Code Quiz

Scaling Before the Train/Test Split

Spot the data-leakage bug in a feature pipeline that scales before splitting the data.

Codepython

import numpy as np
from sklearn.preprocessing import StandardScaler
from sklearn.model_selection import train_test_split
from sklearn.linear_model import LogisticRegression

def build_features(X, y):
    # Standardize features, then split for evaluation
    scaler = StandardScaler()
    X_scaled = scaler.fit_transform(X)

    X_train, X_test, y_train, y_test = train_test_split(
        X_scaled, y, test_size=0.2, random_state=42
    )

    model = LogisticRegression()
    model.fit(X_train, y_train)
    return model.score(X_test, y_test)

What is the bug in this feature pipeline?

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