shap

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SHAP (SHapley Additive exPlanations) is a game theoretic approach to explain the output of any machine learning model. It connects optimal credit allocation with local explanations using the classic Shapley values from game theory and their related extensions (see papers for details and citations).

Install

SHAP can be installed from either PyPI or conda-forge:

GPU support

To enable GPU-accelerated Tree SHAP (GPUTreeExplainer), install the cuda extra:

This requires the CUDA toolkit to be installed on your system.

Supported versions

SHAP follows SPEC 0 for minimum supported dependency versions. We test against the versions specified there and may not fix bugs for older versions.

Contributing

We welcome contributions highly. Feel free to file an issue. Before opening a PR make sure you've read our CONTRIBUTING.md guideline.

Tree ensemble example (XGBoost/LightGBM/CatBoost/scikit-learn/pyspark models)

While SHAP can explain the output of any machine learning model, we have developed a high-speed exact algorithm for tree ensemble methods (see our Nature MI paper). Fast C++ implementations are supported for XGBoost, LightGBM, CatBoost, scikit-learn and pyspark tree models:

import xgboost
import shap

# train an XGBoost model
X, y = shap.datasets.california()
model = xgboost.XGBRegressor().fit(X, y)

# explain the model's predictions using SHAP
# (same syntax works for LightGBM, CatBoost, scikit-learn, transformers, Spark, etc.)
explainer = shap.Explainer(model)
shap_values = explainer(X)

# visualize the first prediction's explanation
shap.plots.waterfall(shap_values[0])

The above explanation shows features each contributing to push the model output from the base value (the average model output over the training dataset we passed) to the model output. Features pushing the prediction higher are shown in red, those pushing the prediction lower are in blue. Another way to visualize the same explanation is to use a force plot (these are introduced in our Nature BME paper):

# visualize the first prediction's explanation with a force plot
shap.plots.force(shap_values[0])

If we take many force plot explanations such as the one shown above, rotate them 90 degrees, and then stack them horizontally, we can see explanations for an entire dataset (in the notebook this plot is interactive):

# visualize all the training set predictions
shap.plots.force(shap_values[:500])

To understand how a single feature effects the output of the model we can plot the SHAP value of that feature vs. the value of the feature for all the examples in a dataset. Since SHAP values represent a feature's responsibility for a change in the model output, the plot below represents the change in predicted house price as the latitude changes. Vertical dispersion at a single value of latitude represents interaction effects with other features. To help reveal these interactions we can color by another feature. If we pass the whole explanation tensor to the color argument the scatter plot will pick the best feature to color by. In this case it picks longitude.

# create a dependence scatter plot to show the effect of a single feature across the whole dataset
shap.plots.scatter(shap_values[:, "Latitude"], color=shap_values)

To get an overview of which features are most important for a model we can plot the SHAP values of every feature for every sample. The plot below sorts features by the sum of SHAP value magnitudes over all samples, and uses SHAP values to show the distribution of the impacts each feature has on the model output. The color represents the feature value (red high, blue low). This reveals for example that higher median incomes increases the predicted home price.

# summarize the effects of all the features
shap.plots.beeswarm(shap_values)

We can also just take the mean absolute value of the SHAP values for each feature to get a standard bar plot (produces stacked bars for multi-class outputs):

shap.plots.bar(shap_values)

Natural language example (transformers)

SHAP has specific support for natural language models like those in the Hugging Face transformers library. By adding coalitional rules to traditional Shapley values we can form games that explain large modern NLP model using very few function evaluations. Using this functionality is as simple as passing a supported transformers pipeline to SHAP:

import transformers
import shap

# load a transformers pipeline model
model = transformers.pipeline('sentiment-analysis', top_k=None)

# explain the model on two sample inputs
explainer = shap.Explainer(model)
shap_values = explainer(["What a great movie! ...if you have no taste."])

# visualize the first prediction's explanation for the POSITIVE output class
shap.plots.text(shap_values[0, :, "POSITIVE"])

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