Error earlier if virtualenv is not installed and DAG has a PythonVirtualenvOperator

[MatrixManAtYrService]

While trying to use @task.virtualenv I got this error:

 File "/Users/matt/src/airflow/airflow/models/taskinstance.py", line 1340, in _execute_task
    result = task_copy.execute(context=context)
  File "/Users/matt/src/airflow/airflow/decorators/base.py", line 138, in execute
    return_value = super().execute(context)
  File "/Users/matt/src/airflow/airflow/operators/python.py", line 350, in execute
    return super().execute(context=serializable_context)
  File "/Users/matt/src/airflow/airflow/operators/python.py", line 152, in execute
    return_value = self.execute_callable()
  File "/Users/matt/src/airflow/airflow/operators/python.py", line 362, in execute_callable
    prepare_virtualenv(
  File "/Users/matt/src/airflow/airflow/utils/python_virtualenv.py", line 95, in prepare_virtualenv
    execute_in_subprocess(virtualenv_cmd)
  File "/Users/matt/src/airflow/airflow/utils/process_utils.py", line 136, in execute_in_subprocess
    with subprocess.Popen(
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/local/Cellar/python@3.9/3.9.4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'virtualenv'

It was helpful enough that I was able to guess that pip install virtualenv would fix things for me, but:

• The error could have come up earlier
• FileNotFoundError made me doubt that it was a packaging issue

This PR checks if virtualenv is importable at init time and throws a more helpful exception.

I'm interested to know if there's a better way to handle things like this, but this is at least better than how it was.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst)
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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 5 days if no further activity occurs. Thank you for your contributions.

[uranusjr]

Technically PythonVirtualenvOperator does not require virtualenv to be installed in the same environment as Airflow, but only that virtualenv command is available in PATH. I would use shutil.which("virtualenv") to detect its existence instead.

[potiuk]

Hey @MatrixManAtYrService - can you please rebase it to the latest main (it should be conflict-less)

[MatrixManAtYrService]

Oops, must've overlooked the earlier notification.

• switched to shutil.which("virtualenv")
• rebased onto main

[potiuk]

Temporary errors only :)