[AIP-31] Add lazy way to get Airflow context from a wrapped function

Description

Currently the only way to access the context object from a PythonOperator wrapped function is by setting provide_context=True in the Operator. That loads it into the kwargs. This makes the signature of the function become difficult to argue about.

Adding a way to lazy get the function when executing the function may be better. Similar to Flask flask.request that get populated when the function is executed.

Use case / motivation

Use the Airflow context in arbitrary function while keeping the signature of the function stable and easy to reason about.

An idea of implementation would be:

from airflow.task import context

@task 
def context_task(...)
    return context['run_date']

This can be populated by enabling a context manager on the PythonOperator.execute function that populates the field (similar to how DagContext works) and removes it afterwards.

[casassg]

I see, I still think we should implement this lazy resolved context. Using the current approach from master means that signatures can mismatch op_args/op_kwargs and would make it difficult to reason about the function signature:

@task 
def simple_task(text:str, execution_date: datetime):
  print(text)
  print(execution_date)

simple_task("hello")

As you can see this will execute correctly even though the function signature has a an extra argument. This leads to confusion in my personal opinion hence my proposed solution.

[casassg]

Any updates on this?