Add `dag_id` and `map_index` to the default template variables

[pedro-cf]

Description

Add dag_id and map_index as default variables to the template variables (accessible via task parameters).

Use case/motivation

Generally useful and specifically easier access to these variables within operators that have serialization issues with the dag_run and ti objects, such as the PythonVirtualEnvOperator.

Related issues

• cannot use get_current_context() in a @task.virtualenv task #34158

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

Code of Conduct

• I agree to follow this project's Code of Conduct

[shahar1]

1. Is it a feature suggestion, or a suggestion for improving the documentation?
   Please clarify the description - it would also be useful to provide some code examples that will explain the motivation.
2. Is this PR a recreation of Add 'dag_id' and 'dag_run_id' to the default template variables #40859? Why was the latter closed?

[pedro-cf]

1. Is it a feature suggestion, or a suggestion for improving the documentation?

Please clarify the description - it would also be useful to provide some code examples that will explain the motivation.

2. Is this PR a recreation of Add 'dag_id' and 'dag_run_id' to the default template variables #40859? Why was the latter closed?

1. Feature Request/Suggestion. I believe I give a solid reason:

Generally useful and specifically easier access to these variables within operators that have serialization issues with the dag_run and ti objects, such as the PythonVirtualEnvOperator.

2. Different variables in this issue.

[shahar1]

1. Is it a feature suggestion, or a suggestion for improving the documentation?

Please clarify the description - it would also be useful to provide some code examples that will explain the motivation.

2. Is this PR a recreation of Add 'dag_id' and 'dag_run_id' to the default template variables #40859? Why was the latter closed?

1. Feature Request/Suggestion. I believe I give a solid reason:

Generally useful and specifically easier access to these variables within operators that have serialization issues with the dag_run and ti objects, such as the PythonVirtualEnvOperator.

2. Different variables in this issue.

There's currently an open PR (#41039) for solving #34158. When this PR is merged, will there still be a justification for the suggested feature?

[pedro-cf]

There's currently an open PR (#41039) for solving #34158. When this PR is merged, will there still be a justification for the suggested feature?

I believe so, these are key variables, more important than some of the variables already available.
It's also a much cleaner way to access these directly.

[uranusjr]

I wonder if we should try to add some more, and remove ti and dag_run altogether. Exposing those objects directly to the user is kind of problematic.

Or, maybe we should make those a proxy instead of the actual object, and remove things like task_id and run_id.