import os
import sys
import tempfile
import mimetypes
import webbrowser

# Import the email modules we'll need
from email import policy
from email.parser import BytesParser

# An imaginary module that would make this work and be safe.
from imaginary import magic_html_parser

# In a real program you'd get the filename from the arguments.
with open('outgoing.msg', 'rb') as fp:
 msg = BytesParser(policy=policy.default).parse(fp)

# Now the header items can be accessed as a dictionary, and any non-ASCII will
# be converted to unicode:
print('To:', msg['to'])
print('From:', msg['from'])
print('Subject:', msg['subject'])

# If we want to print a preview of the message content, we can extract whatever
# the least formatted payload is and print the first three lines. Of course,
# if the message has no plain text part printing the first three lines of html
# is probably useless, but this is just a conceptual example.
simplest = msg.get_body(preferencelist=('plain', 'html'))
print()
print(''.join(simplest.get_content().splitlines(keepends=True)[:3]))

ans = input("View full message?")
if ans.lower()[0] == 'n':
 sys.exit()

# We can extract the richest alternative in order to display it:
richest = msg.get_body()
partfiles = {}
if richest['content-type'].maintype == 'text':
 if richest['content-type'].subtype == 'plain':
 for line in richest.get_content().splitlines():
 print(line)
 sys.exit()
 elif richest['content-type'].subtype == 'html':
 body = richest
 else:
 print("Don't know how to display {}".format(richest.get_content_type()))
 sys.exit()
elif richest['content-type'].content_type == 'multipart/related':
 body = richest.get_body(preferencelist=('html'))
 for part in richest.iter_attachments():
 fn = part.get_filename()
 if fn:
 extension = os.path.splitext(part.get_filename())[1]
 else:
 extension = mimetypes.guess_extension(part.get_content_type())
 with tempfile.NamedTemporaryFile(suffix=extension, delete=False) as f:
 f.write(part.get_content())
 # again strip the <> to go from email form of cid to html form.
 partfiles[part['content-id'][1:-1]] = f.name
else:
 print("Don't know how to display {}".format(richest.get_content_type()))
 sys.exit()
with tempfile.NamedTemporaryFile(mode='w', delete=False) as f:
 # The magic_html_parser has to rewrite the href="cid:...." attributes to
 # point to the filenames in partfiles. It also has to do a safety-sanitize
 # of the html. It could be written using html.parser.
 f.write(magic_html_parser(body.get_content(), partfiles))
webbrowser.open(f.name)
os.remove(f.name)
for fn in partfiles.values():
 os.remove(fn)

# Of course, there are lots of email messages that could break this simple
# minded program, but it will handle the most common ones.