[GitHub] [airflow] KKcorps commented on a change in pull request #6348: [AIRFLOW-XXX] GSoD: Adding 'Create a custom operator' doc

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KKcorps commented on a change in pull request #6348: [AIRFLOW-XXX] GSoD: Adding 'Create a custom operator' doc
URL: https://github.com/apache/airflow/pull/6348#discussion_r335576837
 
 

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 File path: docs/howto/operator/custom-operator.rst
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+ .. 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.
+
+
+Create 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 derived class:
+
+* Constructor - Define the parameters required for the operator. You only need to specify the arguments specific to your operator.
+
+* 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``:
+
+.. code::  python
+
+        class HelloOperator(BaseOperator):
+
+            def __init__(
+                    self,
+                    name: str,
+                    *args, **kwargs) -> None:
+                super().__init__(*args, **kwargs)
+                self.name = name
+
+            def execute(self, context):
+                message = "Hello {}".format(name)
+                print(message)
+                return message
+
+The above operator can now be used as follows:
+
+.. code:: python
+
+    hello_task = HelloOperator(task_id='', dag=dag, 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):
+
+            def __init__(
+                    self,
+                    name: str,
+                    conn_id: str,
+                    database: str,
+                    *args, **kwargs) -> None:
+                super().__init__(*args, **kwargs)
+                self.name = name
+                self.conn_id = 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 ``get_connection`` method from ``BaseHook``. 
+
+
+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']
+
+            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
+
+        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 .
 
 Review comment:
   Wouldn't it be better to make it bold rather than highlight it, since it's not a parameter but a value

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