XCom with return values in Apache Airflow - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push a value to XCom using the return statement in a PythonOperator task.

Apache Airflow

def task_function():
    return [1]

Drag options to blanks, or click blank then click option'

ANone

Bprint('Hello from XCom!')

C'Hello from XCom!'

Dxcom_push('Hello from XCom!')

Attempts:

3 left

💡 Hint

Common Mistakes

Using print instead of return

Calling xcom_push directly inside the function

Returning a value from the task function automatically pushes it to XCom.

2fill in blank

medium

Complete the code to pull the XCom value from a previous task named 'push_task'.

Apache Airflow

from airflow.operators.python import PythonOperator

def pull_task(ti):
    value = ti.xcom_pull(task_ids=[1])
    print(value)

Drag options to blanks, or click blank then click option'

A'push_task'

Bpush_task

C'pull_task'

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable name without quotes

Using the wrong task ID

Task IDs must be passed as strings to xcom_pull.

3fill in blank

hard

Fix the error in the code to correctly return a dictionary as an XCom value.

Apache Airflow

def task_function():
    data = [1]  # dictionary to return
    return data

Drag options to blanks, or click blank then click option'

A'key: value'

B['key', 'value']

C('key', 'value')

D{'key': 'value'}

Attempts:

3 left

💡 Hint

Common Mistakes

Returning a list or tuple instead of a dictionary

Returning a string instead of a dictionary

Returning a dictionary pushes it as an XCom value correctly.

4fill in blank

hard

Fill both blanks to create a PythonOperator that pushes a return value and another that pulls it.

Apache Airflow

push_task = PythonOperator(
    task_id='push_task',
    python_callable=[1],
    dag=dag
)
pull_task = PythonOperator(
    task_id='pull_task',
    python_callable=[2],
    dag=dag
)

Drag options to blanks, or click blank then click option'

Atask_function_push

Btask_function_pull

Ctask_function

Dpull_function

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same function for both tasks

Mixing up push and pull function names

The push task uses the function that returns a value, and the pull task uses the function that pulls it.

5fill in blank

hard

Fill all three blanks to create a DAG with two tasks where the second pulls the XCom value from the first.

Apache Airflow

from airflow import DAG
from airflow.operators.python import PythonOperator
from datetime import datetime

def [1]():
    return 'Data from first task'

def [2](ti):
    data = ti.xcom_pull(task_ids=[3])
    print(f'Received: {data}')

dag = DAG('xcom_example', start_date=datetime(2024, 1, 1), schedule_interval='@daily')

push = PythonOperator(task_id='push', python_callable=[1], dag=dag)
pull = PythonOperator(task_id='pull', python_callable=[2], dag=dag)
push >> pull

Drag options to blanks, or click blank then click option'

Apush_task_function

Bpull_task_function

C'push'

D'pull'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong task_id in xcom_pull

Mismatching function names and task_ids

The first function returns data, the second pulls it using the correct task ID string.

Practice

1. What does returning a value from a PythonOperator task do in Airflow?

easy

A. Automatically pushes the returned value to XCom for other tasks to use.

B. Saves the value only in the task logs without sharing.

C. Deletes any previous XCom values for that task.

D. Triggers the next task immediately without waiting.

Solution

1. Step 1: Understand PythonOperator return behavior

   When a PythonOperator returns a value, Airflow automatically stores it in XCom for that task instance.

2. Step 2: Recognize XCom purpose

   XCom is designed to share data between tasks, so returning a value pushes it to XCom for downstream tasks.

3. Final Answer:

   Automatically pushes the returned value to XCom for other tasks to use. -> Option A

4. Quick Check:

   Return value = XCom push [OK]

Hint: Return values from PythonOperator go to XCom automatically [OK]

Common Mistakes:

• Thinking return values are only logged, not shared
• Assuming return clears previous XCom data
• Believing return triggers next task immediately

2. Which of the following is the correct way to pull an XCom value in a downstream task?

easy

A. ti.xcom_pull(task_ids='task1', key='return_value')

B. ti.xcom_push(key='result')

C. xcom_pull(task_id='task1')

D. ti.pull_xcom(task_id='task1')

Solution

1. Step 1: Identify correct method signature

   The method to pull XCom data is ti.xcom_pull(), which requires task_ids and optionally a key.

