Change error message.

Lib/test/test_codecs.py

@@ -3196,7 +3196,7 @@ def test_multiple_args(self):
 
     # http://bugs.python.org/issue19609
     def test_codec_lookup_failure(self):
-        msg = "^unknown encoding: {}$".format(self.codec_name)
+        msg = f"^encoding '{self.codec_name}' is not registered$"
         with self.assertRaisesRegex(LookupError, msg):
             "str input".encode(self.codec_name)
         with self.assertRaisesRegex(LookupError, msg):

Lib/test/test_io.py

@@ -3939,7 +3939,7 @@ def _to_memoryview(buf):
 
 class CTextIOWrapperTest(TextIOWrapperTest):
     io = io
-    shutdown_error = "LookupError: unknown encoding: ascii"
+    shutdown_error = "LookupError: encoding 'ascii' is not registered"
 
     def test_initialization(self):
         r = self.BytesIO(b"\xc3\xa9\n\n")
@@ -4039,7 +4039,7 @@ def write(self, data):
 
 class PyTextIOWrapperTest(TextIOWrapperTest):
     io = pyio
-    shutdown_error = "LookupError: unknown encoding: ascii"
+    shutdown_error = "LookupError: encoding 'ascii' is not registered"
 
 
 class IncrementalNewlineDecoderTest(unittest.TestCase):

Lib/tokenize.py

@@ -398,10 +398,9 @@ def find_cookie(line):
         except LookupError:
             # This behaviour mimics the Python interpreter
             if filename is None:
-                msg = "unknown encoding: " + encoding
+                msg = f"encoding '{encoding}' is not registered"
             else:
-                msg = "unknown encoding for {!r}: {}".format(filename,
-                        encoding)
+                msg = f"encoding '{encoding}' is not registered for {filename!r}"
             raise SyntaxError(msg)
 
         if bom_found:

Python/codecs.c

@@ -204,7 +204,7 @@ PyObject *_PyCodec_Lookup(const char *encoding)
     if (result == NULL) {
         /* XXX Perhaps we should cache misses too ? */
         PyErr_Format(PyExc_LookupError,
-                     "unknown encoding: %s", encoding);
+                     "encoding '%s' is not registered", encoding);
         goto onError;
     }