How to send Bcc in Python Google API?

Question

I'm using the code described in:

Sending email via gmail & python

to send e-mails. Everything works great. Now I'd like to add a Bcc (one or more) and I don't know how.

1. If I add: message['Bcc'] = 'mail@mail.com' it works but the senders see the Bcc, and I don't want that.

2. I know that using SMTP you can:

    smtp = smtplib.SMTP('smtp.gmail.com', 587)
    smtp.sendmail(user, [to] + bcc, message.as_string())

but I don't want to do it that way.

How can I add Bcc without the sender knowing about it, using google google_auth_oauthlib?

This is the code I'm using

import httplib2
import os
import oauth2client
from oauth2client import client, tools, file
import base64
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from apiclient import errors, discovery
import mimetypes
from email.mime.image import MIMEImage
from email.mime.audio import MIMEAudio
from email.mime.base import MIMEBase

SCOPES = 'https://www.googleapis.com/auth/gmail.send'
CLIENT_SECRET_FILE = 'client_secret.json'
APPLICATION_NAME = 'Gmail API Python Send Email'

def get_credentials():
    home_dir = os.path.expanduser('~')
    credential_dir = os.path.join(home_dir, '.credentials')
    if not os.path.exists(credential_dir):
        os.makedirs(credential_dir)
    credential_path = os.path.join(credential_dir,
                                   'gmail-python-email-send.json')
    store = oauth2client.file.Storage(credential_path)
    credentials = store.get()
    if not credentials or credentials.invalid:
        flow = client.flow_from_clientsecrets(CLIENT_SECRET_FILE, SCOPES)
        flow.user_agent = APPLICATION_NAME
        credentials = tools.run_flow(flow, store)
        print('Storing credentials to ' + credential_path)
    return credentials

def SendMessage(sender, to, subject, msgHtml, msgPlain, attachmentFile=None):
    credentials = get_credentials()
    http = credentials.authorize(httplib2.Http())
    service = discovery.build('gmail', 'v1', http=http)
    if attachmentFile:
        message1 = createMessageWithAttachment(sender, to, subject, msgHtml, msgPlain, attachmentFile)
    else: 
        message1 = CreateMessageHtml(sender, to, subject, msgHtml, msgPlain)
    result = SendMessageInternal(service, "me", message1)
    return result

def SendMessageInternal(service, user_id, message):
    try:
        message = (service.users().messages().send(userId=user_id, body=message).execute())
        print('Message Id: %s' % message['id'])
        return message
    except errors.HttpError as error:
        print('An error occurred: %s' % error)
        return "Error"
    return "OK"

def CreateMessageHtml(sender, to, subject, msgHtml, msgPlain):
    msg = MIMEMultipart('alternative')
    msg['Subject'] = subject
    msg['From'] = sender
    msg['To'] = to
    msg.attach(MIMEText(msgPlain, 'plain'))
    msg.attach(MIMEText(msgHtml, 'html'))
    return {'raw': base64.urlsafe_b64encode(msg.as_string())}

def createMessageWithAttachment(
    sender, to, subject, msgHtml, msgPlain, attachmentFile):
    """Create a message for an email.

    Args:
      sender: Email address of the sender.
      to: Email address of the receiver.
      subject: The subject of the email message.
      msgHtml: Html message to be sent
      msgPlain: Alternative plain text message for older email clients          
      attachmentFile: The path to the file to be attached.

    Returns:
      An object containing a base64url encoded email object.
    """
    message = MIMEMultipart('mixed')
    message['to'] = to
    message['from'] = sender
    message['subject'] = subject

    messageA = MIMEMultipart('alternative')
    messageR = MIMEMultipart('related')

    messageR.attach(MIMEText(msgHtml, 'html'))
    messageA.attach(MIMEText(msgPlain, 'plain'))
    messageA.attach(messageR)

    message.attach(messageA)

    print("create_message_with_attachment: file: %s" % attachmentFile)
    content_type, encoding = mimetypes.guess_type(attachmentFile)

    if content_type is None or encoding is not None:
        content_type = 'application/octet-stream'
    main_type, sub_type = content_type.split('/', 1)
    if main_type == 'text':
        fp = open(attachmentFile, 'rb')
        msg = MIMEText(fp.read(), _subtype=sub_type)
        fp.close()
    elif main_type == 'image':
        fp = open(attachmentFile, 'rb')
        msg = MIMEImage(fp.read(), _subtype=sub_type)
        fp.close()
    elif main_type == 'audio':
        fp = open(attachmentFile, 'rb')
        msg = MIMEAudio(fp.read(), _subtype=sub_type)
        fp.close()
    else:
        fp = open(attachmentFile, 'rb')
        msg = MIMEBase(main_type, sub_type)
        msg.set_payload(fp.read())
        fp.close()
    filename = os.path.basename(attachmentFile)
    msg.add_header('Content-Disposition', 'attachment', filename=filename)
    message.attach(msg)

    return {'raw': base64.urlsafe_b64encode(message.as_string())}


def main():
    to = "to@address.com"
    sender = "from@address.com"
    subject = "subject"
    msgHtml = "Hi<br/>Html Email"
    msgPlain = "Hi\nPlain Email"
    SendMessage(sender, to, subject, msgHtml, msgPlain)
    # Send message with attachment: 
    SendMessage(sender, to, subject, msgHtml, msgPlain, '/path/to/file.pdf')

if __name__ == '__main__':
    main()