Simplify magic number release test

Simplify the magic number release test by removing
EXPECTED_MAGIC_NUMBERS table and making the expected
magic number self-contained within the test.

BPO: 29514

Lib/importlib/_bootstrap_external.py

@@ -248,21 +248,6 @@ def _write_atomic(path, data, mode=0o666):
 #
 # Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
 # in PC/launcher.c must also be updated.
-#
-# In general, the MAGIC_NUMBER should not change for maintenance releases
-# although this may be required under exceptional circumstances. When
-# each release reaches candidate status, an entry in EXPECTED_MAGIC_NUMBERS
-# should be added for this release. This value is tested against the actual
-# MAGIC_NUMBER to ensure that if a change is required within a minor
-# release, the exception is first discussed with python-dev and relevant
-# community stakeholders such as OS distribution package maintainers
-# are properly informed of the change.
-
-EXPECTED_MAGIC_NUMBERS = {
-    (2, 7): 62211,
-    (3, 5): 3350,
-    (3, 6): 3379
-}
 
 MAGIC_NUMBER = (3390).to_bytes(2, 'little') + b'\r\n'
 _RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c

Lib/importlib/util.py

@@ -4,7 +4,6 @@
 from ._bootstrap import _resolve_name
 from ._bootstrap import spec_from_loader
 from ._bootstrap import _find_spec
-from ._bootstrap_external import EXPECTED_MAGIC_NUMBERS
 from ._bootstrap_external import MAGIC_NUMBER
 from ._bootstrap_external import cache_from_source
 from ._bootstrap_external import decode_source

Lib/test/test_importlib/test_util.py

@@ -757,5 +757,49 @@ def test_source_from_cache_path_like_arg(self):
  ) = util.test_both(PEP3147Tests, util=importlib_util)
 
 
+class MagicNumberTests:
+    """
+    Test release compatibility issues relating to importlib
+    """
+    def test_magic_number(self):
+        """
+        Each python minor release should generally have a MAGIC_NUMBER
+        that does not change once the release reaches candidate status.
+
+        Once a release reaches candidate status, the value of the constant
+        EXPECTED_MAGIC_NUMBER in this test should be changed.
+        This test will then check that the actual MAGIC_NUMBER matches
+        the expected value for the release.
+
+        In exceptional cases, it may be required to change the MAGIC_NUMBER
+        for a maintenance release. In this case the change should be
+        discussed in python-dev. If a change is required, community
+        stakeholders such as OS package maintainers must be notified
+        in advance. Such exceptional releases will then require an
+        adjustment to this test case.
+        """
+        if sys.version_info.releaselevel not in ('final', 'candidate'):
+            return
+        EXPECTED_MAGIC_NUMBER = 3379
+        actual = int.from_bytes(self.util.MAGIC_NUMBER[:2], 'little')
+
+        msg = (
+            "Candidate and final releases require the current  "
+            "importlib.util.MAGIC_NUMBER to match the expected "
+            "magic number in this test. Set the expected "
+            "magic number in this test to the current MAGIC_NUMBER to "
+            "continue with the release.\n\n"
+            "Changing the MAGIC_NUMBER for a maintenance release "
+            "requires discussion in python-dev and notification of "
+            "community stakeholders."
+        )
+        self.assertEqual(EXPECTED_MAGIC_NUMBER, actual, msg)
+
+
+Source_MagicNumberTests = util.specialize_class(
+    MagicNumberTests, 'Source', None, util=importlib_util
+)
+
+
 if __name__ == '__main__':
     unittest.main()