[GitHub] kristiany opened a new pull request #4819: [AIRFLOW-3997] Add accessor for optional variables

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kristiany opened a new pull request #4819: [AIRFLOW-3997] Add accessor for optional variables
URL: https://github.com/apache/airflow/pull/4819
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-3997
     - In case you are fixing a typo in the documentation you can prepend your commit with \[AIRFLOW-XXX\], code changes always need a Jira issue.
     - In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal([AIP](https://cwiki.apache.org/confluence/display/AIRFLOW/Airflow+Improvements+Proposals)).
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI changes:
   
   Add a `optional` function for a safe way to retrieve a variable, without exploding if it doesn't exist.  
   
   This will not change existing functions in the `Variable` class. If
   variable `foo` doesn't exist:
   ```
   foo = Variable.get("foo")
   -> KeyError
   ```
   
   For optional `None` is returned instead:
   ```
   foo = Variable.optional("foo")
   if foo is None:
       handle_missing_foo()
   ```
   
   * Add optional accessor to the `Variable` class with support for JSON deserialization and session
   
   
   ### Tests
   
   - [x] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:
   
   * Test the optional accessor with support for JSON deserialization
   * Test the current behaviour for missing variables for the `get` function to raise a `KeyError`
   
   ### Commits
   
   - [x] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [x] In case of new functionality, my PR adds documentation that describes how to use it.
     - When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
     - All the public functions and the classes in the PR contain docstrings that explain what it does
   
   Describe the optional accessor in the doc, in the get variable examples section. Also describe the current behaviour for `get` of raising an error when a variable is missing
   
   ### Code Quality
   
   - [ ] Passes `flake8`
   

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