Model Explainability¶

Supported model types¶

This preview release supports the following Python-native model packages.

• XGBoost

• CatBoost

• LightGBM

• Scikit-learn

The following Snowpark ML modeling classes from snowflake.ml.modeling are supported.

• XGBoost

• LightGBM

• Scikit-learn (except pipeline models)

Explainability is available by default for the above models logged using Snowpark ML 1.6.2 and later. The implementation uses the SHAP library (https://pypi.org/project/shap/).

Logging models with background data¶

Background data, typically a sample of representative data, is an important ingredient of Shapley value-based explanations. Background data gives the Shapley algorithm an idea of what “average” inputs look like to which it can compare individual explanations.

The Shapley value is computed by systematically perturbing input features and replacing them with the background data. Because it reports deviation from background data, it is important to use consistent background data when comparing Shapley values from multiple data sets.

Some tree-based models implicitly encode background data within their structure during training, and may not require explicit background data. Most models, however, require background data to be provided separately for useful explanations, and all models (including tree-based models) can be explained more accurately if you provide background data.

You can provide up to 1,000 rows of background data when logging a model by passing it in the sample_input_data parameter, as shown below.

mv = reg.log_model(
    catboost_model,
    model_name="diamond_catboost_explain_enabled",
    version_name="explain_v0",
    conda_dependencies=["snowflake-ml-python"],
    sample_input_data = xs, # xs will be used as background data
)

You can also provide background data while logging the model with a signature, as shown below.

mv = reg.log_model(
    catboost_model,
    model_name="diamond_catboost_explain_enabled",
    version_name="explain_v0",
    conda_dependencies=["snowflake-ml-python"],
    signatures={"predict": predict_signature, "predict_proba": predict_proba_signature},
    sample_input_data = xs, # xs will be used as background data
    options= {"enable_explainability": True} # you will need to set this flag in order to pass both signatures and background data
)

Retrieving explainability values¶

Models with explainability have a method named explain that returns the Shapley values for the model’s features.

Because Shapley values are explanations of predictions made from specific inputs, you must pass input data to explain to generate the predictions to be explained.

The Snowflake model version object will have a method called explain, and you call it using ModelVersion.run in Python.

reg = Registry(...)
mv = reg.get_model("Explainable_Catboost_Model").default
explanations = mv.run(input_data, function_name="explain")

The following is an example of retrieving the explanation in SQL.

WITH MV_ALIAS AS MODEL DATABASE.SCHEMA.DIAMOND_CATBOOST_MODEL VERSION EXPLAIN_V0
SELECT *,
      FROM DATABASE.SCHEMA.DIAMOND_DATA,
          TABLE(MV_ALIAS!EXPLAIN(CUT, COLOR, CLARITY, CARAT, DEPTH, TABLE_PCT, X, Y, Z));

mv_new = reg.log_model(
    model,
    model_name="model_with_explain_enabled",
    version_name="explain_v0",
    conda_dependencies=["snowflake-ml-python"],
    sample_input_data = xs,
    options={"relax_version": False}
)

Adding explainability to existing models¶

Models that were logged in the registry using a version of Snowpark ML older than 1.6.2 do not have the explainability feature. Since model versions are immutable, you must create a new model version to add explainability to an existing model. You can use ModelVersion.load to retrieve the Python object represeting the model’s implementation, then log that to the registry as a new model version. Be sure to pass your background data as sample_input_data. This approach is shown below.

mv_old = reg.get_model("model_without_explain_enabled").default
model = mv_old.load()
mv_new = reg.log_model(
    model,
    model_name="model_with_explain_enabled",
    version_name="explain_v0",
    conda_dependencies=["snowflake-ml-python"],
    sample_input_data = xs
)

Logging models without explainability¶

Explainability is enabled by default if the model supports it. To log a model version in the registry without explainability, pass False for the enable_explainability option when logging the model, as shown here.

mv = reg.log_model(
    catboost_model,
    model_name="diamond_catboost_explain_enabled",
    version_name="explain_v0",
    conda_dependencies=["snowflake-ml-python"],
    sample_input_data = xs,
    options= {"enable_explainability": False}
)