[3.5] Backport bpo-30876 (GH-2639), bpo-18018 and bpo-26367. (#2677)

* bpo-30876: Relative import from unloaded package now reimports the package
instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.
(cherry picked from commit 8a9cd20edca7d01b68292036029ae3735ce65edd)

* bpo-18018: Import raises ImportError instead of SystemError if a relative
import is attempted without a known parent package.

* bpo-26367: importlib.__init__() raises ImportError like
builtins.__import__() when ``level`` is specified but without an accompanying
package specified.

1 files changed, 3 insertions, 4 deletions

diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
index 9eecbfe..02fe05b 100644
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -925,10 +925,9 @@ def _sanity_check(name, package, level):
     if level > 0:
         if not isinstance(package, str):
             raise TypeError('__package__ not set to a string')
-        elif package not in sys.modules:
-            msg = ('Parent module {!r} not loaded, cannot perform relative '
-                   'import')
-            raise SystemError(msg.format(package))
+        elif not package:
+            raise ImportError('attempted relative import with no known parent '
+                              'package')
     if not name and level == 0:
         raise ValueError('Empty module name')