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dlt adds Reverse ETL - build a custom destination in minutes

· 8 min read
Adrian Brudaru

Why Python is the right approach for doing Reverse ETL

Reverse ETL is generally about putting data into a business application. This data would often come from a SQL database used as a middle layer for data integrations and calculations.

That’s fine - but nowadays most data people speak Python, and the types of things we want to put into an operational application don’t always come from a DB, they often come from other business applications, or from things like a dataframe on which we did some scoring, etc.

reverse etl

The full potential of Reverse ETL is in the flexibility of sources

SQL databases are a good start, but in reality very often our data source is something else. More often than not, it’s a Python analyst’s implementation of some scoring or some business calculation.

Other times, it’s a business application - for example, we might have a form that sends the response data to a webhook, from where it could end up in Salesforce, DWH, and Slack as a notification. And of course, if this is done by a data person it will be done in Python.

Such, it follows that if we want to cater to the data crowd, we need to be Pythonic.

There’s synergy with ETL

Reverse ETL is ultimately ETL. Data is extracted from a source, its transformed, and then loaded to a destination. The challenges are similar, the most notable difference being that pulling data from a strongly typed environment like a DB and converting it to weakly typed JSON is MUCH easier than the other way around. You can argue that Reverse ETL is simpler than ETL.

Flavors of Reverse ETL

Just like we have ETL and ELT, we also have flavors of Reverse ETL

  • Reverse ETL or TEL: Transform the data to a specification, read it from DB, and send it to an application.
  • Tool Reverse ETL or ETL: Extract from DB, map fields to destination in the tool, load to destination.
  • Pythonic Freestyle Reverse ETL: You extract data from wherever you want and put it anywhere except storage/DB. Transformations are optional.

Examples of Python reverse ETL

  • Read data from Mongo, do anomaly detection, and notify anomalies to Slack.
  • Read membership data from Stripe, calculate the chance to churn, and upload to CRM for account managers.
  • Capture a form response with a webhook and send the information to CRM, DWH, and Slack.

Add python? - new skills unlocked!

So why is it much better to do reverse ETL in Python?

  • Live Streaming and Flexibility: Python's ability to handle live data streams and integrate with various APIs and services surpasses the capabilities of SQL-based data warehouses designed for batch processing.
  • End-to-End Workflow: Employing Python from data extraction to operational integration facilitates a streamlined workflow, enabling data teams to maintain consistency and efficiency across the pipeline.
  • Customization and Scalability: Python's versatility allows for tailored solutions that can scale with minimal overhead, reducing the reliance on additional tools and simplifying maintenance.
  • Collaboration and Governance: By keeping the entire data workflow within Python, teams can ensure better governance, compliance, and collaboration, leveraging common tools and repositories.

Example: Building a Custom Destination and a pipeline in under 1h

Documentation used: Building a destination: docs SQL source: docs In this example, you will see why it’s faster to build a custom destination than set up a separate tool.

dlt allows you to define custom destination functions. You'll write a function that extracts the relevant data from your dataframe and formats it for the Notion API.

This example assumes you have set up Google Sheets API access and obtained the necessary credentials to authenticate.

Step 1: Setting Up Google Sheets API (10min)

  1. Enable the Google Sheets API in the Google Developers Console.
  2. Download the credentials JSON file.
  3. Share the target Google Sheet with the email address found in your credentials JSON file.

Step 2: Define the Destination method in its own file (20min)

Install the required package for the Google API client:

pip install --upgrade google-api-python-client google-auth-httplib2 google-auth-oauthlib

Here’s how to define a destination function to update a Google Sheet. In our case we wrote a slightly complex function that checks the headers and aligns the columns with the existing ones before inserting:

import dlt
from google.oauth2.service_account import Credentials
from googleapiclient.discovery import build

def google_sheets(items,
sheets_id: str = dlt.config.value,
credentials_json: dict = dlt.secrets.value,
range_name: str = 'Sheet1'):
Send data to a Google Sheet.
:param items: Batch of items to send.
:param table_schema: Schema of the table (unused in this example but required by dlt).
:param sheets_id: ID of the Google Sheet, retrieved from config.
:param credentials_json: Google Service Account credentials, retrieved from secrets.
:param range_name: The specific range within the Sheet where data should be appended.
credentials = Credentials.from_service_account_info(credentials_json)
service = build('sheets', 'v4', credentials=credentials)

