XCom with return values in Apache Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

XCom Push and Pull Output

Given the following Airflow task code, what will be the output of the XCom pull in the downstream task?

Apache Airflow

def push_function(ti):
    ti.xcom_push(key='result', value=42)

def pull_function(ti):
    value = ti.xcom_pull(key='result', task_ids='push_task')
    print(value)

# Assume push_function runs as 'push_task' and pull_function runs as 'pull_task' downstream.

A 0

B None

C KeyError

D 42

💡 Hint

Remember that xcom_push stores a value with a key, and xcom_pull retrieves it using the same key and task id.

The push_function stores the integer 42 with key 'result'. The pull_function retrieves it using the same key and task id, so it prints 42.

🧠 Conceptual

intermediate

XCom Return Value Behavior

What happens when a PythonOperator task returns a value in Airflow?

A The return value is ignored and not stored anywhere.

B The return value is automatically pushed to XCom with key 'return_value'.

C The return value causes the task to fail.

D The return value is stored in a log file only.

💡 Hint

Think about how Airflow captures outputs from PythonOperator tasks.

PythonOperator automatically pushes the return value of the Python callable to XCom with the key 'return_value'. This allows downstream tasks to pull it easily.

❓ Troubleshoot

advanced

XCom Pull Returns None

You have two tasks: 'task_a' pushes a value to XCom with key 'data'. 'task_b' tries to pull this value but gets None. What is the most likely cause?

A 'task_a' did not push any value to XCom.

B The key used in xcom_pull is misspelled.

C All of the above.

D 'task_b' is running before 'task_a' completes.

💡 Hint

Consider timing, key names, and whether the push actually happened.

If 'task_b' runs before 'task_a' finishes, the value won't be available yet. If the key is wrong or 'task_a' never pushed, the pull returns None. All these cause None.

🔀 Workflow

advanced

XCom Usage in Task Dependencies

Which Airflow DAG setup correctly ensures that a value pushed by 'task1' is available to 'task2' via XCom?

A 'task1' >> 'task2' with 'task2' pulling XCom using task_ids='task1'.

B 'task2' >> 'task1' with 'task2' pulling XCom using task_ids='task1'.

C 'task1' and 'task2' run in parallel with no dependencies.

D 'task1' pushes XCom but 'task2' does not pull it.

💡 Hint

Think about task order and how XCom pull uses task_ids.

Setting 'task1' upstream of 'task2' ensures 'task1' runs first and pushes the value. 'task2' can then pull it using task_ids='task1'.

✅ Best Practice

expert

Best Practice for Large Data in XCom

What is the best practice when you need to share large data between Airflow tasks using XCom?

A Store the large data in external storage (e.g., S3) and push only a reference or path via XCom.

B Push the entire large data directly via XCom for simplicity.

C Split the large data into many small pieces and push each piece separately via XCom.

D Avoid using XCom and rewrite tasks to not share data.

💡 Hint

Consider XCom size limits and performance.

XCom is not designed for large data. Best practice is to store big data externally and pass only a small reference or path through XCom to keep performance and reliability.