From a1814637d8be7132d33792f0fa2f517f4dbe3cce Mon Sep 17 00:00:00 2001
From: Matthias Bussonnier <bussonniermatthias@gmail.com>
Date: Tue, 14 Feb 2017 16:54:59 -0800
Subject: [PATCH 1/3] bpo-29546: Improve from-import error message with
 location

Add location information like canonical module name where identifier
cannot be found and file location if available.

First iteration of this was gh-91
---
 Doc/whatsnew/3.7.rst             |  3 +++
 Lib/test/test_import/__init__.py | 14 +++++++++++++-
 Misc/NEWS                        |  2 ++
 Python/ceval.c                   | 17 ++++++++++++++---
 4 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
index 21621c5ee4eac9..f6367fd1f4a672 100644
--- a/Doc/whatsnew/3.7.rst
+++ b/Doc/whatsnew/3.7.rst
@@ -83,6 +83,9 @@ Other Language Changes
   whitespace, not only spaces.
   (Contributed by Robert Xiao in :issue:`28927`.)
 
+* :exc:`ImportError` now displays module name and module ``__file__`` path when
+  ``from ... import ...`` fails. :issue:`29546`.
+
 
 New Modules
 ===========
diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index df678f17f18e4e..ace1a46f9b91a7 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -85,6 +85,15 @@ def test_from_import_missing_attr_has_name_and_path(self):
             from os import i_dont_exist
         self.assertEqual(cm.exception.name, 'os')
         self.assertEqual(cm.exception.path, os.__file__)
+        self.assertRegex(str(cm.exception), "cannot import name 'i_dont_exist' from 'os' \(.*/Lib/os.py\)")
+
+    def test_from_import_missing_attr_has_name_and_so_path(self):
+        import _opcode
+        with self.assertRaises(ImportError) as cm:
+            from _opcode import i_dont_exist
+        self.assertEqual(cm.exception.name, '_opcode')
+        self.assertEqual(cm.exception.path, _opcode.__file__)
+        self.assertRegex(str(cm.exception), "cannot import name 'i_dont_exist' from '_opcode' \(.*\.(so|dll)\)")
 
     def test_from_import_missing_attr_has_name(self):
         with self.assertRaises(ImportError) as cm:
@@ -365,9 +374,12 @@ def __getattr__(self, _):
         module_name = 'test_from_import_AttributeError'
         self.addCleanup(unload, module_name)
         sys.modules[module_name] = AlwaysAttributeError()
-        with self.assertRaises(ImportError):
+        with self.assertRaises(ImportError) as cm:
             from test_from_import_AttributeError import does_not_exist
 
+        self.assertEqual(str(cm.exception),
+            "cannot import name 'does_not_exist' from '<unknown module name>' (unknown location)")
+
 
 @skip_if_dont_write_bytecode
 class FilePermissionTests(unittest.TestCase):
diff --git a/Misc/NEWS b/Misc/NEWS
index 0c2122f8ca04d1..0d4451f05ca51b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -21,6 +21,8 @@ Core and Builtins
 
 - bpo-29546: Set the 'path' and 'name' attribute on ImportError for ``from ... import ...``.
 
+- bpo-29546: Improve from-import error message with location
+
 - Issue #29319: Prevent RunMainFromImporter overwriting sys.path[0].
 
 - Issue #29337: Fixed possible BytesWarning when compare the code objects.
diff --git a/Python/ceval.c b/Python/ceval.c
index 69c93838419609..bc22218d6ca6f9 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -4995,7 +4995,7 @@ import_from(PyObject *v, PyObject *name)
 {
     PyObject *x;
     _Py_IDENTIFIER(__name__);
-    PyObject *fullmodname, *pkgname, *pkgpath;
+    PyObject *fullmodname, *pkgname, *pkgpath, *pkgname_or_unknown;
 
