bpo-20490: Improve circular import error message (GH-15308)

Author: asottile

Lib/test/test_import/__init__.py

@@ -1324,6 +1324,16 @@ def test_crossreference2(self):
         self.assertIn('partially initialized module', errmsg)
         self.assertIn('circular import', errmsg)
 
+    def test_circular_from_import(self):
+        with self.assertRaises(ImportError) as cm:
+            import test.test_import.data.circular_imports.from_cycle1
+        self.assertIn(
+            "cannot import name 'b' from partially initialized module "
+            "'test.test_import.data.circular_imports.from_cycle1' "
+            "(most likely due to a circular import)",
+            str(cm.exception),
+        )
+
 
 if __name__ == '__main__':
     # Test needs to be a package, so we can do relative imports.

Lib/test/test_import/data/circular_imports/from_cycle1.py

@@ -0,0 +1,2 @@
+from .from_cycle2 import a
+b = 1

Lib/test/test_import/data/circular_imports/from_cycle2.py

@@ -0,0 +1,2 @@
+from .from_cycle1 import b
+a = 1

Misc/NEWS.d/next/Core and Builtins/2019-08-15-12-48-36.bpo-20490.-hXeEn.rst

@@ -0,0 +1,2 @@
+Improve import error message for partially initialized module on circular
+``from`` imports - by Anthony Sottile.

Python/ceval.c

@@ -5233,10 +5233,17 @@ import_from(PyThreadState *tstate, PyObject *v, PyObject *name)
         PyErr_SetImportError(errmsg, pkgname, NULL);
     }
     else {
-        errmsg = PyUnicode_FromFormat(
-            "cannot import name %R from %R (%S)",
-            name, pkgname_or_unknown, pkgpath
-        );
+        _Py_IDENTIFIER(__spec__);
+        PyObject *spec = _PyObject_GetAttrId(v, &PyId___spec__);
+        Py_XINCREF(spec);
+        const char *fmt =
+            _PyModuleSpec_IsInitializing(spec) ?
+            "cannot import name %R from partially initialized module %R "
+            "(most likely due to a circular import) (%S)" :
+            "cannot import name %R from %R (%S)";
+        Py_XDECREF(spec);
+
+        errmsg = PyUnicode_FromFormat(fmt, name, pkgname_or_unknown, pkgpath);
         /* NULL checks for errmsg and pkgname done by PyErr_SetImportError. */
         PyErr_SetImportError(errmsg, pkgname, pkgpath);
     }