Testing custom operators in Apache Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple custom operator execution

Given the following Airflow custom operator code snippet, what will be the output when the operator runs successfully?

Apache Airflow

from airflow.models import BaseOperator
from airflow.utils.decorators import apply_defaults

class HelloOperator(BaseOperator):
    @apply_defaults
    def __init__(self, name: str, **kwargs):
        super().__init__(**kwargs)
        self.name = name

    def execute(self, context):
        message = f"Hello, {self.name}!"
        print(message)
        return message

# Assume this operator is instantiated and run with name='Airflow'

A Hello, Airflow!

B Hello, name!

C Error: missing required argument

D None

💡 Hint

Look at the execute method and what it prints and returns.

The execute method prints and returns the message with the name passed during initialization. Since name='Airflow', the output is Hello, Airflow!.

❓ Configuration

intermediate

Correct DAG configuration to use a custom operator

Which DAG configuration snippet correctly uses a custom operator MyOperator with task_id task1?

Apache Airflow

from airflow import DAG
from datetime import datetime
from my_operators import MyOperator

default_args = {
    'start_date': datetime(2024, 1, 1),
}

dag = DAG('my_dag', default_args=default_args, schedule_interval='@daily')

# Fill in the correct task definition below

A task1 = MyOperator(task_id='task1')

B task1 = MyOperator(dag=dag)

C task1 = MyOperator('task1', dag=dag)

D task1 = MyOperator(task_id='task1', dag=dag)

💡 Hint

Remember that task_id is a required named argument for operators.

The correct way to instantiate an operator in a DAG is to provide task_id as a named argument and assign the dag. Option D does this correctly.

❓ Troubleshoot

advanced

Error raised when execute method is missing

What error will Airflow raise if a custom operator class does not implement the execute method?

Apache Airflow

from airflow.models import BaseOperator

class BrokenOperator(BaseOperator):
    pass

# When Airflow tries to run BrokenOperator, what happens?

A AttributeError: 'BrokenOperator' object has no attribute 'execute'

B TypeError: Can't instantiate abstract class BrokenOperator with abstract methods execute

C RuntimeError: execute method missing

D No error, runs with default execute

💡 Hint

BaseOperator uses Python's abstract base class mechanism to enforce execute implementation.

BaseOperator declares execute as an abstract method. Not implementing it causes a TypeError when instantiating the class.

🔀 Workflow

advanced

Order of execution in DAG with custom operators

Given a DAG with two custom operators op1 and op2 where op2 depends on op1, what is the correct order of execution?

Apache Airflow

op1 = MyOperator(task_id='op1', dag=dag)
op2 = MyOperator(task_id='op2', dag=dag)
op1 >> op2

A Both run in parallel

B 2,1

C 1,2

D No guaranteed order

💡 Hint

The >> operator sets task dependencies in Airflow.

op1 >> op2 means op1 must finish before op2 starts, so op1 runs first, then op2.

✅ Best Practice

expert

Best practice for unit testing a custom operator's execute method

Which approach is best for unit testing the execute method of a custom Airflow operator?

A Mock Airflow context and dependencies, then call <code>execute</code> directly and assert results

B Run the operator in a real Airflow environment and check logs manually

C Test only the DAG, not the operator code

D Use print statements inside <code>execute</code> and verify output visually

💡 Hint

Unit tests should be automated and isolated from external systems.

Mocking the Airflow context and dependencies allows isolated, automated testing of the execute method without needing a full Airflow environment.