SMTPDFix: Test email, locally

A simple SMTP server based on aiosmtpd for use as a fixture with pytest that supports encryption and authentication. All this does is receives messages and appends them to a list as an email.Message.

⚠ 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 smtpdfix

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",
            "smtpdfix",
        ],
    },
)

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 = "from.addr@example.org"
    to_addrs = "to.addr@example.org"
    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_ = "from.addr@example.org"
    to_ = "to.addr@example.org"
    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

As of version 0.2.7 the plugin automatically registers and it is not necessary to include it manually by adding pytest_plugins = "smtpdfix" to the module or conftest.py.

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.

Not as a fixture

In some situations it may be desirable to not use the fixture which is initialized before entering the test. This can be accomplished by using the SMTPDFix class.

from smtplib import SMTP

from smtpdfix import SMTPDFix


def test_smtpdfix():
    hostname, port = "127.0.0.1", 8025

    with SMTPDFix(hostname, port) as smtpd:
        with SMTP(hostname, port) as client:
            from_addr = "foo@example.org"
            to_addrs = "bar@example.org"
            msg = (f"From: {from_addr}\r\n"
                   f"To: {to_addrs}\r\n"
                   f"Subject: Foo\r\n\r\n"
                   f"Foo bar")

            client.sendmail(from_addr, to_addrs, msg)

        assert len(smtpd.messages) == 1

Configuration

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

If environment variables are included in a .env file they'll be loaded automatically.

Resolving certificate and key paths

In order to resolve the certificate and key paths for the SSL/TLS context SMTPDFix does the following:

1. On initialization of a smtpdfix.Config the ssl_cert_path is set by the SMTPD_SSL_CERTS_PATH environment variable and the ssl_cert_files is set to a tuple of (SMTPD_SSL_CERT_FILE and SMTPD_SSL_KEY_FILE). If the environment variables are not set the deafults are used.
2. If an SSL Context is needed, when the smptdfix.AuthController is initialized it will attempt to find the files in the following sequence for both the certificate file and the key file:
   1. If the value in ssl_cert_files is None then None is returned. Setting the key file to be none assumes that it has been included in the certificate file.
   2. If the value in ssl_cert_files is a valid path to a file then this is returned.
   3. ssl_cert_path and the value from ssl_cert_files are joined and returned if it a valid path to a file.
   4. A FileNotFoundError is raised.

An example, assuming that the certificate and key are written in a single PEM file located at ./certificates/localhost.cert.pem would be:

from smtplib import STMP_SSL


def test_custom_certificate(smtpd):
    smtpd.config.ssl_cert_files = "./certificates/localhost.cert.pem"
    smtpd.config.use_ssl = True

    from_ = "from.addr@example.org"
    to_ = "to.addr@example.org"
    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

Alternatives

Many libraries for sending email have built-in methods for testing and using these methods should generally be prefered over SMTPDFix. 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 -p no:smtpd --cov

Before submitting a pull request with your changes you should ensure that all imports are sorted and that the code passes linting with flake8.

$ isort .
$ flake8 .

We include a pre-commit configuration file to automate checks and clean up imports before pushing code. In order to install pre-commit git hooks:

$ pip install pre-commit
$ pre-commit install

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. Issue #10
• Currently no support for termination through signals. Issue #4
• If the fixture start exceeds the ready_timeout and aborts the host and port are not consistently released and subsequent uses may result in an error. Issue #80

Written with ☕ and ❤ in Montreal, QC