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

Author: zooba

Lib/test/test_import/__init__.py

@@ -1326,6 +1326,16 @@ 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

@@ -5236,10 +5236,17 @@ 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);
  }