# Fetch existing headers from the sheet
existing_headers_result = service.spreadsheets().values().get(
spreadsheetId=sheets_id, range="Sheet1!A1:1"
existing_headers = existing_headers_result.get('values', [[]])[0] if existing_headers_result.get('values') else []

# Determine new headers from items
new_keys = set().union(*(d.keys() for d in items))
# Identify headers that need to be added (not already existing)
headers_to_add = [key for key in new_keys if key not in existing_headers]
# New comprehensive headers list, preserving the order of existing headers and adding new ones at the end
comprehensive_headers = existing_headers + headers_to_add

# If there are headers to add, update the first row with comprehensive headers
if headers_to_add:
update_body = {'values': [comprehensive_headers]}
spreadsheetId=sheets_id, range="Sheet1!A1",
valueInputOption='RAW', body=update_body

# Prepare the data rows according to the comprehensive headers list
values = []
for item in items:
row = [item.get(header, "") for header in comprehensive_headers] # Fill missing keys with empty string

body = {'values': values}

# Append the data rows
if values:
append_body = {'values': values}
append_result = service.spreadsheets().values().append(
spreadsheetId=sheets_id, range=range_name,
valueInputOption='RAW', insertDataOption='INSERT_ROWS', body=append_body
print(f"{append_result.get('updates').get('updatedRows')} rows have been added to the sheet.")

Step 3: Configure secrets (5min)

For the custom destination, you can follow this example. Configure the source as instructed in the source documentation.


credentials_json = '''
"type": "service_account",
"project_id": "your_project_id",
"private_key_id": "your_private_key_id",


sheets_id = "1xj6APSKhepp8-sJIucbD9DDx7eyBt4UI2KlAYaQ9EKs"

Step 4: Running the pipeline in

Now, assuming you have a source function dict_row_generator(), you can set up and run your pipeline as follows:

# ... destination code from above

# pass some destination arguments explicitly (`range_name`)
pipeline = dlt.pipeline("my_google_sheets_pipeline", destination=google_sheets(range_name="named_range"))

# Use the source function and specify the resource "people_report"
def dict_row_generator():
yield {"row": 1, 'a': "a"}
yield {"row": 2, 'b': "b"}
yield {"row": 3, 'c': "c"}
yield {"row": 1, 'a': 1}
yield {"row": 2, 'b': 2}
yield {"row": 3, 'c': 3}

# Now, run the pipeline with the specified source
info =

In this setup, append_to_google_sheets acts as a custom destination within your dlt pipeline, pushing the fetched data to the specified Google Sheet. This method enables streamlined and secure data operations, fully utilizing Python's capabilities for Reverse ETL processes into Google Sheets.

What does dlt do for me here?

Using dlt for reverse ETL instead of plain Python, especially with its @dlt.destination decorator, provides a structured framework that streamlines the process of data integrating into various destinations. Here’s how the dlt decorator specifically aids you compared to crafting everything from scratch in plain Python:

Faster time to Production grade pipelines

The @dlt.destination decorator significantly reduces the need for custom boilerplate code. It provides a structured approach to manage batch processing, error handling, and retries, which would otherwise require complex custom implementations in plain Python. This built-in functionality ensures reliability and resilience in your data pipelines.

Focus on custom business logic and adding value

The flexibility of creating custom destinations with dlt shifts the focus from the possibilities to the necessities of your specific use case. This empowers you to concentrate on implementing the best solutions for your unique business requirements.

Scalability and efficient resource use

dlt facilitates efficient handling of large data loads through chunking and batching, allowing for optimal use of computing resources. This means even small worker machines can stream data effectively into your chosen endpoint instead of wasting a large machine waiting for the network. The library design supports easy scaling and adjustments. Making changes to batch sizes or configurations is straightforward, ensuring your data pipelines can grow and evolve with minimal effort. This approach simplifies maintenance and ensures that once a solution is implemented, it's broadly applicable across your projects.

In Conclusion

Reverse ETL is just a piece of the ETL puzzle. It could be done cleaner and better when done in Python end to end.

Tools will always appeal to the non-technical folks. However, anyone with the ability to do Python pipelines can do Reverse ETL pipelines too, bringing typical benefits of code vs tool to a dev team - customisation, collaboration, best practices, etc.

So read more about how to built a dlt destination and consider giving it a try in your next reverse ETL pipeline.

This demo works on codespaces. Codespaces is a development environment available for free to anyone with a Github account. You'll be asked to fork the demo repository and from there the README guides you with further steps.
The demo uses the Continue VSCode extension.

Off to codespaces!


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