The following will cause a KeyError on email.message.get()

import email
import email.policy

text = "Subject: =?us-ascii?X?somevalue?="
eml = email.message_from_string(text, policy=email.policy.default)
eml.get('Subject')

This is caused by the fact that the code in _encoded_words.py assumes the content-transfer-encoding of an encoded-word is always 'q' or 'b' (after lowercasing): https://github.com/python/cpython/blob/aca8c406ada3bb547765b262bed3ac0cc6be8dd3/Lib/email/_encoded_words.py#L178

I realise it's probably a silly edge case and I haven't (yet) encountered something like this in the wild, but it does seem contrary to the spirit of the email library to raise an exception like this that can propagate all the way to email.message.get().

Hello,

I am not a email expert, but according to RFC 1342 the enconding can be either "B" or "Q". So, I think is reasonable that when a not correct enconding is set, should be raise an exception


I think that we can improve the message raising a more specific Exception

I agree with you that according to the RFC, the cte can of course only be "B" or "Q". My point is that, in my example, if you try to do that you get a KeyError propagating all the way down to email.message.get(), which I believe is incorrect. 

Consider an encoded word which is syntactically incorrect in a different way, like  if for instance it's missing the terminating '?=':

'=?UTF-8?Q?somevalue'

Currently, this case will cause _encoded_words.py to throw a ValueError on this line:

_, charset, cte, cte_string, _ = ew.split('?')

Which is then caught by _header_value_parser.get_encoded_word() and handled appropriately.

To me this is the same kind of thing. I agree that an exception should be thrown, I just don't think it should propagate all the way back to the caller of email.message.get().

On a separate note, I agree with you that perhaps _encoded_words.decode() should throw more specific exceptions instead of ValueError and KeyError but that's a separate thing. I can fix that if you prefer.

Hi Andrei sorry for my last message. Now I understand perfectly your idea and your PR. IMO this is a correct patch.

New changeset 65dcc8a8dc41d3453fd6b987073a5f1b30c5c0fd by Abhilash Raj (Andrei Troie) in branch 'master':
bpo-38332: Catch KeyError from unknown cte in encoded-word. (GH-16503)
https://github.com/python/cpython/commit/65dcc8a8dc41d3453fd6b987073a5f1b30c5c0fd

New changeset febe359559781019c0c8432a2f768809d00af6af by Miss Islington (bot) in branch '3.7':
bpo-38332: Catch KeyError from unknown cte in encoded-word. (GH-16503)
https://github.com/python/cpython/commit/febe359559781019c0c8432a2f768809d00af6af

New changeset e540bb546163f108c7c304f2e6865efaa78cd4c2 by Miss Islington (bot) in branch '3.8':
bpo-38332: Catch KeyError from unknown cte in encoded-word. (GH-16503)
https://github.com/python/cpython/commit/e540bb546163f108c7c304f2e6865efaa78cd4c2