Close #17828: better handling of codec errors

- output type errors now redirect users to the type-neutral
  convenience functions in the codecs module
- stateless errors that occur during encoding and decoding
  will now be automatically wrapped in exceptions that give
  the name of the codec involved

1 files changed, 18 insertions, 9 deletions

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 224a80b45e..77898165a7 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -3054,8 +3054,10 @@ PyUnicode_Decode(const char *s,
         goto onError;
     if (!PyUnicode_Check(unicode)) {
         PyErr_Format(PyExc_TypeError,
-                     "decoder did not return a str object (type=%.400s)",
-                     Py_TYPE(unicode)->tp_name);
+                     "'%.400s' decoder returned '%.400s' instead of 'str'; "
+                     "use codecs.decode() to decode to arbitrary types",
+                     encoding,
+                     Py_TYPE(unicode)->tp_name, Py_TYPE(unicode)->tp_name);
         Py_DECREF(unicode);
         goto onError;
     }
@@ -3113,8 +3115,10 @@ PyUnicode_AsDecodedUnicode(PyObject *unicode,
         goto onError;
     if (!PyUnicode_Check(v)) {
         PyErr_Format(PyExc_TypeError,
-                     "decoder did not return a str object (type=%.400s)",
-                     Py_TYPE(v)->tp_name);
+                     "'%.400s' decoder returned '%.400s' instead of 'str'; "
+                     "use codecs.decode() to decode to arbitrary types",
+                     encoding,
+                     Py_TYPE(unicode)->tp_name, Py_TYPE(unicode)->tp_name);
         Py_DECREF(v);
         goto onError;
     }
@@ -3425,7 +3429,8 @@ PyUnicode_AsEncodedString(PyObject *unicode,
         PyObject *b;
 
         error = PyErr_WarnFormat(PyExc_RuntimeWarning, 1,
-            "encoder %s returned bytearray instead of bytes",
+            "encoder %s returned bytearray instead of bytes; "
+            "use codecs.encode() to encode to arbitrary types",
             encoding);
         if (error) {
             Py_DECREF(v);
@@ -3438,8 +3443,10 @@ PyUnicode_AsEncodedString(PyObject *unicode,
     }
 
     PyErr_Format(PyExc_TypeError,
-                 "encoder did not return a bytes object (type=%.400s)",
-                 Py_TYPE(v)->tp_name);
+                 "'%.400s' encoder returned '%.400s' instead of 'bytes'; "
+                 "use codecs.encode() to encode to arbitrary types",
+                 encoding,
+                 Py_TYPE(v)->tp_name, Py_TYPE(v)->tp_name);
     Py_DECREF(v);
     return NULL;
 }
@@ -3465,8 +3472,10 @@ PyUnicode_AsEncodedUnicode(PyObject *unicode,
         goto onError;
     if (!PyUnicode_Check(v)) {
         PyErr_Format(PyExc_TypeError,
-                     "encoder did not return an str object (type=%.400s)",
-                     Py_TYPE(v)->tp_name);
+                     "'%.400s' encoder returned '%.400s' instead of 'str'; "
+                     "use codecs.encode() to encode to arbitrary types",
+                     encoding,
+                     Py_TYPE(v)->tp_name, Py_TYPE(v)->tp_name);
         Py_DECREF(v);
         goto onError;
     }