bpo-37409: fix relative import with no parent (#14956)

Relative imports use resolve_name to get the absolute target name,
which first seeks the current module's absolute package name from the globals:
If __package__ (and __spec__.parent) are missing then
import uses __name__, truncating the last segment if
the module is a submodule rather than a package __init__.py
(which it guesses from whether __path__ is defined).

The __name__ attempt should fail if there is no parent package (top level modules),
if __name__ is '__main__' (-m entry points), or both (scripts).
That is, if both __name__ has no subcomponents and the module does not seem
to be a package __init__ module then import should fail.

1 files changed, 4 insertions, 0 deletions

diff --git a/Lib/test/test_builtin.py b/Lib/test/test_builtin.py
index 7b3c054..185d8f7 100644
--- a/Lib/test/test_builtin.py
+++ b/Lib/test/test_builtin.py
@@ -160,6 +160,10 @@ class BuiltinTest(unittest.TestCase):
         self.assertRaises(TypeError, __import__, 1, 2, 3, 4)
         self.assertRaises(ValueError, __import__, '')
         self.assertRaises(TypeError, __import__, 'sys', name='sys')
+        # Relative import outside of a package with no __package__ or __spec__ (bpo-37409).
+        self.assertRaises(ImportError, __import__, '',
+                          {'__package__': None, '__spec__': None, '__name__': '__main__'},
+                          locals={}, fromlist=('foo',), level=1)
         # embedded null character
         self.assertRaises(ModuleNotFoundError, __import__, 'string\x00')