Default args and DAG parameters in Apache Airflow - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the start_date of the DAG?

Given this Airflow DAG snippet, what is the effective start_date of the DAG tasks?

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

dag = DAG(
  'example_dag',
  default_args=default_args,
  start_date=datetime(2024, 2, 1),
  schedule_interval='@daily'
)

Apache Airflow

from datetime import datetime
from airflow import DAG

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

dag = DAG(
  'example_dag',
  default_args=default_args,
  start_date=datetime(2024, 2, 1),
  schedule_interval='@daily'
)

A The start_date is the current date when DAG runs

B 2024-02-01

C The DAG has no start_date, so it will fail

D 2024-01-01

💡 Hint

The DAG parameter start_date overrides the one in default_args.

When both DAG and default_args specify start_date, the DAG's own start_date takes precedence for scheduling.

🧠 Conceptual

intermediate

What happens if a task has no start_date but DAG has one?

In Airflow, if a DAG has a start_date defined but a task inside it does not specify any start_date, what will be the task's effective start_date?

Choose the correct behavior.

A The task inherits the DAG's start_date

B The task will fail to run due to missing start_date

C The task uses the current date as start_date

D The task uses the earliest start_date from default_args

💡 Hint

Tasks inherit missing parameters from DAG or default_args.

If a task does not specify start_date, it inherits the DAG's start_date or from default_args if provided.

❓ Configuration

advanced

Identify the error in this DAG default_args usage

Examine the following DAG code snippet. What error will occur when Airflow tries to parse this DAG?

default_args = {
  'owner': 'airflow',
  'start_date': '2024-01-01'
}

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

Apache Airflow

from datetime import datetime
from airflow import DAG
default_args = {
  'owner': 'airflow',
  'start_date': '2024-01-01'
}
dag = DAG(
  'faulty_dag',
  default_args=default_args,
  schedule_interval='@daily'
)

A No error, DAG will run fine

B SyntaxError due to missing comma

C TypeError: start_date must be a datetime object, not string

D ValueError: schedule_interval is invalid

💡 Hint

Check the type of the start_date value.

Airflow requires start_date to be a datetime object, not a string. Using a string causes a TypeError.

🔀 Workflow

advanced

Order the steps to override default_args in a DAG

Put these steps in the correct order to override a default argument in an Airflow DAG.

Steps:
1. Pass the new argument value directly in the DAG constructor
2. Define default_args dictionary with base parameters
3. Create the DAG object with default_args parameter
4. Import required modules and datetime

A 1,2,3,4

B 4,2,3,1

C 1,3,2,4

D 2,1,4,3

💡 Hint

Start with imports, then define defaults, create DAG, then override.

First import modules, then define default_args, create DAG with default_args, finally override parameters in DAG constructor.

❓ Troubleshoot

expert

Why does this DAG fail to schedule despite correct start_date?

You have this DAG snippet:

default_args = {
  'owner': 'airflow',
  'start_date': datetime(2024, 4, 1)
}

dag = DAG(
  'test_dag',
  default_args=default_args,
  schedule_interval='@daily',
  catchup=False
)

# Task definition omitted for brevity

The DAG never runs any tasks even after 2024-04-01. What is the most likely reason?

Apache Airflow

from datetime import datetime
from airflow import DAG
default_args = {
  'owner': 'airflow',
  'start_date': datetime(2024, 4, 1)
}
dag = DAG(
  'test_dag',
  default_args=default_args,
  schedule_interval='@daily',
  catchup=False
)

A schedule_interval '@daily' is invalid and prevents scheduling

B catchup=False disables all runs permanently

C Missing task definitions cause DAG to not run

D The DAG start_date is in the future relative to the scheduler's current date

💡 Hint

Check the system date vs start_date.

If the DAG's start_date is in the future relative to the scheduler's current date, no runs are triggered yet.