2. Step 2: Match correct parameters

   Correct usage is ti.xcom_pull(task_ids='task1', key='return_value') to get the returned value from task1.

3. Final Answer:

   ti.xcom_pull(task_ids='task1', key='return_value') -> Option A

4. Quick Check:

   Use ti.xcom_pull with task_ids and key [OK]

Hint: Use ti.xcom_pull(task_ids, key) to get XCom data [OK]

Common Mistakes:

• Using xcom_push instead of xcom_pull to get data
• Wrong parameter names like task_id instead of task_ids
• Calling non-existent methods like ti.pull_xcom

3. Given this DAG snippet:

def task1():
    return {'value': 42}

def task2(ti):
    data = ti.xcom_pull(task_ids='task1')
    print(data)

What will task2 print when executed?

medium

A. None

B. {'return_value': {'value': 42}}

C. 42

D. Error: xcom_pull missing key

Solution

1. Step 1: Understand task1 return value

   task1 returns a dictionary {'value': 42}, which is pushed to XCom automatically.

2. Step 2: Analyze task2 xcom_pull call

   task2 calls ti.xcom_pull(task_ids='task1') without a key, so it pulls all XCom values from task1 as a dictionary {'return_value': {'value': 42}}.

3. Final Answer:

   {'return_value': {'value': 42}} -> Option B

4. Quick Check:

   Pull without key returns XComs dict [OK]

Hint: Pull without key returns XComs dict [OK]

Common Mistakes:

• Expecting only the inner value without specifying key
• Assuming None if key is not given
• Thinking it raises an error without key

4. What is wrong with this code snippet?

def task1():
    return 100

def task2(ti):
    value = ti.xcom_pull(task_ids='task1', key='value')
    print(value)

medium

A. task2 must use task_id instead of task_ids in xcom_pull.

B. task1 should push to XCom manually using xcom_push.

C. task2 pulls with wrong key; default return key is 'return_value'.

D. task1 return value must be a string, not int.

Solution

1. Step 1: Check task1 return and XCom key

   task1 returns 100, which is stored in XCom with the default key 'return_value'.

2. Step 2: Analyze task2 xcom_pull key usage

   task2 tries to pull with key='value', which does not exist, so it will return None.

3. Final Answer:

   task2 pulls with wrong key; default return key is 'return_value'. -> Option C

4. Quick Check:

   Default XCom key for return is 'return_value' [OK]

Hint: Default XCom key for return is 'return_value' [OK]

Common Mistakes:

• Thinking return requires manual xcom_push
• Using wrong key name in xcom_pull
• Confusing task_id and task_ids parameter names

5. You want to pass a dictionary from task1 to task3 through task2 without modifying task2. Which is the best way to do this using XCom return values?

hard

A. Use Airflow Variables instead of XCom for this data passing.

B. Have task1 push manually; task2 returns the same dictionary to pass it on.

C. Modify task2 to pull from task1 and push to task3 manually.

D. Have task1 return the dictionary; task3 pulls it directly using ti.xcom_pull(task_ids='task1').

Solution

1. Step 1: Understand XCom chaining

   XCom stores data per task instance; tasks can pull data from any previous task by specifying task_ids.

2. Step 2: Avoid modifying task2

   Since task2 should not be changed, task3 can pull data directly from task1's XCom return value.

3. Final Answer:

   Have task1 return the dictionary; task3 pulls it directly using ti.xcom_pull(task_ids='task1'). -> Option D

4. Quick Check:

   Pull directly from original task to avoid modifying intermediates [OK]

Hint: Pull XCom directly from original task to skip intermediate tasks [OK]

Common Mistakes:

• Thinking data must pass through every intermediate task
• Modifying tasks unnecessarily to forward XCom
• Using Variables instead of XCom for task-to-task data