AIP-72: Allow pushing and pulling XCom from Task Context

[kaxil]

Part of #44481

There is a lot of cleanup to do but I wanted to get a basic DAG that uses XCom working first.

Example DAG used: tutorial_dag

from __future__ import annotations

# [START tutorial]
# [START import_module]
import json
import textwrap

import pendulum

# The DAG object; we'll need this to instantiate a DAG
from airflow.models.dag import DAG

# Operators; we need this to operate!
from airflow.providers.standard.operators.python import PythonOperator

# [END import_module]

# [START instantiate_dag]
with DAG(
    "tutorial_dag",
    # [START default_args]
    # These args will get passed on to each operator
    # You can override them on a per-task basis during operator initialization
    default_args={"retries": 2},
    # [END default_args]
    description="DAG tutorial",
    schedule=None,
    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
    catchup=False,
    tags=["example"],
) as dag:
    # [END instantiate_dag]
    # [START documentation]
    dag.doc_md = __doc__
    # [END documentation]

    # [START extract_function]
    def extract(**kwargs):
        ti = kwargs["ti"]
        data_string = '{"1001": 301.27, "1002": 433.21, "1003": 502.22}'
        ti.xcom_push("order_data", data_string)

    # [END extract_function]

    # [START transform_function]
    def transform(**kwargs):
        ti = kwargs["ti"]
        extract_data_string = ti.xcom_pull(task_ids="extract", key="order_data")
        order_data = json.loads(extract_data_string)

        total_order_value = 0
        for value in order_data.values():
            total_order_value += value

        total_value = {"total_order_value": total_order_value}
        total_value_json_string = json.dumps(total_value)
        ti.xcom_push("total_order_value", total_value_json_string)

    # [END transform_function]

    # [START load_function]
    def load(**kwargs):
        ti = kwargs["ti"]
        total_value_string = ti.xcom_pull(task_ids="transform", key="total_order_value")
        total_order_value = json.loads(total_value_string)

        print(total_order_value)

    # [END load_function]

    # [START main_flow]
    extract_task = PythonOperator(
        task_id="extract",
        python_callable=extract,
    )
    extract_task.doc_md = textwrap.dedent(
        """\
    #### Extract task
    A simple Extract task to get data ready for the rest of the data pipeline.
    In this case, getting data is simulated by reading from a hardcoded JSON string.
    This data is then put into xcom, so that it can be processed by the next task.
    """
    )

    transform_task = PythonOperator(
        task_id="transform",
        python_callable=transform,
    )
    transform_task.doc_md = textwrap.dedent(
        """\
    #### Transform task
    A simple Transform task which takes in the collection of order data from xcom
    and computes the total order value.
    This computed value is then put into xcom, so that it can be processed by the next task.
    """
    )

    load_task = PythonOperator(
        task_id="load",
        python_callable=load,
    )
    load_task.doc_md = textwrap.dedent(
        """\
    #### Load task
    A simple Load task which takes in the result of the Transform task, by reading it
    from xcom and instead of saving it to end user review, just prints it out.
    """
    )

    extract_task >> transform_task >> load_task

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[ashb]

A few nits