[Python-checkins] bpo-20490: Improve circular import error message (GH-15308)

[Steve Dower]

https://github.com/python/cpython/commit/65366bc8bdc4716ebc361e622590b45a6e5aef07
commit: 65366bc8bdc4716ebc361e622590b45a6e5aef07
branch: master
author: Anthony Sottile <asottile at umich.edu>
committer: Steve Dower <steve.dower at python.org>
date: 2019-09-09T08:17:50-07:00
summary:

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

files:
A Lib/test/test_import/data/circular_imports/from_cycle1.py
A Lib/test/test_import/data/circular_imports/from_cycle2.py
A Misc/NEWS.d/next/Core and Builtins/2019-08-15-12-48-36.bpo-20490.-hXeEn.rst
M Lib/test/test_import/__init__.py
M Python/ceval.c

diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index 88746b4315f9..f167c84f1b28 100644
--- a/Lib/test/test_import/__init__.py
+++ b/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.
diff --git a/Lib/test/test_import/data/circular_imports/from_cycle1.py b/Lib/test/test_import/data/circular_imports/from_cycle1.py
new file mode 100644
index 000000000000..aacfd5f46fca
--- /dev/null
+++ b/Lib/test/test_import/data/circular_imports/from_cycle1.py
@@ -0,0 +1,2 @@
+from .from_cycle2 import a
+b = 1
diff --git a/Lib/test/test_import/data/circular_imports/from_cycle2.py b/Lib/test/test_import/data/circular_imports/from_cycle2.py
new file mode 100644
index 000000000000..62a66e1cfd02
--- /dev/null
+++ b/Lib/test/test_import/data/circular_imports/from_cycle2.py
@@ -0,0 +1,2 @@
+from .from_cycle1 import b
+a = 1
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-08-15-12-48-36.bpo-20490.-hXeEn.rst b/Misc/NEWS.d/next/Core and Builtins/2019-08-15-12-48-36.bpo-20490.-hXeEn.rst
new file mode 100644
index 000000000000..dfee480e1aa7
--- /dev/null
+++ b/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.
diff --git a/Python/ceval.c b/Python/ceval.c
index f9e03b3097f0..d280d7977607 100644
--- a/Python/ceval.c
+++ b/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);
     }