bpo-38735: Don't fail when importing from / with sys.pycache_prefix set (GH-30456)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/importlib/_bootstrap_external.py

@@ -297,7 +297,8 @@ def cache_from_source(path, debug_override=None, *, optimization=None):
         # Strip initial drive from a Windows path. We know we have an absolute
         # path here, so the second part of the check rules out a POSIX path that
         # happens to contain a colon at the second character.
-        if head[1] == ':' and head[0] not in path_separators:
+        # Slicing avoids issues with an empty (or short) `head`.
+        if head[1:2] == ':' and head[0:1] not in path_separators:
             head = head[2:]
 
         # Strip initial path separator from `head` to complete the conversion

Lib/test/test_importlib/test_util.py

@@ -580,6 +580,18 @@ def test_cache_from_source_respects_pycache_prefix_relative(self):
                 self.util.cache_from_source(path, optimization=''),
                 os.path.normpath(expect))
 
+    @unittest.skipIf(sys.implementation.cache_tag is None,
+                     'requires sys.implementation.cache_tag to not be None')
+    def test_cache_from_source_in_root_with_pycache_prefix(self):
+        # Regression test for gh-82916
+        pycache_prefix = os.path.join(os.path.sep, 'tmp', 'bytecode')
+        path = 'qux.py'
+        expect = os.path.join(os.path.sep, 'tmp', 'bytecode',
+                              f'qux.{self.tag}.pyc')
+        with util.temporary_pycache_prefix(pycache_prefix):
+            with os_helper.change_cwd('/'):
+                self.assertEqual(self.util.cache_from_source(path), expect)
+
     @unittest.skipIf(sys.implementation.cache_tag is None,
                      'requires sys.implementation.cache_tag to not be None')
     def test_source_from_cache_inside_pycache_prefix(self):

Misc/NEWS.d/next/Library/2022-01-07-16-56-57.bpo-38735.NFfJX6.rst

@@ -0,0 +1,2 @@
+Fix failure when importing a module from the root directory on unix-like
+platforms with sys.pycache_prefix set.