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Description:

pytestsmtpd 0.1.0

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 = "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

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_ = "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

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_path
SMTPD_SSL_CERTS_PATH
./certs/
The path to the key and certificate in PEM format for encryption with SSL/TLS or StartTLS.

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

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. smtpdfix Issue #10
Currently no support for termination through signals. smtpdfix Issue #4

Written with ☕ and ❤ in Montreal, QC