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Machine Learning Academy · Leçon

Visualiser et interpréter les arbres de décision

Exportez et affichez un arbre avec plot_tree de sklearn, lisez les règles de décision et extrayez l’importance des caractéristiques pour les rapports destinés aux parties prenantes.

Visualiser et interpréter les arbres de décision est une leçon Machine Learning Academy gratuite sur CoddyKit. Ceci est la leçon 4 sur 4. Tu peux lire la leçon complète ci-dessous gratuitement — puis la pratiquer en direct dans le navigateur avec un éditeur de code intégré et un tuteur IA 24/7. Elle fait partie du parcours d'apprentissage Machine Learning Academy, et ta progression se synchronise sur le web et l'application CoddyKit. Le cours Machine Learning Academy comprend 4 leçons au total.

Certaines parties de cette leçon n'ont pas encore été traduites et s'affichent en anglais.

Why Tree Visualisation Matters

Decision trees are often called white-box models because their decision logic is fully transparent. Visualising a trained tree lets you: verify the model is making decisions based on sensible features, explain predictions to non-technical stakeholders, identify potential data quality issues (e.g., a feature that should not be important appearing at the root), and debug unexpected behaviour. Visualisation turns the tree's mathematical structure into a human-readable flowchart that domain experts can validate against their knowledge.

from sklearn.tree import DecisionTreeClassifier, plot_tree
from sklearn.datasets import load_iris
import matplotlib.pyplot as plt

X, y = load_iris(return_X_y=True)
feature_names = load_iris().feature_names
class_names   = load_iris().target_names

tree = DecisionTreeClassifier(max_depth=3, random_state=42)
tree.fit(X, y)

plt.figure(figsize=(14, 6))
plot_tree(tree,
          feature_names=feature_names,
          class_names=class_names,
          filled=True,      # Color by majority class
          rounded=True,     # Rounded boxes
          fontsize=10)
plt.title('Iris Decision Tree (depth=3)')
plt.show()

Reading a Node in plot_tree Output

Each node in the plot_tree output shows four pieces of information: (1) The split condition (e.g., petal length <= 2.45), (2) The Gini impurity of the node, (3) The number of samples that reached this node during training, and (4) The class distribution as a list of sample counts per class. Leaf nodes show all four but no split condition — the majority class is the prediction. Node colour intensity indicates purity: darker = more samples of the dominant class.

# Interpreting node output from plot_tree:
#
# petal length (cm) <= 2.45     <- split condition
# gini = 0.667                  <- impurity before split
# samples = 150                 <- training samples reaching node
# value = [50, 50, 50]          <- samples per class [setosa, versicolor, virginica]
# class = setosa                <- majority class (prediction if leaf)

print('Gini 0.667 = equal 3-class split (maximum 3-class impurity)')
print('samples=150 at root = all training samples')
print('value=[50,50,50] = perfectly balanced classes')

Exporting Tree as Text with export_text

For logging, reports, or environments without graphical display, export_text() produces a text-based representation of the tree. Each level of indentation represents one split level. The pipe character shows branches, and leaf lines show the predicted class. This format is useful for embedding decision rules in documentation, saving to log files, or displaying in command-line environments. It also allows comparing tree structures numerically across different hyperparameter configurations.

from sklearn.tree import DecisionTreeClassifier, export_text
from sklearn.datasets import load_iris

X, y = load_iris(return_X_y=True)
tree = DecisionTreeClassifier(max_depth=3, random_state=42)
tree.fit(X, y)

text_repr = export_text(
    tree,
    feature_names=list(load_iris().feature_names)
)
print(text_repr)

Exporting to Graphviz DOT Format

export_graphviz() generates a DOT language file that can be rendered as a high-quality SVG or PNG using Graphviz. This is ideal for presentation-quality tree diagrams and for large trees that need scrolling to view. The DOT file can also be converted to a PDF or embedded in reports. In Jupyter, use graphviz.Source(dot_data) to render inline. This approach gives full control over font size, colour scheme, and layout — important when the tree is shared with business stakeholders.

from sklearn.tree import export_graphviz
from sklearn.datasets import load_iris
from sklearn.tree import DecisionTreeClassifier
import graphviz

X, y = load_iris(return_X_y=True)
tree = DecisionTreeClassifier(max_depth=3, random_state=42)
tree.fit(X, y)

dot_data = export_graphviz(
    tree,
    out_file=None,
    feature_names=load_iris().feature_names,
    class_names=load_iris().target_names,
    filled=True, rounded=True,
    special_characters=True
)
# graph = graphviz.Source(dot_data)  # Renders in Jupyter
# graph.render('iris_tree', format='png')  # Save as PNG

Feature Importances: What Drove the Model?

