the-easy-tree 0.1.3

Decision tree algorithm implementation for regression and classification with minimal boilerplate code.

easy-tree

Decision tree algorithm implementation for regression and classification with minimal boilerplate code.

• Does not require data transform and can work natively with missing values.
• Supports lazy and eager data frames from polars.
• Prefers splits that lead to balanced tree.
• Single class for both classification and regression.
• Easy to extract and use rules corresponding to leaves.
• Save and load trained model.
• Little niceties:
  • Supports training straight from the CSV-file. One-line training is also possible.
  • Simple library to build and apply logical expressions (in the easy_tree/logic.py).

The implementation does not provide many parameters on purpose. The main goal is to provide a robust decision tree model without exhaustive tweaking of those parameters for quite often marginal gains and questionable stability.

The API resembles that of scikit-learn and should be familiar for most people.

Quick start

Training a regression tree model

import easy_tree as et
import polars as pl


# load lazy data frame and extract target as polars series.
df = pl.scan_csv("tests/data/titanic_train.csv")
y_true = df.select("Fare").collect().to_series()

# train a tree, no data preprocessing is required!
tree = et.DecisionTree()
tree.fit(data=df, y_true=y_true)

# compute predictions and MSE.
pred = tree.predict(df)
mse = (pred - y_true).pow(2).mean()
print(f"MSE = {mse}")

Currently, the following parameters are exposed for tuning:

• max_depth -- maximal tree depth. Default value is 4.
• min_leaf_size -- minimal number of samples in the leaf. Default is 10.

Training a classification tree

If target series has dtype String then classification model is trained.

import easy_tree as et
import polars as pl


# load data
df = pl.scan_csv("tests/data/titanic_train.csv")
y_true = df.select("Survived").collect().to_series().cast(str)  # target has dtype `String`

# train a tree using the same class
tree = et.DecisionTree()
tree.fit(data=df, y_true=y_true)

# compute prediction and accuracy
pred = tree.predict(df)  # prediction has dtype `String`
accuracy = (pred == y_true).mean()
print(f"Accuracy = {accuracy}")

Feature importances

Feature importances are available via the feature_importances_ property.

# feature importances
print({name: f"{importance:.3f}" for name, importance in tree.feature_importances_.items()})
# {'Sex': '0.631', 'Pclass': '0.208', 'Cabin': '0.093', 'Embarked': '0.034', 'Parch': '0.015', 'Age': '0.012', 'Fare': '0.007'}

Working with tree leaves

The trained DecisionTree object has a property leaves that contains list of, well, tree leaves. All tree nodes can be accessed via the method get_nodes.

Each leaf has a property full_condition that represent "path" from the root of the tree to that leaf.

An example below prints out conditions for the classification tree trained on the Titanic data from the example above.

print("\n".join(str(leaf.full_condition) for leaf in tree.leaves))
# ((Sex != male) AND (NOT ((Pclass > 2.0) AND (Pclass != None)))) AND (NOT ((Fare > 49.5042) AND (Fare != None)))
# ((Sex != male) AND (NOT ((Pclass > 2.0) AND (Pclass != None)))) AND ((Fare > 49.5042) AND (Fare != None))
# ((Sex != male) AND ((Pclass > 2.0) AND (Pclass != None))) AND (Embarked != S)
# ((Sex != male) AND ((Pclass > 2.0) AND (Pclass != None))) AND (Embarked == S)
# ((Sex == male) AND (Cabin != None)) AND (NOT ((Age > 37.0) AND (Age != None)))
# ((Sex == male) AND (Cabin != None)) AND ((Age > 37.0) AND (Age != None))
# ((Sex == male) AND (Cabin == None)) AND (NOT ((Parch > 0.0) AND (Parch != None)))
# ((Sex == male) AND (Cabin == None)) AND ((Parch > 0.0) AND (Parch != None))

Checking target statistics for leafs

Each node has attribute target_stats which is computed during training from training data and has the following fields:

• score_reduction -- how much the node's split contributed to the score reduction
• mean -- [regression] mean value of the target in the node
• var -- [regression] target variance in the node
• distr -- [classification] dictionary in the format {"class_name": count} for target classes

Example for regression (target: "Fare"):

print(tree.leaves[0].target_stats)
# TargetStats(score_reduction=307.79988432965, mean=75.86526103896104, var=4915.438094119127, distr=None)

Example for binary classification (target: "Survived", cast to str):

print(tree.leaves[0].target_stats)
# TargetStats(score_reduction=0.029410696893090726, mean=None, var=None, distr={'0': 7, '1': 79})

Example for non-binary classification (target: "Embarked"):

print(tree.leaves[0].target_stats)
# TargetStats(score_reduction=0.11205789293238799, mean=None, var=None, distr={'Q': 24, 'C': 7, 'S': 6})

Applying leaf conditions

Each leaf can be applied to the data in order to compute a mask corresponding to that leaf.

df = pl.scan_csv("tests/data/titanic_train.csv")
mask = tree.leaves[0].full_condition.apply(df)  # boolean mask corresponding to the leaf samples.
masked_df = df.filter(mask)  # subset of the data samples, that "belong" to the leaf.

