Access dynamic values in Airflow variables - airflow

Question

I am trying to achieve a way to access dynamic values in Airflow Variables.
Like
Just want to ask is there any way to insert the DAG_NAME and DateTime.now value at run-time which was defined in the DAG file
So the final result would be something like this "Started 0_dag_1 on 22-Sept-2021 12:00:00"

Answer 1

This is not built-in in airflow, so those variables are not automatically expanded when you use them.
But it's Python. You can do everything. But you just have to realise that Airflow is designed for people who know Python and can write their own custom Python code to extend built-in capabilities of Airflow. You can do it by custom operators of yours or via macros.
You can write the code to do that in your own operators (or implement it in your Python callables if you use PythonOperator) to process your variable via JINJA template and pass the context to the template. You can even write a common code for that that will be re-used by a number of custom operators.
This is nothing airflow-specific (except that you can reuse context that you get in execute method of airflow, where you have all the same fields and variables. Jinja documented here https://jinja.palletsprojects.com/en/3.0.x/ and you can find examples how Airflow does it in the code:
https://github.com/apache/airflow/blob/bbb5fe28066809e26232e403417c92c53cfa13d3/airflow/models/baseoperator.py#L1099
Also (as Elad mentioned in the comment) you could encapsulate similar code in custom macros (that you can add via plugins) and use those macros instead.
https://airflow.apache.org/docs/apache-airflow/stable/plugins.html but this is a little more involved.

Answer 2

For your use case it's best to use user defined macro and not Variables.
Variables are stored in the database as string which means that you will need to read the record and then run a logic to replace placeholders.
Macros saves you that trouble.
A possible solution is:
from datetime import datetime
from airflow.operators.bash import BashOperator
from airflow import DAG
def macro_string(dag):
now = datetime.now().strftime('%d-%b-%Y %H:%M:%S')
return f'<p> Started {dag.dag_id} on { now }</p>'
dag = DAG(
dag_id='macro_example',
schedule_interval='0 21 * * *',
start_date=datetime(2021, 1, 1),
user_defined_macros={
'mymacro': macro_string,
},
)
task = BashOperator(
task_id='bash_mymacro',
bash_command='echo "{{ mymacro(dag) }}"',
dag=dag,
)