 .. Licensed to the Apache Software Foundation (ASF) under one
    or more contributor license agreements.  See the NOTICE file
    distributed with this work for additional information
    regarding copyright ownership.  The ASF licenses this file
    to you under the Apache License, Version 2.0 (the
    "License"); you may not use this file except in compliance
    with the License.  You may obtain a copy of the License at

 ..   http://www.apache.org/licenses/LICENSE-2.0

 .. Unless required by applicable law or agreed to in writing,
    software distributed under the License is distributed on an
    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    KIND, either express or implied.  See the License for the
    specific language governing permissions and limitations
    under the License.


Creating a custom Operator
==========================


Airflow allows you to create new operators to suit the requirements of you or your team. 
The extensibility is one of the many reasons which makes Apache Airflow powerful. 

You can create any operator you want by extending the :class:`airflow.models.baseoperator.BaseOperator`

There are two methods that you need to override in a derived class:

* Constructor - Define the parameters required for the operator. You only need to specify the arguments specific to your operator.
  Use ``@apply_defaults`` decorator function to fill unspecified arguments with ``default_args``. You can specify the ``default_args``
  in the dag file. See :ref:`Default args <default-args>` for more details.

* Execute - The code to execute when the runner calls the operator. The method contains the 
  airflow context as a parameter that can be used to read config values.

Let's implement an example ``HelloOperator`` in a new file ``hello_operator.py``:

.. code::  python
        
        from airflow.models.baseoperator import BaseOperator
        from airflow.utils.decorators import apply_defaults
        
        class HelloOperator(BaseOperator):

            @apply_defaults
            def __init__(
                    self,
                    name: str,
                    *args, **kwargs) -> None:
                super().__init__(*args, **kwargs)
                self.name = name

            def execute(self, context):
                message = "Hello {}".format(self.name)
                print(message)
                return message

.. note::

    For imports to work, you should place the file in a directory that
    is present in the ``PYTHONPATH`` env. Airflow adds ``dags/``, ``plugins/``, and ``config/`` directories
    in the Airflow home to ``PYTHONPATH`` by default. e.g., In our example, 
    the file is placed in the ``custom_operator`` directory.

You can now use the derived custom operator as follows:

.. code:: python

    from custom_operator.hello_operator import HelloOperator

    with dag:
        hello_task = HelloOperator(task_id='sample-task', name='foo_bar')

Hooks
^^^^^
Hooks act as an interface to communicate with the external shared resources in a DAG.
For example, multiple tasks in a DAG can require access to a MySQL database. Instead of
creating a connection per task, you can retrieve a connection from the hook and utilize it.
Hook also helps to avoid storing connection auth parameters in a DAG. 
See :doc:`connection/index` for how to create and manage connections.

Let's extend our previous example to fetch name from MySQL:

.. code:: python

    class HelloDBOperator(BaseOperator):

            @apply_defaults
            def __init__(
                    self,
                    name: str,
                    mysql_conn_id: str,
                    database: str,
                    *args, **kwargs) -> None:
                super().__init__(*args, **kwargs)
                self.name = name
                self.mysql_conn_id = mysql_conn_id
                self.database = database

            def execute(self, context):
                hook = MySqlHook(mysql_conn_id=self.mysql_conn_id,
                         schema=self.database)
                sql = "select name from user"
                result = hook.get_first(sql)
                message = "Hello {}".format(result['name'])
                print(message)
                return message

When the operator invokes the query on the hook object, a new connection gets created if it doesn't exist. 
The hook retrieves the auth parameters such as username and password from Airflow
backend and passes the params to the :py:func:`airflow.hooks.base_hook.BaseHook.get_connection`. 
You should create hook only in the ``execute`` method or any method which is called from ``execute``.
The constructor gets called whenever Airflow parses a DAG which happens frequently.
The ``execute`` gets called only during a DAG run.

User interface
^^^^^^^^^^^^^^^
Airflow also allows the developer to control how the operator shows up in the DAG UI.
Override ``ui_color`` to change the background color of the operator in UI. 
Override ``ui_fgcolor`` to change the color of the label.

.. code::  python

        class HelloOperator(BaseOperator):
            ui_color = '#ff0000'
            ui_fgcolor = '#000000'
            ....

Templating
^^^^^^^^^^^
You can use :ref:`Jinja templates <jinja-templating>` to parameterize your operator.
Airflow considers the field names present in ``template_fields``  for templating while rendering
the operator.

.. code:: python
    
        class HelloOperator(BaseOperator):
            
            template_fields = ['name']
            
            @apply_defaults
            def __init__(
                    self,
                    name: str,
                    *args, **kwargs) -> None:
                super().__init__(*args, **kwargs)
                self.name = name

            def execute(self, context):
                message = "Hello from {}".format(name)
                print(message)
                return message

You can use the template as follows:

.. code:: python

        with dag:
            hello_task = HelloOperator(task_id='task_id_1', dag=dag, name='{{ task_id }}')

In this example, Jinja looks for the ``name`` parameter and substitutes ``{{ task_id }}`` with
``task_id_1``.


The parameter can also contain a file name, for example, a bash script or a SQL file. You need to add
the extension of your file in ``template_ext``. If a ``template_field`` contains a string ending with
the extension mentioned in ``template_ext``, Jinja reads the content of the file and replace the templates
with actual value. Note that Jinja substitutes the operator attributes and not the args. 

.. code:: python

        class HelloOperator(BaseOperator):
            
            template_fields = ['guest_name']
            
            @apply_defaults
            def __init__(
                    self,
                    name: str,
                    *args, **kwargs) -> None:
                super().__init__(*args, **kwargs)
                self.guest_name = name

In the example, the ``template_fields`` should be ``['guest_name']`` and not  ``['name']``


Define an operator extra link
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For your operator, you can :doc:`Define an extra link <define_extra_link>` that can
redirect users to external systems. For example, you can add a link that redirects
the user to the operator's manual.