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Version: 1.0 prerelease

About Great Expectations

Great Expectations (GX) is the leading tool for validating and documenting your data. GX 1.0 is the open source Python library that supports this tool. With GX 1.0 you can further customize, automate, and expand on GX's processes to suite your specialized use cases.

Software developers have long known that automated testing is essential for managing complex codebases. GX brings the same discipline, confidence, and acceleration to data science and data engineering teams.

Why use Great Expectations?​

With GX, you can assert what you expect from the data you load and transform, and catch data issues quickly – Expectations are basically unit tests for your data. Not only that, but GX also creates data documentation and data quality reports from those Expectations. Data science and data engineering teams use GX to:

  • Test data they ingest from other teams or vendors and ensure its validity.
  • Validate data they transform as a step in their data pipeline in order to ensure the correctness of transformations.
  • Prevent data quality issues from slipping into data products.
  • Streamline knowledge capture from subject-matter experts and make implicit knowledge explicit.
  • Develop rich, shared documentation of their data.

To learn more about how data teams are using GX, see Case studies from Great Expectations.

Key Features​

GX provides a unique framework for describing your data and validating it to ensure that it meets the standards you've defined. In the process, GX will generate human-readable reports about the state of your data. Additionally, GX's support for multiple backends ensures you can run your validations on data stored in different formats with minimal effort.

Expectations​

Expectations are assertions about your data. In GX, those assertions are expressed in a declarative language in the form of simple, human-readable Python methods. For example, in order to assert that you want the column “passenger_count” to be between 1 and 6, you can say:

Python code
expect_column_values_to_be_between(
    column="passenger_count",
    min_value=1,
    max_value=6
)

GX then uses this statement to validate whether the column passenger_count in a given table is indeed between 1 and 6, and returns a success or failure result. The library currently provides several dozen highly expressive built-in Expectations, and allows you to write custom Expectations.

Data validation​

Once you’ve created your Expectations, GX can load any batch or several batches of data to validate with your suite of Expectations. GX tells you whether each Expectation in an Expectation Suite passes or fails, and returns any unexpected values that failed a test, which can significantly speed up debugging data issues!

Data Docs​

GX renders Expectations in a clean, human-readable format called Data Docs. These HTML docs contain both your Expectation Suites and your data Validation Results each time validation is run – think of it as a continuously updated data quality report. The following image shows a sample Data Doc:

Screenshot of Data Docs

Support for various Data Sources and Store backends​

GX currently supports native execution of Expectations against various Data Sources, such as Pandas dataframes, Spark dataframes, and SQL databases via SQLAlchemy. This means you’re not tied to having your data in a database in order to validate it: You can also run GX against CSV files or any piece of data you can load into a dataframe.

GX is highly configurable. It allows you to store all relevant metadata, such as the Expectations and Validation Results in file systems, database backends, as well as cloud storage such as S3 and Google Cloud Storage, by configuring metadata Stores.

What does Great Expectations NOT do?​

GX is NOT a pipeline execution framework.

GX integrates seamlessly with DAG execution tools such as Airflow, dbt, Prefect, Dagster, and Kedro. GX does not execute your pipelines for you, but instead, validation can simply be run as a step in your pipeline.

GX is NOT a data versioning tool.

GX does not store data itself. Instead, it deals in metadata about data: Expectations, Validation Results, etc. If you want to bring your data itself under version control, check out tools like: DVC, Quilt, and lakeFS.

GX currently works best in a Python environment.

GX is Python-based. You can invoke it from the command line without using a Python programming environment, but if you’re working in another ecosystem, other tools might be a better choice. If you’re running in a pure R environment, you might consider assertR as an alternative. Within the TensorFlow ecosystem, TFDV fulfills a similar function as GX.

Community Resources​

GX is committed to supporting and growing the GX community. It’s not enough to build a great tool. GX wants to build a great community as well.

Open source doesn’t always have the best reputation for being friendly and welcoming, and that makes us sad. Everyone belongs in open source, and GX is dedicated to making you feel welcome.

Contact Great Expectations​

Join the GX public Slack channel. Before you post for the first time, review the Slack Guidelines.

Ask a question​

Slack is good for that, too: join Slack and read How to write a good question in Slack. You can also use GitHub Discussions.

File a bug report or feature request​

If you have bugfix or feature request, see upvote an existing issue or open a new issue.

Contribute code or documentation​

To make a contribution to GX, see Contribute.

Not interested in managing your own configuration or infrastructure?​

Learn more about GX Cloud, a fully managed SaaS offering. Sign up for the weekly cloud workshop! You’ll get to preview the latest features and participate in the private Alpha program!