[AIRFLOW-2099] Handle getsource() calls gracefully

[night0wl]

There are several scenarios where Task Instance view tries to render
Python callables where 'x' is not the correct artefact to target.

This commit adds a helper fuction to test for known scenarios, and
derives the source from the correct artifact or as a default returns
'No source available for '. This means that even in unknown or
unfixable edge cases, the Task Instance view still renders instead of
displaying an exception.

Make sure you have checked all steps below.

JIRA

• My PR addresses the following Airflow JIRA issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-2099

Description

• Here are some details about my PR, including screenshots of any UI changes:

1. Adds a wrapper function for getting Python source code.
2. It tests for/catches known anomalies and implements a work around, finally returning 'source code not available for ' if no option works.

Tests

• My PR adds the following unit tests

1. test_dag_view_task_with_python_operator_using_partial
2. test_dag_view_task_with_python_operator_using_instance

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
  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"

• Passes git diff upstream/master -u -- "*.py" | flake8 --diff

[codecov-io]

Codecov Report

Merging #3571 into master will decrease coverage by 58.94%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master    #3571       +/-   ##
===========================================
- Coverage   76.83%   17.89%   -58.95%     
===========================================
  Files         204      204               
  Lines       15492    15510       +18     
===========================================
- Hits        11904     2775     -9129     
- Misses       3588    12735     +9147

Impacted Files	Coverage Δ
airflow/www/views.py	0% <ø> (-68.95%)	⬇️
airflow/www/utils.py	0% <0%> (-87.06%)	⬇️
airflow/operators/email_operator.py	0% <0%> (-100%)	⬇️
airflow/www/forms.py	0% <0%> (-100%)	⬇️
...le_dags/example_passing_params_via_test_command.py	0% <0%> (-100%)	⬇️
...w/example_dags/example_latest_only_with_trigger.py	0% <0%> (-100%)	⬇️
airflow/example_dags/example_docker_operator.py	0% <0%> (-100%)	⬇️
airflow/www_rbac/blueprints.py	0% <0%> (-100%)	⬇️
airflow/www/validators.py	0% <0%> (-100%)	⬇️
airflow/operators/sensors.py	0% <0%> (-100%)	⬇️
... and 160 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9ebb04a...331bb51. Read the comment docs.

[ashb]

Could you add some tests that directly call wwwutils.get_python_source with one case for each type?

[night0wl]

@ashb no problem, they should go in tests/www/test_utils.py in the UtilsTest TestCase I assume.

"One case for each type", this would be 4; one for callable class, one for method, one for partial function, and one for None

[ashb]

Yes to both questions.

[ashb]

Remove the print and squash to one commit in this case please :)

[ashb]

Probably shouldn't leave this in here :)

[night0wl]

oops!

[night0wl]

@ashb any idea what's up with codecov? I rebased again onto master to make sure everything was up-to-date.

[ashb]

Codecov has a bit of a "wobble" and goes up and down in small numbers - I've never dug into why but it doesn't worry me.

Oh, that it hasn't run. 🤷 - again not concerning me much.