Get a file from sftp then email it as attachment in airflow

Question

I'm working on a dag that get file from sftp then attach the file in the email using airflow.

The process:

1. Copy file from sftp to ./my_path/my_file.txt locally.
2. Use the path to find the file and attach in the email.

After further investigation, I need to make sure that both tasks to run with the same worker since I want to save the file locally rather than in GCS.

Hence, I used SubDagOperator for this. Based on this answer.

The code is running fine until I added EmailOperator. This send_email task failed because it cannot find the path of the file.

import os
from datetime import datetime, timedelta
from airflow import DAG
from airflow.providers.sftp.operators.sftp import SFTPOperator
from airflow.providers.google.cloud.transfers.sftp_to_gcs import SFTPToGCSOperator
from airflow.operators.dummy import DummyOperator
from airflow.operators.subdag_operator import SubDagOperator
from airflow.operators.email import EmailOperator


default_args = {
    "owner": "my_email@example.com",
    "start_date": datetime(2021, 11, 1),
    "retries": 1,
    "retry_delay": timedelta(seconds=60),
    "depends_on_past": False,
    "email": ["my_email@example.com],
    "email_on_failure": True,
    "email_on_retry": False,
    "catchup": False
}

PARENT_DAG_NAME = "parent_dag_v3"
CHILD_DAG_NAME = "run_child_dag"

dag = DAG(
    PARENT_DAG_NAME,
    default_args=default_args,
    description="My awesome pipeline",
    schedule_interval="0 7 * * *"
)


def child_dags(parent_dag_name, child_dag_name, args):
    child_dag = DAG(
        "{0}.{1}".format(parent_dag_name, child_dag_name),
        default_args=args,
        description="My child DAG",
        schedule_interval=None
        )

    copy_file = SFTPOperator(
        task_id="download_my_files",
        ssh_conn_id="=my_conn",
        local_filepath="./my_path/my_file.txt",
        remote_filepath="/IN/my_file.txt",
        operation="get",
        create_intermediate_dirs=True,
        dag=child_dag
    )


    send_email = EmailOperator(
    to=["user_email@example.com"],
    task_id="send_email",
    subject="Testing onlyyy",
    html_content="blahblah falafel wooooo",
    files=["./my_path/my_file.txt"], # STUCK HERE
    dag=child_dag)

    copy_file >> send_email

    return child_dag


run_child_dag = SubDagOperator(
    task_id = CHILD_DAG_NAME,
    subdag=child_dags(parent_dag_name = PARENT_DAG_NAME,
        child_dag_name = CHILD_DAG_NAME,  # Must be the same as task ID
        args = dag.default_args),
    dag=dag
)


start = DummyOperator(task_id="start", dag=dag)

start >> run_child_dag

Main question is, how do I find the right path to be used in EmailOperator?

If there is any better way of doing this using GCS for example, I'm open for suggestions.