pytest-smtpd

⚠ Not intended for use with production systems. ⚠

This fixture is intended to address cases where to test an application that sends an email, it needs to be intercepted for subsequent processing. For example, sending an email with a code for password reset or two-factor authentication. This fixture allows a test to trigger the email being sent, ensure that it's sent, and read the email.

Installing

To install using pip, first upgrade pip to the latest version to avoid any issues installing cryptography:

python -m pip install --upgrade pip
pip install pytest-smtpd

Or, if you're using setuptools, it can be included in the extras_require argument of a setup.py file:

setup(
    ...
    extras_require={
        "test": [
            "pytest",
            "pytest-smtpd",
        ],
    },
)

and then installed with pip (-e assumes that you want your project to be editable):

python -m pip install --upgrade pip
pip install -e .[test]

Using

The SMTPDFix plugin, smtpd, automatically registers for use with pytest when you install smtpdfix. To use it simply add to your test method.

from smtplib import SMTP


def test_sendmail(smtpd):
    from_addr = "[email protected]"
    to_addrs = "[email protected]"
    msg = (f"From: {from_addr}\r\n"
           f"To: {to_addrs}\r\n"
           f"Subject: Foo\r\n\r\n"
           f"Foo bar")

    with SMTP(smtpd.hostname, smtpd.port) as client:
        client.sendmail(from_addr, to_addrs, msg)

    assert len(smtpd.messages) == 1

To use STARTTLS:

from smtplib import SMTP


def test_sendmail(smtpd):
    smptd.config.use_starttls = True
    from_ = "[email protected]"
    to_ = "[email protected]"
    msg = (f"From: {from_}\r\n"
           f"To: {to_}\r\n"
           f"Subject: Foo\r\n\r\n"
           f"Foo bar")

    with SMTP(smtpd.hostname, smtpd.port) as client:
        client.starttls()  # Note that you need to call starttls first.
        client.sendmail(from_addr, to_addrs, msg)

    assert len(smtpd.messages) == 1

To use TLS encryption on the connections:

from smtplib import STMP_SSL  # Note the different client class.


def test_custom_certificate(smtpd):
    smtpd.config.use_ssl = True

    from_ = "[email protected]"
    to_ = "[email protected]"
    msg = (f"From: {from_}\r\n"
           f"To: {to_}\r\n"
           f"Subject: Foo\r\n\r\n"
           f"Foo bar")

    with SMTP_SSL(smtpd.hostname, smtpd.port) as client:
        client.sendmail(from_addr, to_addrs, msg)

    assert len(smtpd.messages) == 1

The certificates included with the fixture will work for addresses localhost, localhost.localdomain, 127.0.0.1, 0.0.0.1, ::1. If using other addresses the key (key.pem) and certificate (cert.pem) must be in a location specified under SMTP_SSL_CERTS_PATH.

Configuration

Configuration is handled through properties in the config of the fixture and are initially set from environment variables:

Property	Variable	Default	Description
host	SMTPD_HOST	127.0.0.1 or ::1	The hostname that the fixture will listen on.
port	SMTPD_PORT	a random free port	The port that the fixture will listen on.
ready_timeout	SMTPD_READY_TIMEOUT	10.0	The seconds the server will wait to start before raising a TimeoutError.
login_username	SMTPD_LOGIN_NAME	user	Username for default authentication.
login_password	SMTPD_LOGIN_PASSWORD	password	Password for default authentication.
use_ssl	SMTPD_USE_SSL	False	Whether the fixture should use fixed TLS/SSL for transactions. If using smtplib requires that SMTP_SSL be used instead of SMTP.
use_starttls	SMTPD_USE_STARTTLS	False	Whether the fixture should use StartTLS to encrypt the connections. If using smtplib requires that SMTP.starttls() is called before other commands are issued. Overrides use_tls as the preferred method for securing communications with the client.
enforce_auth	SMTPD_ENFORCE_AUTH	False	If set to true then the fixture refuses MAIL, RCPT, DATA commands until authentication is completed.
ssl_cert_files	SMTPD_SSL_CERT_FILE and SMTPD_SSL_KEY_FILE	("cert.pem", None)	A tuple of the path for the certificate file and key file in PEM format.

Alternatives

Many libraries for sending email have built-in methods for testing and using these methods should generally be prefered over pytest-smtpd. Some known solutions:

• fastapi-mail: has a record_messsages() method to intercept the mail. Instructions on how to suppress the sending of mail and implement it can be seen at https://sabuhish.github.io/fastapi-mail/example/#unittests-using-fastapimail
• flask-mail: has a method to suppress sending and capture the email for testing purposes. Instructions

Developing

To develop and test smtpdfix you will need to install pytest-asyncio to run asynchronous tests, isort to sort imports and flake8 to lint. To install in a virtual environment for development:

python -m venv venv
./venv/scripts/activate
pip install -e .[dev]

Code is tested using tox:

tox

Quick tests can be handled by running pytest directly:

pytest

Known Issues

• Firewalls may interfere with the operation of the smtp server.
• Authenticating with LOGIN and PLAIN mechanisms fails over TLS/SSL, but works with STARTTLS. smtpdfix Issue #10
• Currently no support for termination through signals. smtpdfix Issue #4

Written with ☕ and ❤ in Montreal, QC