如何获取Gmail API消息的文本/纯文本部分

import base64
from email import policy
from email.parser import BytesParser

def get_message_text(service, user_id, msg_id):
    message = service.users().messages().get(userId=user_id, id=msg_id).execute()
    payload = message['payload']
    
    # 查找纯文本部分
    for part in payload['parts']:
        if part['mimeType'] == 'text/plain':
            body = base64.urlsafe_b64decode(part['body']['data'].encode('UTF-8'))
            return body.decode()
    
    # 如果没有找到纯文本部分，尝试解析整个邮件的MIME结构
    if 'parts' in payload:
        msg_bytes = base64.urlsafe_b64decode(payload['body']['data'].encode('UTF-8'))
    else:
        msg_bytes = base64.urlsafe_b64decode(message['payload']['body']['data'].encode('UTF-8'))
    
    msg = BytesParser(policy=policy.default).parsebytes(msg_bytes)
    for part in msg.walk():
        if part.get_content_type() == 'text/plain':
            return part.get_payload(decode=True).decode()

# 使用示例
# service = 已授权的Gmail API服务对象
# user_id = 'me'
# msg_id = '邮件的ID'
# text = get_message_text(service, user_id, msg_id)
# print(text)