How to send email with BytesIO attachment via SMTP in Python

This example details how to send an email in Python, with an attachment from a io.BytesIO instance instead of reading the attachment from a file on the filesystem:

#!/usr/bin/env python3
__author__ = "Uli Köhler"
__license__ = "CC0 1.0 Universal (public domain)"
__version__ = "1.0"
import smtplib
import mimetypes
from io import BytesIO
from email.message import EmailMessage

# Create message and set text content
msg = EmailMessage()
msg['Subject'] = 'This email contains an attachment'
msg['From'] = '[email protected]'
msg['To'] = '[email protected]'
# Set text content
msg.set_content('Please see attached file')

def attach_bytesio_to_email(email, buf, filename):
    """Attach a file identified by filename, to an email message"""
    # Reset read position & extract data
    buf.seek(0)
    binary_data = buf.read()
    # Guess MIME type or use 'application/octet-stream'
    maintype, _, subtype = (mimetypes.guess_type(filename)[0] or 'application/octet-stream').partition("/")
    # Add as attachment
    email.add_attachment(binary_data, maintype=maintype, subtype=subtype, filename=filename)

# Attach files
buf = BytesIO()
buf.write(b"This is a test text")
attach_bytesio_to_email(msg, buf, "test.txt")

def send_mail_smtp(mail, host, username, password):
    s = smtplib.SMTP(host)
    s.starttls()
    s.login(username, password)
    s.send_message(msg)
    s.quit()

send_mail_smtp(msg, 'smtp.my-domain.com', '[email protected]', 'sae7ooka0S')

The script from above is using the following utility functions:

def attach_bytesio_to_email(email, buf, filename):
    """Attach a file identified by filename, to an email message"""
    # Reset read position & extract data
    buf.seek(0)
    binary_data = buf.read()
    # Guess MIME type or use 'application/octet-stream'
    maintype, _, subtype = (mimetypes.guess_type(filename)[0] or 'application/octet-stream').partition("/")
    # Add as attachment
    email.add_attachment(binary_data, maintype=maintype, subtype=subtype, filename=filename

def send_mail_smtp(mail, host, username, password):
    s = smtplib.SMTP(host)
    s.starttls()
    s.login(username, password)
    s.send_message(msg)
    s.quit()

which you can use in your code directly. The easiest way to initialize the email message is using

# Create message and set text content
msg = EmailMessage()
msg['Subject'] = 'This email contains an attachment'
msg['From'] = '[email protected]'
msg['To'] = '[email protected]'
# Set text content
msg.set_content('Please see attached file')

and then attaching your BytesIO instance (named buf) using

attach_bytesio_to_email(msg, buf, "test.txt")

and once you’re finished with adding attachments, sending the message using

send_mail_smtp(msg, 'smtp.my-domain.com', '[email protected]', 'sae7ooka0S')