After training, tree.feature_importances_ reveals the relative contribution of each input feature to the model's predictions. Features used at the root and upper levels typically have high importance because their splits affect all training samples. Features used only in deep leaves have low importance. Plotting feature importances as a bar chart is a standard step in any tree-based analysis — it confirms that the model relies on sensible, domain-relevant features rather than spurious correlates that happen to work on training data.

from sklearn.tree import DecisionTreeClassifier
from sklearn.datasets import load_breast_cancer
import pandas as pd
import matplotlib.pyplot as plt

X, y = load_breast_cancer(return_X_y=True)
features = load_breast_cancer().feature_names

tree = DecisionTreeClassifier(max_depth=5, random_state=42)
tree.fit(X, y)

imp = pd.Series(tree.feature_importances_, index=features).sort_values(ascending=False)

imp.head(10).plot(kind='barh')
plt.title('Top 10 Feature Importances')
plt.xlabel('Importance')
plt.gca().invert_yaxis()
plt.show()

print('Top feature:', imp.index[0], '(importance:', imp.iloc[0].round(3), ')')

Tracing a Single Prediction

The decision_path() method returns a sparse indicator matrix showing which nodes each sample visits. Combined with tree.tree_, you can reconstruct the exact sequence of decisions for any prediction. This is the foundation of automated explanation systems: for each prediction, you can generate a human-readable list of rules like 'petal length was 1.4 cm (≤2.45), so went left; ended at leaf predicting setosa.' This level of transparency is required in regulated domains where every decision must be auditable.

from sklearn.tree import DecisionTreeClassifier
from sklearn.datasets import load_iris
import numpy as np

X, y = load_iris(return_X_y=True)
features = load_iris().feature_names
tree = DecisionTreeClassifier(max_depth=3, random_state=42)
tree.fit(X, y)

# Trace the path for sample 0
node_indicator = tree.decision_path(X[[0]])
nodes_visited = node_indicator.indices

T = tree.tree_
for node in nodes_visited[:-1]:  # All except leaf
    feature = features[T.feature[node]]
    threshold = T.threshold[node]
    val = X[0, T.feature[node]]
    direction = 'left (<= )' if val <= threshold else 'right (> )'
    print(f'Node {node}: {feature} = {val:.2f}, threshold={threshold:.2f} -> {direction}')

Interpreting Split Conditions for Stakeholders

When communicating tree decisions to non-technical stakeholders, convert the mathematical split conditions into plain-language statements. Instead of 'petal length (cm) <= 2.45: gini=0.0, samples=50', say 'If the petal is shorter than 2.45 cm, the flower is almost certainly a Setosa.' Frame each branch in terms of the business meaning of the feature. Decision trees are uniquely suited to stakeholder communication among ML models because every decision corresponds to a testable, interpretable business rule.

# Human-readable rule extraction from a trained tree
from sklearn.tree import _tree

def extract_rules(tree, feature_names, class_names):
    T = tree.tree_
    rules = []
    
    def recurse(node, path):
        if T.feature[node] != _tree.TREE_UNDEFINED:
            feat = feature_names[T.feature[node]]
            thresh = T.threshold[node]
            recurse(T.children_left[node],  path + [f'{feat} <= {thresh:.2f}'])
            recurse(T.children_right[node], path + [f'{feat} > {thresh:.2f}'])
        else:
            majority_class = class_names[T.value[node].argmax()]
            rules.append(' AND '.join(path) + f' => {majority_class}')
    
    recurse(0, [])
    return rules

Partial Dependence Plots for Individual Features

While feature importances show which features matter most, Partial Dependence Plots (PDP) show how a feature influences the prediction. A PDP marginalises over all other features and plots the model's predicted output as a function of one (or two) features. For decision trees, PDPs produce step-function shapes reflecting the axis-aligned split thresholds. Scikit-learn's PartialDependenceDisplay generates these plots directly from a fitted tree, making it easy to explain individual feature effects to domain experts.

from sklearn.inspection import PartialDependenceDisplay
from sklearn.tree import DecisionTreeClassifier
from sklearn.datasets import load_iris
import matplotlib.pyplot as plt

X, y = load_iris(return_X_y=True)
features = load_iris().feature_names

tree = DecisionTreeClassifier(max_depth=4, random_state=42)
tree.fit(X, y)

# PDP for the two most important features
fig, ax = plt.subplots(figsize=(10, 4))
PartialDependenceDisplay.from_estimator(
    tree, X, features=[2, 3],  # petal length and petal width
    feature_names=features, ax=ax
)
plt.tight_layout()
plt.show()

Comparing Tree Structures Across Hyperparameters

Visualising how the tree structure changes with depth helps build intuition. A depth-1 tree (stump) has one split and two leaves — the most important single feature. A depth-2 tree refines both branches with a second level of questions. Comparing trees at depth 1, 3, and 5 on the same dataset shows how the model builds increasingly complex decision logic. If the depth-5 tree uses the same features as the depth-3 tree at its top levels, those features are genuinely important. If new, obscure features appear at depth 5, they are likely capturing noise.

from sklearn.tree import DecisionTreeClassifier, export_text
from sklearn.datasets import load_iris

X, y = load_iris(return_X_y=True)
feat_names = list(load_iris().feature_names)

for depth in [1, 3, 5]:
    tree = DecisionTreeClassifier(max_depth=depth, random_state=42)
    tree.fit(X, y)
    print(f'\n--- max_depth={depth}, leaves={tree.get_n_leaves()} ---')
    print(export_text(tree, feature_names=feat_names)[:300])

Using Trees to Generate Business Rules

One of the most valuable uses of decision trees in industry is generating explicit business rules that can be implemented in rule engines, spreadsheets, or legacy systems that cannot run ML models. Each path from root to leaf is a complete IF-THEN rule. These rules can be translated into SQL WHERE clauses, Python dictionaries, or credit-scoring scorecards. By carefully controlling tree depth and minimum sample constraints, you can generate a small, high-accuracy rule set that a business analyst can manually review, approve, and maintain.

