Modify SmtpNotifier to accept template with defaults

[vchiapaikeo]

closes: #35381

Creating a TemplatedSmtpNotifier class that simplifies sending emails based on the discussion in #35381. This class simply extends from the SmtpNotifier and defaults a number of the required arguments (like email_from, subject, and html_content). The latter two variables are pulled from airflow/settings.py.

On the template itself, I was initially thinking of having at least three distinct templates following #35381 (comment) but it's simple to add a condition to detect whether we are sending an SLA miss or a failure/success/retry email so I decided on using a single template. Happy to change that approach if the community thinks having distinct templates are cleaner.

Will work on tests after the approach is generally agreed upon.

Examples

Sample dag used to test emails -

importdatetimefromairflowimportDAGfromairflow.operators.bashimportBashOperatorfromairflow.providers.smtp.notifications.smtpimportSmtpNotifierfromdatetimeimporttimedeltawithDAG(
dag_id="my_dag",
start_date=datetime.datetime(2021, 1, 1),
schedule="*/5 * * * *",
max_active_tasks=1,
max_active_runs=1,
sla_miss_callback=SmtpNotifier(from_email=None, to="xyz@gmail.com"),
):
BashOperator(
task_id="task",
bash_command="sleep 180 & exit 0",
retries=2,
sla=timedelta(seconds=30),
on_success_callback=SmtpNotifier(from_email=None, to="xyz@gmail.com"),
on_failure_callback=SmtpNotifier(from_email=None, to="xyz@gmail.com"),
on_retry_callback=SmtpNotifier(from_email=None, to="xyz@gmail.com"),
)

on_failure_callback / on_success_callback / on_retry_callback -

sla_miss_callback -

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[vchiapaikeo]

cc @eladkal , @potiuk , @BasPH, @hussein-awala - would love to hear what you think when you have a moment

[hussein-awala]

Well done! I just thought about it after seeing your change, and I think doing what @eladkal suggests is not complicated; you can just add a default value for subject and html_content in the SmtpNotifier operator without needing to create a new one, and it will be fully backward compatible.

[potiuk]

LGTM. @hussein-awala@eladkal ?

[stephenatgithub]

Please notice that chicken-egg provider is encountered.

apache-airflow-providers-smtp 1.6.0 does not work with airflow 2.8.1
because modified code in airflow/settings.py is not yet released in airflow 2.8.1

I manually downgraded apache-airflow-providers-smtp to 1.5.0 and it worked fine with airflow 2.8.1 now

I was using this constraint file during installation.

Should change it to apache-airflow-providers-smtp==1.5.0 manually and then specify your own constraints

[potiuk]

This is backward compatibility issue that should be fixed (it was not caught because the settings import was a local import and that's why it passed compatibility checks. Will make a PR fixing it in a moment.

[potiuk]

Fix in #37701

[stephenatgithub]

Wow... really quick responses and immediate actions. Thank you everyone in this airflow community.