mlleak 0.1.1

ML Data Leakage & Split Sanity Checker - Detect duplicates, time leakage, and group leakage in train/test splits

mlleak 🔍

ML Data Leakage & Split Sanity Checker

A focused Python package to detect data leakage and bad train/test splits before they ruin your ML models.

What Problems Does mlleak Solve?

Data leakage is one of the most insidious bugs in machine learning. It causes:

• 🎯 Unrealistically high validation scores
• 💥 Catastrophic production failures
• ⏰ Wasted time debugging mysterious model behavior

mlleak catches these issues before you train your model.

When to Use mlleak

🔴 Critical Use Cases (MUST USE)

1. Recommendation Systems

• Problem: Same users appearing in both train and test splits
• Impact: Model learns user preferences directly instead of generalizing
• Solution: Use group_cols=["user_id"] to detect user overlap

2. Fraud Detection

• Problem: Same fraudsters/accounts in training and testing data
• Impact: Perfect fraud detection in validation, fails in production
• Solution: Use group_cols=["account_id", "device_id"] to ensure no overlap

3. Time-Series Forecasting

• Problem: Test data from earlier time periods than training data
• Impact: Model "predicts" the past, fails on actual future data
• Solution: Use time_col="timestamp" to validate temporal ordering

4. Medical Diagnosis

• Problem: Same patients appear in both train and test with different scans
• Impact: Model memorizes patients instead of learning disease patterns
• Solution: Use group_cols=["patient_id"] to prevent patient leakage

5. E-commerce Personalization

• Problem: Same customers/sessions in both splits
• Impact: Overfitted recommendations that don't generalize
• Solution: Use group_cols=["customer_id", "session_id"]

🟡 Important Use Cases (SHOULD USE)

6. Customer Churn Prediction

• Ensure customers don't appear in both splits
• Validate temporal ordering (test must be after train)

7. Image Classification with Augmentation

• Detect if augmented versions of same image appear in both sets
• Critical for medical imaging, satellite imagery

8. Natural Language Processing

• Check for duplicate text samples
• For user-generated content: ensure same authors don't overlap

9. Financial Modeling

• Validate temporal splits for stock/price predictions
• Check for duplicate transactions or entities

10. A/B Testing Data

• Ensure same users/sessions don't appear in both groups
• Validate experiment integrity

ML Problem Types Where Leakage Occurs

Problem Type	Leakage Risk	What to Check	mlleak Parameters
Classification	Medium	Duplicate rows	mlleak.report(train, test, target="label")
Regression	Medium	Duplicate rows	mlleak.report(train, test, target="value")
Time-Series	HIGH	Temporal order	time_col="date"
Recommendations	HIGH	User/item overlap	group_cols=["user_id", "item_id"]
Fraud Detection	CRITICAL	Entity overlap	group_cols=["account_id", "user_id"]
Medical ML	CRITICAL	Patient overlap	group_cols=["patient_id"]
NLP (user content)	High	Author overlap	group_cols=["author_id"]
Computer Vision	Medium-High	Duplicate images	Default duplicate check
Forecasting	HIGH	Temporal order	time_col="timestamp"
Personalization	HIGH	Customer overlap	group_cols=["customer_id"]

Real-World Example Scenarios

❌ Bad: Model Looks Perfect But Fails in Production

# Split without checking - DISASTER WAITING TO HAPPEN
train, test = train_test_split(df, test_size=0.2)
model.fit(train[features], train['target'])
score = model.score(test[features], test['target'])
# Score: 0.98! 🎉 (But it's a lie...)

✅ Good: Validate Split Before Training

# Check for leakage FIRST
import mlleak

mlleak.report(
    train, 
    test, 
    target='target',
    group_cols=['user_id'],
    time_col='date'
)
# Risk Score: 85/100 - HIGH RISK!
# Found: 200 overlapping users
# FIX THE SPLIT BEFORE TRAINING! 🛑

Who Should Use mlleak?

✅ Data Scientists - Before model training
✅ ML Engineers - In production pipelines
✅ Research Teams - Validating experimental splits
✅ Kaggle Competitors - Ensuring fair validation
✅ Anyone doing train/test splits - Prevention is better than debugging

Installation

pip install mlleak

Quick Start

import pandas as pd
import mlleak

# Option 1: From CSV files
train_df = pd.read_csv('train.csv')
test_df = pd.read_csv('test.csv')

# Option 2: From existing DataFrames (any source)
# train_df and test_df can come from:
# - Database queries
# - In-memory variables
# - sklearn train_test_split
# - Any pandas DataFrame source

# Run the sanity check
mlleak.report(
    train_df, 
    test_df,
    target="target_column",
    group_cols=["user_id"],  # Optional: columns that shouldn't overlap
    time_col="timestamp"      # Optional: time column for temporal validation
)

What It Detects

1. Duplicate Rows 🔄

Finds exact row duplicates across train and test sets.

mlleak.report(train_df, test_df)

2. Time Leakage ⏰

Detects if test data has timestamps earlier than training data (future leakage).

mlleak.report(train_df, test_df, time_col="date")

3. Group Leakage 👥

Ensures group identifiers (like user_id, customer_id) don't appear in both splits.

mlleak.report(train_df, test_df, group_cols=["user_id", "session_id"])

Example Output

══════════════════════════════════════════════════
         ML DATA LEAKAGE REPORT
══════════════════════════════════════════════════

DATASET INFO:
  Train samples: 8000
  Test samples:  2000
  Total features: 15

LEAKAGE CHECKS:
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

✓ Duplicate Rows: PASS
  No duplicate rows found between splits

✗ Time Leakage: FAIL
  Found 150 test samples with timestamps before training data
  Latest train date: 2024-06-30
  Earliest test date: 2024-05-15

✗ Group Leakage: FAIL
  Found 45 overlapping groups in 'user_id'
  This means same users appear in both train and test

OVERALL RISK SCORE: 68/100 (HIGH RISK ⚠️)

RECOMMENDATIONS:
  • Remove overlapping user_id groups from test set
  • Ensure test data is strictly after training data

API Reference

mlleak.report()

mlleak.report(
    train_df: pd.DataFrame,
    test_df: pd.DataFrame,
    target: str = None,
    group_cols: List[str] = None,
    time_col: str = None,
    verbose: bool = True
) -> dict

Parameters:

• train_df: Training DataFrame
• test_df: Test DataFrame
• target: (Optional) Name of target column to exclude from duplicate checks
• group_cols: (Optional) Columns representing groups that shouldn't overlap
• time_col: (Optional) Time/date column for temporal validation
• verbose: (Optional) Print report to console (default: True)

Returns:

• Dictionary with detailed findings and risk score

Use Cases

✅ Before training: Validate your train/test split
✅ Code reviews: Ensure proper data splitting practices
✅ CI/CD pipelines: Automated leakage detection
✅ Debugging: Understand why your model performs too well

Best Practices

1. Always run mlleak before training production models
2. Check group leakage for user-based data (recommendations, fraud detection)
3. Validate time splits for time-series and sequential data
4. Fix issues immediately - high risk scores indicate serious problems

Contributing

Contributions welcome! This is a focused tool - new features should align with the core mission of detecting data leakage.

License

MIT License - see LICENSE file for details

Credits

Built for ML practitioners who care about model reliability.