[AIRFLOW-3865] Added api to get python code of a given Dag id

[ramandumcs]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/projects/AIRFLOW/issues/AIRFLOW-3865
  • 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.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  It exposes an Api to get code of a given DagId

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:
  PR added a UT to test get_dag_code Api

Commits

• 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":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• 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

Code Quality

• Passes flake8

[ashb]

This is a potentially expensive operation (as it will load all dags from disk!)

Could you instead do something like in #4407 where we load the dag model from the DB instead.

[ramandumcs]

Thanks @ashb . Incorporated it

[ramandumcs]

@ashb could you please review it and provide your feedback/comments

[codecov-io]

Codecov Report

Merging #4687 into master will decrease coverage by 0.03%.
The diff coverage is 26.92%.

@@            Coverage Diff             @@
##           master    #4687      +/-   ##
==========================================
- Coverage   74.35%   74.31%   -0.04%     
==========================================
  Files         430      431       +1     
  Lines       27964    27988      +24     
==========================================
+ Hits        20792    20800       +8     
- Misses       7172     7188      +16

Impacted Files	Coverage Δ
airflow/api/common/experimental/get_code.py	25% <25%> (ø)
airflow/www/api/experimental/endpoints.py	85.2% <30%> (-2.97%)	⬇️
airflow/models/__init__.py	92.74% <0%> (-0.06%)	⬇️
airflow/contrib/hooks/databricks_hook.py	95.57% <0%> (ø)	⬆️
airflow/executors/celery_executor.py	76.92% <0%> (ø)	⬆️
airflow/contrib/hooks/bigquery_hook.py	58.06% <0%> (ø)	⬆️
airflow/contrib/hooks/spark_submit_hook.py	80.08% <0%> (ø)	⬆️
airflow/contrib/utils/mlengine_operator_utils.py
...rflow/contrib/utils/mlengine_prediction_summary.py
...rflow/contrib/operators/mlengine_operator_utils.py	95.23% <0%> (ø)
... and 4 more

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