无法将IMAPInputStream强制转换为Multipart

from email.parser import Parser
from email.header import decode_header
from email.utils import parseaddr

def parse_imap_input(imap_input):
    msg = Parser().parsestr(imap_input)
    msg_body = b''
    if msg.is_multipart():
        for part in msg.walk():
            ctype = part.get_content_type()
            if ctype == 'text/plain':
                msg_body = part.get_payload()
                break
    else:
        msg_body = msg.get_payload()

    headers = {}
    for key, value in msg.items():
        decoded_value = decode_header(value)[0]
        if key == 'Subject':
            headers['Subject'] = decoded_value.decode('utf-8')
        elif key == 'From':
            headers['From'] = parseaddr(decoded_value)[1]
        elif key == 'To':
            headers['To'] = parseaddr(decoded_value)[1]
        elif key == 'Date':
            headers['Date'] = decoded_value.decode('utf-8')

    return headers, msg_body