 
 @contextmanager
 def uncache(*names):
     """Uncache a module from sys.modules.
 
     A basic sanity check is performed to prevent uncaching modules that either
     cannot/shouldn't be uncached.
 
     """
@@ skipping 92 matching lines @@
                 self.module_code[fullname]()
             return self.modules[fullname]
 
     def __enter__(self):
         self._uncache = uncache(*self.modules.keys())
         self._uncache.__enter__()
         return self
 
     def __exit__(self, *exc_info):
         self._uncache.__exit__(None, None, None)