Parsing an invalid email message id can throw a header parser error. A bug in parse_message_ still tries to append the unset token to a variable.

File "/opt/python-3.8.0/lib/python3.8/email/_header_value_parser.py", line 2116, in parse_message_
id
    message_id.append(token)
UnboundLocalError: local variable 'token' referenced before assignment

Version 3.7 is not affected.

Thanks for the report. Can you please attach a sample script to reproduce this error?

This was also reported in issue38708 with the original code added in issue35805. Commenting out the except clause also doesn't raise any error in test suite so I assume the code path was not tested. Maybe the script could be added as part of the test suite.

$ git diff
diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index 1668b4a14e..9b6ca3a268 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -2110,10 +2110,10 @@ def parse_message_id(value):
     message_id = MessageID()
     try:
         token, value = get_msg_id(value)
+        message_id.append(token)
     except errors.HeaderParseError:
         message_id.defects.append(errors.InvalidHeaderDefect(
             "Expected msg-id but found {!r}".format(value)))
-    message_id.append(token)
     return message_id

 #

More tests are always good :)

The "correct" solution here (as far as I remember, its has been a while since I've had time to even looked at the _header_value_parser code) would be to add a new 'invalid-msg-id' token, and do this:

    message_id = MessageID()
    try:
        token, value = get_msg_id(value)
        message_id.append(token)
    except HeaderParseError as ex:
        message_id = InvalidMessageID(value)
        message_id.defects.append(InvalidHeaderDefect(
            f"Invalid msg_id: {ex}"))
    return message_id

Actually, the success path there should also check that value is empty, and if it is not register a defect for that as well.

New changeset bb815499af855b1759c02535f8d7a9d0358e74e8 by Miss Islington (bot) (Claudiu Popa) in branch 'master':
bpo-38698: Prevent UnboundLocalError to pop up in parse_message_id (GH-17277)
https://github.com/python/cpython/commit/bb815499af855b1759c02535f8d7a9d0358e74e8

New changeset e21aa61e96f8343200e765d119ebe778873a6bf1 by Miss Islington (bot) in branch '3.8':
bpo-38698: Prevent UnboundLocalError to pop up in parse_message_id (GH-17277)
https://github.com/python/cpython/commit/e21aa61e96f8343200e765d119ebe778873a6bf1

New changeset 68157da8b42b26408af5d157d2dba4fcf29c6320 by Abhilash Raj in branch 'master':
bpo-38698: Add a new InvalidMessageID token to email header parser. (GH-17503)
https://github.com/python/cpython/commit/68157da8b42b26408af5d157d2dba4fcf29c6320

New changeset f66f4a09d0b6817fe6a86a567fd506aa223f1563 by Miss Islington (bot) in branch '3.8':
bpo-38698: Add a new InvalidMessageID token to email header parser. (GH-17503)
https://github.com/python/cpython/commit/f66f4a09d0b6817fe6a86a567fd506aa223f1563

Closing this as fixed.