bpo-38691: importlib ignores PYTHONCASEOK if -E is used (GH-18627)

The importlib module now ignores the PYTHONCASEOK
environment variable when the -E or -I command line
options are being used.

Doc/library/functions.rst

@@ -1835,6 +1835,9 @@ are always available.  They are listed here in alphabetical order.
       Negative values for *level* are no longer supported (which also changes
       the default value to 0).
 
+   .. versionchanged:: 3.9
+      When the command line options :option:`-E` or :option:`-I` are being used,
+      the environment variable :envvar:`PYTHONCASEOK` is now ignored.
 
 .. rubric:: Footnotes
 

Doc/whatsnew/3.9.rst

@@ -645,6 +645,9 @@ Changes in the Python API
   since the *buffering* parameter has been removed.
   (Contributed by Victor Stinner in :issue:`39357`.)
 
+* The :mod:`importlib` module now ignores the :envvar:`PYTHONCASEOK`
+  environment variable when the :option:`-E` or :option:`-I` command line
+  options are being used.
 
 CPython bytecode changes
 ------------------------

Lib/importlib/_bootstrap_external.py

@@ -34,8 +34,8 @@ def _make_relax_case():
             key = b'PYTHONCASEOK'
 
         def _relax_case():
-            """True if filenames must be checked case-insensitively."""
-            return key in _os.environ
+            """True if filenames must be checked case-insensitively and ignore environment flags are not set."""
+            return not sys.flags.ignore_environment and key in _os.environ
     else:
         def _relax_case():
             """True if filenames must be checked case-insensitively."""

Lib/test/test_importlib/extension/test_case_sensitivity.py

@@ -1,7 +1,7 @@
 from importlib import _bootstrap_external
 from test import support
 import unittest
-
+import sys
 from .. import util
 
 importlib = util.import_importlib('importlib')
@@ -21,6 +21,7 @@ class ExtensionModuleCaseSensitivityTest(util.CASEOKTestBase):
                                            self.machinery.EXTENSION_SUFFIXES))
         return finder.find_module(bad_name)
 
+    @unittest.skipIf(sys.flags.ignore_environment, 'ignore_environment flag was set')
     def test_case_sensitive(self):
         with support.EnvironmentVarGuard() as env:
             env.unset('PYTHONCASEOK')
@@ -28,6 +29,7 @@ class ExtensionModuleCaseSensitivityTest(util.CASEOKTestBase):
             loader = self.find_module()
             self.assertIsNone(loader)
 
+    @unittest.skipIf(sys.flags.ignore_environment, 'ignore_environment flag was set')
     def test_case_insensitivity(self):
         with support.EnvironmentVarGuard() as env:
             env.set('PYTHONCASEOK', '1')

Lib/test/test_importlib/source/test_case_sensitivity.py

@@ -1,4 +1,6 @@
 """Test case-sensitivity (PEP 235)."""
+import sys
+
 from .. import util
 
 importlib = util.import_importlib('importlib')
@@ -38,6 +40,7 @@ class CaseSensitivityTest(util.CASEOKTestBase):
             insensitive_finder = self.finder(insensitive_path)
             return self.find(sensitive_finder), self.find(insensitive_finder)
 
+    @unittest.skipIf(sys.flags.ignore_environment, 'ignore_environment flag was set')
     def test_sensitive(self):
         with test_support.EnvironmentVarGuard() as env:
             env.unset('PYTHONCASEOK')
@@ -47,6 +50,7 @@ class CaseSensitivityTest(util.CASEOKTestBase):
             self.assertIn(self.name, sensitive.get_filename(self.name))
             self.assertIsNone(insensitive)
 
+    @unittest.skipIf(sys.flags.ignore_environment, 'ignore_environment flag was set')
     def test_insensitive(self):
         with test_support.EnvironmentVarGuard() as env:
             env.set('PYTHONCASEOK', '1')

Misc/NEWS.d/next/Library/2020-02-23-02-09-03.bpo-38691.oND8Sk.rst

@@ -0,0 +1,3 @@
+The :mod:`importlib` module now ignores the :envvar:`PYTHONCASEOK`
+environment variable when the :option:`-E` or :option:`-I` command line
+options are being used.