     x = PyObject_GetAttr(v, name);
     if (x != NULL || !PyErr_ExceptionMatches(PyExc_AttributeError))
@@ -5022,12 +5022,23 @@ import_from(PyObject *v, PyObject *name)
     return x;
  error:
     pkgpath = PyModule_GetFilenameObject(v);
+    if (pkgname == NULL) {
+        pkgname_or_unknown = PyUnicode_FromString("<unknown module name>");
+    } else {
+        pkgname_or_unknown = pkgname;
+    }
 
     if (pkgpath == NULL || !PyUnicode_Check(pkgpath)) {
         PyErr_Clear();
-        PyErr_SetImportError(PyUnicode_FromFormat("cannot import name %R", name), pkgname, NULL);
+        PyErr_SetImportError(
+            PyUnicode_FromFormat("cannot import name %R from %R (unknown location)",
+                name, pkgname_or_unknown),
+            pkgname, NULL);
     } else {
-        PyErr_SetImportError(PyUnicode_FromFormat("cannot import name %R", name), pkgname, pkgpath);
+        PyErr_SetImportError(
+            PyUnicode_FromFormat("cannot import name %R from %R (%S)",
+                name, pkgname_or_unknown, pkgpath),
+            pkgname, pkgpath);
     }
 
     return NULL;

From 6ec81c713c8fb904a02b432e980db307c6f9bf56 Mon Sep 17 00:00:00 2001
From: Matthias Bussonnier <bussonniermatthias@gmail.com>
Date: Fri, 17 Feb 2017 17:23:25 -0800
Subject: [PATCH 2/3] Update reference counting

from Barry Warsaw:

- It's possible the code jumps to error: after pkgname is decref'd. I
  think that decref should be deferred until just before the good-path
  return.

- pkgname_or_unknown will be a new reference if pkgname is NULL, or it
  will steal the pkgname reference if it's not. In either case, the error
  stanza should decref pkgname_or_unknown, which will take care of
  ensuring the object gets decref for any goto error path.

- pkgpath doesn't get decref'd. I think you should Py_XDECREF it before
  the return NULL just in case pkgpath is itself NULL.
---
 Python/ceval.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Python/ceval.c b/Python/ceval.c
index bc22218d6ca6f9..d77871d6f30cfa 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -5009,7 +5009,6 @@ import_from(PyObject *v, PyObject *name)
         goto error;
     }
     fullmodname = PyUnicode_FromFormat("%U.%U", pkgname, name);
-    Py_DECREF(pkgname);
     if (fullmodname == NULL) {
         return NULL;
     }
@@ -5018,6 +5017,7 @@ import_from(PyObject *v, PyObject *name)
     if (x == NULL) {
         goto error;
     }
+    Py_DECREF(pkgname);
     Py_INCREF(x);
     return x;
  error:
@@ -5041,6 +5041,8 @@ import_from(PyObject *v, PyObject *name)
             pkgname, pkgpath);
     }
 
+    Py_XDECREF(pkgname_or_unknown);
+    Py_XDECREF(pkgpath);
     return NULL;
 }
 

From 1b6e462f8d5cf4fc0076ba4bdbe71cabe01c2fb6 Mon Sep 17 00:00:00 2001
From: Matthias Bussonnier <bussonniermatthias@gmail.com>
Date: Mon, 20 Feb 2017 15:08:27 -0800
Subject: [PATCH 3/3] Handle if PyUnicode_FromString fails.

---
 Python/ceval.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Python/ceval.c b/Python/ceval.c
index d77871d6f30cfa..81c89dfadfaac0 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -5024,6 +5024,10 @@ import_from(PyObject *v, PyObject *name)
     pkgpath = PyModule_GetFilenameObject(v);
     if (pkgname == NULL) {
         pkgname_or_unknown = PyUnicode_FromString("<unknown module name>");
+        if (pkgname_or_unknown == NULL) {
+            Py_XDECREF(pkgpath);
+            return NULL;
+        }
     } else {
         pkgname_or_unknown = pkgname;
     }

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