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Pixel Importances with a Parallel Forest of Trees in Scikit-learn

This example shows the use of forests of trees to evaluate the importance of the pixels in an image classification task (faces). The hotter the pixel, the more important.

The code below also illustrates how the construction and the computation of the predictions can be parallelized within multiple jobs.

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Version

In [1]:
import sklearn
sklearn.__version__
Out[1]:
'0.18.1'

Imports

This tutorial imports fetch_olivetti_faces and ExtraTreesClassifier.

In [2]:
import plotly.plotly as py
import plotly.graph_objs as go

from time import time
import matplotlib.pyplot as plt

from sklearn.datasets import fetch_olivetti_faces
from sklearn.ensemble import ExtraTreesClassifier

Calculations

In [3]:
# Number of cores to use to perform parallel fitting of the forest model
n_jobs = 1

# Load the faces dataset
data = fetch_olivetti_faces()
X = data.images.reshape((len(data.images), -1))
y = data.target

mask = y < 5  # Limit to 5 classes
X = X[mask]
y = y[mask]

# Build a forest and compute the pixel importances
print("Fitting ExtraTreesClassifier on faces data with %d cores..." % n_jobs)
t0 = time()
forest = ExtraTreesClassifier(n_estimators=1000,
                              max_features=128,
                              n_jobs=n_jobs,
                              random_state=0)

forest.fit(X, y)
print("done in %0.3fs" % (time() - t0))
importances = forest.feature_importances_
importances = importances.reshape(data.images[0].shape)
Fitting ExtraTreesClassifier on faces data with 1 cores...
done in 2.753s

Plot Pixel Importances

In [4]:
trace = go.Heatmap(z=importances, 
                   colorscale='Hot',
                   showscale=False)

layout = go.Layout(title="Pixel importances with forests of trees",
                   yaxis=dict(autorange='reversed'))

fig = go.Figure(data=[trace], layout=layout)

py.iplot(fig)
Out[4]:
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