bpo-30876: Relative import from unloaded package now reimports the package (#2639)

instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.

Lib/importlib/_bootstrap.py

@@ -919,10 +919,6 @@ def _sanity_check(name, package, level):
         elif not package:
             raise ImportError('attempted relative import with no known parent '
                               'package')
-        elif package not in sys.modules:
-            msg = ('Parent module {!r} not loaded, cannot perform relative '
-                   'import')
-            raise SystemError(msg.format(package))
     if not name and level == 0:
         raise ValueError('Empty module name')
 

Lib/test/test_import/__init__.py

@@ -23,8 +23,9 @@ from test.support import (
     EnvironmentVarGuard, TESTFN, check_warnings, forget, is_jython,
     make_legacy_pyc, rmtree, run_unittest, swap_attr, swap_item, temp_umask,
     unlink, unload, create_empty_file, cpython_only, TESTFN_UNENCODABLE,
-    temp_dir)
+    temp_dir, DirsOnSysPath)
 from test.support import script_helper
+from test.test_importlib.util import uncache
 
 
 skip_if_dont_write_bytecode = unittest.skipIf(
@@ -670,11 +671,11 @@ class RelativeImportTests(unittest.TestCase):
 
         # Check relative import fails with only __package__ wrong
         ns = dict(__package__='foo', __name__='test.notarealmodule')
-        self.assertRaises(SystemError, check_relative)
+        self.assertRaises(ModuleNotFoundError, check_relative)
 
         # Check relative import fails with __package__ and __name__ wrong
         ns = dict(__package__='foo', __name__='notarealpkg.notarealmodule')
-        self.assertRaises(SystemError, check_relative)
+        self.assertRaises(ModuleNotFoundError, check_relative)
 
         # Check relative import fails with package set to a non-string
         ns = dict(__package__=object())
@@ -689,6 +690,20 @@ class RelativeImportTests(unittest.TestCase):
             self.fail("explicit relative import triggered an "
                       "implicit absolute import")
 
+    def test_import_from_non_package(self):
+        path = os.path.join(os.path.dirname(__file__), 'data', 'package2')
+        with uncache('submodule1', 'submodule2'), DirsOnSysPath(path):
+            with self.assertRaises(ImportError):
+                import submodule1
+            self.assertNotIn('submodule1', sys.modules)
+            self.assertNotIn('submodule2', sys.modules)
+
+    def test_import_from_unloaded_package(self):
+        with uncache('package2', 'package2.submodule1', 'package2.submodule2'), \
+             DirsOnSysPath(os.path.join(os.path.dirname(__file__), 'data')):
+            import package2.submodule1
+            package2.submodule1.submodule2
+
 
 class OverridingImportBuiltinTests(unittest.TestCase):
     def test_override_builtin(self):

Lib/test/test_import/data/package2/submodule1.py

@@ -0,0 +1,3 @@
+import sys
+sys.modules.pop(__package__, None)
+from . import submodule2

Lib/test/test_importlib/import_/test___package__.py

@@ -81,7 +81,7 @@ class Using__package__:
 
     def test_bad__package__(self):
         globals = {'__package__': '<not real>'}
-        with self.assertRaises(SystemError):
+        with self.assertRaises(ModuleNotFoundError):
             self.__import__('', globals, {}, ['relimport'], 1)
 
     def test_bunk__package__(self):

Misc/NEWS.d/next/Core and Builtins/2017-07-11-06-31-32.bpo-30876.x35jZX.rst

@@ -0,0 +1,3 @@
+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.

Python/import.c

@@ -1344,7 +1344,6 @@ resolve_name(PyObject *name, PyObject *globals, int level)
     PyObject *abs_name;
     PyObject *package = NULL;
     PyObject *spec;
-    PyInterpreterState *interp = PyThreadState_GET()->interp;
     Py_ssize_t last_dot;
     PyObject *base;
     int level_up;
@@ -1448,12 +1447,6 @@ resolve_name(PyObject *name, PyObject *globals, int level)
                 "attempted relative import with no known parent package");
         goto error;
     }
-    else if (PyDict_GetItem(interp->modules, package) == NULL) {
-        PyErr_Format(PyExc_SystemError,
-                "Parent module %R not loaded, cannot perform relative "
-                "import", package);
-        goto error;
-    }
 
     for (level_up = 1; level_up < level; level_up += 1) {
         last_dot = PyUnicode_FindChar(package, '.', 0, last_dot, -1);