Defining your first asset

In this course, you’ll use data from NYC OpenData to analyze New York City taxi rides. The first asset you’ll define uses data from TLC Trip Record Data, which contains trip records for several types of vehicles. However, we’ll focus on trip data for yellow cabs in this asset.

Your first asset, which you’ll name taxi_trips_file, will retrieve the yellow taxi trip data for March 2023 and save it to a location on your local machine.

1. First, navigate to and open the assets/trips.py file in your Dagster project. This is where you’ll write your asset code.

2. At the top of the trips.py file, confirm the following imports already exist:

   import requests
   from . import constants
   

3. Below the imports, let's define a function that takes no inputs and returns nothing (type-annoted with None). Add the following code to create a function to do this named taxi_trips_file:

   def taxi_trips_file() -> None:
       """
         The raw parquet files for the taxi trips dataset. Sourced from the NYC Open Data portal.
       """
       month_to_fetch = '2023-03'
       raw_trips = requests.get(
           f"https://d37ci6vzurychx.cloudfront.net/trip-data/yellow_tripdata_{month_to_fetch}.parquet"
       )
   
       with open(constants.TAXI_TRIPS_TEMPLATE_FILE_PATH.format(month_to_fetch), "wb") as output_file:
           output_file.write(raw_trips.content)
   

4. To turn the function into an asset in Dagster, you’ll need to do two things:

   1. Import asset from the Dagster library:

      from dagster import asset
      

   2. Add the @asset decorator before the taxi_trips_file function. At this point, your code should look like this:

      import requests
      from . import constants
      from dagster import asset
      
      @asset
      def taxi_trips_file() -> None:
          """
            The raw parquet files for the taxi trips dataset. Sourced from the NYC Open Data portal.
          """
          month_to_fetch = '2023-03'
          raw_trips = requests.get(
              f"https://d37ci6vzurychx.cloudfront.net/trip-data/yellow_tripdata_{month_to_fetch}.parquet"
          )
      
          with open(constants.TAXI_TRIPS_TEMPLATE_FILE_PATH.format(month_to_fetch), "wb") as output_file:
              output_file.write(raw_trips.content)
      

That’s it - you’ve created your first Dagster asset! Using the @asset decorator, you can easily turn any existing Python function into a Dagster asset.

Questions about the -> None bit? That's a Python feature called type annotation. In this case, it's saying that the function returns nothing. You can learn more about type annotations in the Python documentation. We highly recommend using type annotations in your code to make it easier to read and understand.