How to attach large files to an email using Python - Gmail API

The issue you're having here is that your MediaUpload is a single attachment.

Instead of uploading a single attachment as a resumable MediaUpload, you need to upload the entire RFC822 message as a resumable MediaUpload.

In other words:

import ...
...
from io import BytesIO
from googleapiclient.http import MediaIoBaseUpload

SCOPES = [ 'scopes' ]

creds = get_credentials_somehow()
gmail = get_authed_service_somehow()

msg = create_rfc822_message(headers, email_body)
to_attach = get_attachment_paths_from_dir('../reports/tps/memos/2019/04')
add_attachments(msg, to_attach)

media = MediaIoBaseUpload(BytesIO(msg.as_bytes()), mimetype='message/rfc822', resumable=True)
body_metadata = {} # no thread, no labels in this example
resp = gmail.users().messages().send(userId='me', body=body_metadata, media_body=media).execute()
print(resp)
# { "id": "some new id", "threadId": "some new thread id", "labelIds": ["SENT"]}

I pieced this together from your provided code, reviewing this GitHub issue and Google's Inbox-to-Gmail email importer, specificially this bit.

When sending replies to existing messages, you will almost certainly have some sort of metadata that you should provide to help Gmail keep track of your new response and the original conversation. Namely, instead of an empty body parameter, you would pass informative metadata such as

body_metadata = { 'labelIds': [
                    "your label id here",
                    "another label id" ],
                  'threadId': "some thread id you took from the message you're replying to"
                }

Other good refs:

• API Client's Gmail PyDoc
• Actual code used

You mention the attachment being larger than 10Mb, but you don't mention it being smaller than 25Mb: there's a limitation to gmail that attachments can't be larger than 25Mb, so if this is your case, there's simply no way to get this done, as it is beyond gmail limitations.

The explanation can be found here.

Can you confirm that your attachment is not too large?