# Generate SQL-like rules from a trained decision tree
from sklearn.tree import _tree

def tree_to_sql(tree, feature_names, class_names):
    T = tree.tree_
    rules = []
    
    def traverse(node, conditions):
        if T.feature[node] != _tree.TREE_UNDEFINED:
            fname = feature_names[T.feature[node]]
            thresh = T.threshold[node]
            traverse(T.children_left[node],
                     conditions + [f'{fname} <= {thresh:.3f}'])
            traverse(T.children_right[node],
                     conditions + [f'{fname} > {thresh:.3f}'])
        else:
            pred = class_names[T.value[node].argmax()]
            where = ' AND '.join(conditions)
            rules.append(f'WHEN {where} THEN {pred!r}')
    
    traverse(0, [])
    return 'CASE\n  ' + '\n  '.join(rules) + '\nEND',

Saving Tree Visualisations to Files

Saving tree visualisations to files makes them shareable in reports, presentations, and model documentation. With plot_tree and matplotlib, save as PNG or SVG using plt.savefig(). With Graphviz, render to PDF directly. For interactive exploration in Jupyter notebooks, inline SVG provides the clearest output because it is infinitely scalable — useful for deep trees that would be blurry as a fixed-resolution PNG. For stakeholder deliverables, always export at high DPI (300+) or as vector SVG format so the text in nodes remains crisp when zoomed.

from sklearn.tree import DecisionTreeClassifier, plot_tree
from sklearn.datasets import load_iris
import matplotlib.pyplot as plt

X, y = load_iris(return_X_y=True)
tree = DecisionTreeClassifier(max_depth=3, random_state=42)
tree.fit(X, y)

# Save as high-DPI PNG for reports
fig, ax = plt.subplots(figsize=(16, 8))
plot_tree(tree, feature_names=load_iris().feature_names,
          class_names=load_iris().target_names,
          filled=True, rounded=True, ax=ax, fontsize=10)
fig.savefig('iris_decision_tree.png', dpi=200, bbox_inches='tight')
fig.savefig('iris_decision_tree.svg', format='svg', bbox_inches='tight')
print('Saved PNG and SVG tree visualisations')

Quick Check

Test your understanding of Machine Learning with Python concepts from this lesson.

Lesson Recap

In this lesson you learned: how to visualise decision trees using plot_tree, export_text, and export_graphviz, how to read node information (split condition, Gini, samples, value), and how to extract feature importances and decision paths for stakeholder communication. Next up we explore Naive Bayes — a probabilistic classifier that applies Bayes' theorem to make predictions.

Questions Fréquemment Posées

La leçon « Visualiser et interpréter les arbres de décision » est-elle gratuite ?

Oui — le texte complet de « Visualiser et interpréter les arbres de décision » est gratuit à lire ici sur le web. Pour la pratiquer de manière interactive (un éditeur de code intégré et un tuteur IA 24/7) et déverrouiller le reste du cours Machine Learning Academy, passe à CoddyKit PRO. Le cours Machine Learning Academy comprend 4 leçons au total.

Qu'est-ce que j'apprendrai dans « Visualiser et interpréter les arbres de décision » ?

Exportez et affichez un arbre avec plot_tree de sklearn, lisez les règles de décision et extrayez l’importance des caractéristiques pour les rapports destinés aux parties prenantes. Tu pratiques Machine Learning Academy avec du code pratique que tu exécutes directement dans le navigateur, et un tuteur IA 24/7 répond à tes questions au fur et à mesure que tu avances dans la leçon.

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Aucune expérience préalable n'est requise. Machine Learning Academy sur CoddyKit est structuré pour les débutants jusqu'aux apprenants avancés, donc tu peux commencer ici ou depuis le début et avancer à ton rythme. Ceci est la leçon 4 sur 4.

Combien de temps prend la leçon « Visualiser et interpréter les arbres de décision » ?

La plupart des leçons CoddyKit prennent environ 5–10 minutes. Chacune est courte et interactive, tu progresses régulièrement et tu repiques exactement où tu t'es arrêté sur le web et l'app.

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Toutes les leçons de ce cours

  1. Construire un arbre : divisions, nœuds et feuilles
  2. Impureté de Gini et gain d’information
  3. Contrôler la profondeur de l’arbre pour éviter le surajustement
  4. Visualiser et interpréter les arbres de décision
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