Save and load model as json

Use methods save and load in order to save and load the decision tree

tree.save("tree.json")
tree2 = et.DecisionTree.load("tree.json")
assert all(tree.predict(df) == tree2.predict(df))  # `tree2` produces the same predictions.

Train straight from CSV-file

Decision tree can be trained by providing filename and name of the target column as data and y_true respectively.

In that case the type of the model (regression or classification) is defined based on the target-column type, detected by polars.

tree = et.DecisionTree()
tree.fit(data="tests/data/titanic_train.csv", y_true="Fare")  # will train a regression, as "Fare" is auto-detected as numerical column.

# -----------------

tree = et.DecisionTree()
tree.fit(data="tests/data/titanic_train.csv", y_true="Survived")  # will train a regression, as "Survived" is auto-detected as numerical column

# -----------------

tree = et.DecisionTree()
tree.fit(data="tests/data/titanic_train.csv", y_true="Embarked")  # will train a classification tree, as "Embarked" is auto-detected as String-valued column

One-liner is also possible:

tree = et.DecisionTree().fit(data="tests/data/titanic_train.csv", y_true="Embarked")

Small tool to handle logical expressions

Defined in the easy_tree/logic.py and supports:

• Atomic expression, such as A <= 5 and C != "foo".
• Connectives AND, OR, and NOT.
• Comparison with None, so that bar != None and bar == None are possible.
• Serialization and deserialization of expressions.
• Making compound expressions using class ExpressionBuilder.
• Export to polars.Expr class which can be applied to the polars data frame.

Some examples (more can be found in the tests/test_logic.py):

from easy_tree.logic import AtomicExpression, AndExpression, OrExpression, NotExpression, ExpressionBuilder, Operator

atomic = AtomicExpression(colname="Age", operator=Operator.greater, rhs=30)  # Age > 30
atomic_2 = AtomicExpression(colname="Embarked", operator=Operator.equal, rhs="C")  # AtomicExpression(colname="Embarked", operator=Operator.equal, rhs="C") 
and_expr = AndExpression(left=atomic, right=atomic_2)  # (Age > 30) AND (Embarked == C)
or_expr = OrExpression(left=and_expr, right=AtomicExpression(colname="Age", operator=Operator.not_equal, rhs=None))  # ((Age > 30) AND (Embarked == C)) OR (Age != None)
expr_builder = (ExpressionBuilder(atomic)
                .and_(atomic_2)
                .or_(AtomicExpression(colname="Age", operator=Operator.not_equal, rhs=None))
                .not_())
# use property `current` to access the resulting expression
expr_builder.current  # NOT (((Age > 30) AND (Embarked == C)) OR (Age != None))

NOT is a bit special, because it can affect the comparison operator. AtomicExpression and NotExpression have method not_() that handle the negation and can avoid double negation.

not_expr_1 = NotExpression(atomic_2)  # NOT (Embarked == C)
not_expr_1_1 = atomic_2.not_()  # Embarked != C
not_expr_1_2 = ExpressionBuilder(atomic_2).not_().current  # Embarked != C
not_expr_1_3 = NotExpression(not_expr_1)  # NOT (NOT (Embarked == C))
not_expr_1_4 = not_expr_1.not_()  # Embarked == C
not_expr_1_5 = ExpressionBuilder(atomic_2).not_().not_().current  # Embarked == C
not_expr_2 = NotExpression(atomic)  # NOT (Age > 30)
not_expr_2_1 = atomic.not_()  # Age <= 30
not_expr_2_2 = ExpressionBuilder(atomic).not_().current  # Age <= 30

Currently NOT does not expand for AND and OR connectives (De Morgan's laws are not implemented). The following expressions are equivalent:

not_expr_3 = NotExpression(and_expr)  # NOT ((Age > 30) AND (Embarked == C)), 
not_expr_3_1 = OrExpression(left=and_expr.left.not_(), right=and_expr.right.not_())  # (Age <= 30) OR (Embarked != C)