Create your first Expectations

Expectations are the key concept in Great Expectations.

Each Expectation is a declarative, machine-verifiable assertion about the expected format, content, or behavior of your data. Great Expectations comes with dozens of built-in Expectations, and it’s possible to develop your own custom Expectations, too.

Admonition from Mr. Dickens.

“Take nothing on its looks; take everything on evidence. There’s no better rule.”

The CLI will help you create your first Expectation Suite. Suites are simply collections of Expectations. In order to create a new suite, we will use the scaffold command to automatically create an Expectation Suite called taxi.demo with the help of a built-in profiler. Type the following into your terminal:

great_expectations suite scaffold taxi.demo

You will see the following output:

Heads up! This feature is Experimental. It may change. Please give us your feedback!

Would you like to:
    1. choose from a list of data assets in this datasource
    2. enter the path of a data file
: 1

Which data would you like to use?
    1. yellow_tripdata_sample_2019-01 (file)
    2. yellow_tripdata_sample_2019-02 (file)
    Don't see the name of the data asset in the list above? Just type it
: 1

What just happened?

You may now wonder why we chose the first file in this step. Here’s an explanation: Recall that our data directory contains two CSV files: yellow_tripdata_sample_2019-01 and yellow_tripdata_sample_2019-02.

  • yellow_tripdata_sample_2019-01 contains the 2019 taxi data. Since we want to build an Expectation Suite based on what we know about our taxi data from the January 2019 data set, we want to use it for profiling.

  • yellow_tripdata_sample_2019-02 contains the February 2019 data, which we consider the “new” data set that we want to validate before using it in production. We’ll use it later when showing you how to validate data.

Makes sense, right?

After selecting the table, Great Expectations will open a Jupyter notebook which will take you through the next part of the scaffold workflow.

Warning

Don’t execute the Jupyter notebook cells just yet!

Creating Expectations in Jupyter notebooks

Notebooks are a simple way of interacting with the Great Expectations Python API. You could also just write all this in plain Python code, but for convenience, Great Expectations provides you some boilerplate code in notebooks.

Since notebooks are often less permanent, creating Expectations in a notebook also helps reinforce that the source of truth about Expectations is the Expectation Suite, not the code that generates the Expectations.

../../../_images/jupyter_scaffold.gif

Let’s scroll through the notebook and see what’s happening in each cell:

  1. The first cell does several things: It imports all the relevant libraries, loads a Data Context, and creates what we call a Batch of your data and Expectation Suite.

  2. The second cell allows you to specify which columns you want to run the automated Profiler on. Remember how we want to add some tests on the passenger_count column to ensure that its values range between 1 and 6? Let’s uncomment just this one line:

    included_columns = [
        # 'vendor_id',
        # 'pickup_datetime',
        # 'dropoff_datetime',
        'passenger_count',
        ...
    ]
    
  3. The next cell passes the Profiler config to the BasicSuiteBuilderProfiler, which will then profile the data and create the relevant Expectations to add to your taxi.demo suite.

  4. The last cell does several things again: It saves the Expectation Suite to disk, runs the validation against the loaded data batch, and then builds and opens Data Docs, so you can look at the validation results. We will explain the validation step later in the “Validate your data” section.

Let’s execute all the cells and wait for Great Expectations to open a browser window with Data Docs. Go to the next step in the tutorial for an explanation of what you see in Data Docs!