mitogen: Delegate modules in ModulePolicy.unsuitables to Python's importers

These are not modules that Mitogen should block import of., the previous name
Importer.ALWAYS_BLACKLIST was a misnomer. It conflated their unsuitablility
for serving by Mitogen with the concept of modules that should be blocked from
import.

Mitogen should have no opinion about whether they're imported from a local
source. It is attempting to handle their import with Mitogen that we wish to
avoid.

With this cStringIO  (& probably other platform/version specific modules) can
be included regardless.

Author: moreati

docs/changelog.rst

@@ -25,6 +25,8 @@ In progress (unreleased)
   module overrides and blocks. Improve error messages for denied modules.
 * :gh:issue:`1451` :mod:`mitogen`: Move ``Importer.ALWAYS_BLACKLIST``
   to :attr:`mitogen.core.ImportPolicy.unsuitables`
+* :gh:issue:`1451` :mod:`mitogen`: Always delegate modules in
+  :attr:`mitogen.core.ImportPolicy.unsuitables` to Python's own importers
 
 
 v0.3.41 (2026-02-10)

mitogen/core.py

@@ -1323,11 +1323,11 @@ class ImportPolicy(object):
     unsuitables = set([
         '__builtin__',  # Python 2.x built-in Imported as __builtins__.
         'builtins',     # Python 3.x built-in. Imported as __builtins__.
+        'cStringIO',    # Python 2.x extension
         'msvcrt',       # Windows only. Imported by subprocess in some versions.
         'org',          # Jython only. Imported by copy, pickle, & xml.sax.
         'thread',       # Python 2.x built-in. Renamed to _thread in 3.x
     ])
-    if sys.version_info >= (3, 0): unsuitables.add('cStringIO')
 
     def __init__(self, overrides=(), blocks=()):
         self.overrides = set(overrides)
@@ -1348,6 +1348,9 @@ def denied_raise(self, fullname):
     def overriden(self, fullname):
         return bool(self.overrides.intersection(module_lineage(fullname)))
 
+    def unsuited(self, fullname):
+        return bool(self.unsuitables.intersection(module_lineage(fullname)))
+
     def __repr__(self):
         args = (type(self).__name__, self.overrides, self.blocks)
         return '%s(overrides=%r, blocks=%r)' % args
@@ -1472,6 +1475,9 @@ def find_module(self, fullname, path=None):
         if hasattr(_tls, 'running'):
             return None
 
+        if self.policy.unsuited(fullname):
+            return None
+
         _tls.running = True
         try:
             #_v and self._log.debug('Python requested %r', fullname)
@@ -1521,6 +1527,10 @@ def find_spec(self, fullname, path, target=None):
         if fullname.endswith('.'):
             return None
 
+        if self.policy.unsuited(fullname):
+            log.debug('Skipping %s. It is unsuited.')
+            return None
+
         pkgname, _, modname = fullname.rpartition('.')
         if pkgname and modname not in self._present.get(pkgname, ()):
             log.debug('Skipping %s. Parent %s has no submodule %s',

tests/importer_test.py

@@ -23,7 +23,7 @@ class ImporterMixin(testlib.RouterMixin):
     def setUp(self):
         super(ImporterMixin, self).setUp()
         self.context = mock.Mock()
-        self.policy = mock.Mock()
+        self.policy = mitogen.core.ImportPolicy()
         self.importer = mitogen.core.Importer(self.router, self.context, '', self.policy)
 
         # TODO: this is a horrendous hack. Without it, we can't deliver a
@@ -309,6 +309,16 @@ def test_overrides_and_blocks(self):
         self.assertTrue(policy.denied('__builtin__'))
         self.assertTrue(policy.denied('builtins'))
 
+    def test_unsuited(self):
+        policy = mitogen.core.ImportPolicy(
+            overrides=['pkg'],
+            blocks=['pkg'],
+        )
+        self.assertTrue(policy.unsuited('__builtin__'))
+        self.assertTrue(policy.unsuited('builtins'))
+        self.assertFalse(policy.unsuited('pkg'))
+        self.assertFalse(policy.unsuited('otherpkg'))
+
 
 class Python24LineCacheTest(testlib.TestCase):
     # TODO: mitogen.core.Importer._update_linecache()

tests/responder_test.py

@@ -8,6 +8,7 @@
 except ImportError:
     import mock
 
+import mitogen.core
 import mitogen.master
 import testlib
 
@@ -20,7 +21,7 @@ class NeutralizeMainTest(testlib.RouterMixin, testlib.TestCase):
 
     def call(self, *args, **kwargs):
         router = mock.Mock()
-        policy = mock.Mock()
+        policy = mitogen.core.ImportPolicy()
         return self.klass(router, policy).neutralize_main(*args, **kwargs)
 
     def test_missing_exec_guard(self):
@@ -120,7 +121,7 @@ def test_obviously_missing(self):
         )
         msg.router = router
 
-        policy = mock.Mock()
+        policy = mitogen.core.ImportPolicy()
         responder = mitogen.master.ModuleResponder(router, policy)
         responder._on_get_module(msg)
         self.assertEqual(1, len(router._async_route.mock_calls))
@@ -159,7 +160,7 @@ def test_ansible_six_messed_up_path(self):
         )
         msg.router = router
 
-        policy = mock.Mock()
+        policy = mitogen.core.ImportPolicy()
         responder = mitogen.master.ModuleResponder(router, policy)
         responder._on_get_module(msg)
         self.assertEqual(1, len(router._async_route.mock_calls))