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A Python smtplib wrapper for Fastmail

Sometimes I want to send email from a Python script on my Mac. Up to now, my approach has been to shell out to osascript, and use AppleScript to invoke to compose and send the message. This is sub-optimal on several levels:

Plus it was a bit buggy and unreliable. Not a great solution.

My needs are fairly basic: I just want to be able to send a message from my email address, with a bit of body text and a subject, and optionally an attachment or two. And I’m only sending messages from one email provider, Fastmail.

Since the Python standard library includes smtplib, I decided to give that a try.

After a bit of mucking around, I came up with this wrapper:

from email import encoders
from email.mime.base import MIMEBase
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
import smtplib

class FastmailSMTP(smtplib.SMTP_SSL):
    """A wrapper for handling SMTP connections to Fastmail."""

    def __init__(self, username, password):
        super().__init__('', port=465)
        self.login(username, password)

    def send_message(self, *,
        msg_root = MIMEMultipart()
        msg_root['Subject'] = subject
        msg_root['From'] = from_addr
        msg_root['To'] = ', '.join(to_addrs)

        msg_alternative = MIMEMultipart('alternative')

        if attachments:
            for attachment in attachments:
                prt = MIMEBase('application', "octet-stream")
                prt.set_payload(open(attachment, "rb").read())
                    'Content-Disposition', 'attachment; filename="%s"'
                    % attachment.replace('"', ''))

        self.sendmail(from_addr, to_addrs, msg_root.as_string())

lines 7–12 create a subclass of smtplib.SMTP_SSL, and uses the supplied credentials to log into Fastmail. Annoyingly, this subclassing is broken on Python 2, because SMTP_SSL is an old-style class, and so super() doesn’t work. I only use Python 3 these days, so that’s okay for me, but you’ll need to change that if you want a backport.

For getting my username/password into the script, I use the keyring module. It gets them from the system keychain, which feels pretty secure. My email credentials are important – I don’t just want to store them in an environment variable or a hard-coded string.

lines 14–19 defines a convenience wrapper for sending a message. The * in the arguments list denotes the end of positional arguments – all the remaining arguments have to be called as keyword arguments. This is a new feature in Python 3, and I really like it, especially for functions with lots of arguments. It helps enforce clarity in the calling code.

In lines 20–23, I’m setting up a MIME message with my email headers. I deliberately use a multi-part MIME message so that I can add attachments later, if I want.

Then I add the body text. With MIME, you can send multiple versions of the body: a plain text and an HTML version, and the recipient’s client can choose which to display. In practice, I almost always use plaintext email, so that’s all I’ve implemented. If you want HTML, see Stack Overflow.

Then lines 29–37 add the attachments – if there are any. Note that I use None as the default value for the attachments argument, not an empty list – this is to avoid any gotchas around mutable default arguments.

Finally, on line 39, I call the sendmail method from the SMTP class, which actually dispatches the message into the aether.

The nice thing about subclassing the standard SMTP class is that I can use my wrapper class as a drop-in replacement. Like so:

with FastmailSMTP(user, pw) as server:
                        to_addrs=['', ''],
                        msg='Hello world from Python!',
                        subject='Sent from smtplib',

I think this is a cleaner interface to email. Mucking about with MIME messages and SMTP is a necessary evil, but I don’t always care about those details. If I’m writing a script where email support is an orthogonal feature, it’s nice to have them abstracted away.