Styjun

In the program code is "unicode is not defined" returned. A change from unicode to str returns "str is not supported". What is wrong or missing?

for header in [ 'subject' ]:
 dh = email.header.decode_header(msg[header])
 default_charset = 'ASCII'
 print('%-8s: %s' % (header.upper(), ''.join([ unicode(t[0], t[1] or default_charset) for t in dh ])))

The unicode builtin function does not exist in Python 3 - this is why you get the exception NameError: name 'unicode' is not defined. In Python 3 the equivalent of unicode is str.

Like unicode, str accepts an encoding argument and will try to decode a bytestring using the provided encoding. If you pass a str instance to str for decoding you will get TypeError: decoding str is not supported.

The output of email.header.decode_header could include both str and bytes instances, so your comprehension needs to be able to handle both:

print('%-8s: %s' % ('subject'.upper(), ''.join(t[0] if isinstance(t[0], str) else str(t[0], t[1] or default_charset) for t in dh)))

(In Python 3 it's probably best to set default_charset to 'utf-8').

Finally, if you control how the message object is being created, you can have the headers automatically decoded by specifying a policy object when creating the message (Python 3.5+).

>>> from email.policy import default
>>> with open('message.eml', 'rb') as f:
... msg = email.message_from_bytes(f.read(), policy=default)
>>>

>>> for x in msg.raw_items():print(x)
... 
('Subject', 'Ayons asperges pour le =?utf-8?q?d=C3=A9jeuner?=')
('From', '=?utf-8?q?Pep=C3=A9?= Le Pew <pepe@example.com>')
('To', 'Penelope Pussycat <penelope@example.com>,n Fabrette Pussycat <fabrette@example.com>')
('Content-Type', 'text/plain; charset="utf-8"')
('Content-Transfer-Encoding', 'quoted-printable')
('MIME-Version', '1.0')
>>> msg['from']
'Pepé Le Pew <pepe@example.com>'
>>> msg['subject']
'Ayons asperges pour le déjeuner'

(Message data taken from the email examples).