PyChase: AST-Powered Duplicate Code Detection for Python

The Problem: Hidden Code Duplication

Every codebase accumulates copy-paste clones. They start innocently — “I’ll just duplicate this function and tweak it” — but over months they compound into a maintenance tax. When a bug is fixed in one copy, the other 5 copies stay broken. When a feature needs to change, you hunt down every variant by hand.

Traditional duplicate detectors fall into two camps:

1. Line-based tools (Duplo, Simian) — compare raw text, miss everything after variable renaming
2. Token-based tools (jscpd, PMD CPD) — slightly better, but still fail when identifiers change
3. Generic AST tools (SonarQube) — heavy infrastructure, Python support is an afterthought

PyChase takes a different approach: normalized AST fingerprints with MinHash + LSH.

What PyChase Catches

Clone Type Description Example PyChase Type-1 Exact copy-paste Same code, same names ✅ 1.000 score Type-2 Renamed identifiers calculate_total → compute_sum ✅ 0.786 score Type-3 Modified logic Added/removed statements, reordered ✅ 0.620 score

How It Works

1. Parse to AST

Each .py file is parsed into a Python Abstract Syntax Tree using the standard ast module.

2. Normalize

Variable names, function names, attribute names, string literals, numbers, and docstrings are replaced with generic placeholders. This strips everything except structure:

# These two produce the SAME normalized AST:
def calculate_total(items, rate):
    subtotal = 0
    for item in items:
        subtotal += item.price
    tax = subtotal * rate
    return subtotal + tax

def compute_sum(products, factor):
    result = 0
    for product in products:
        result += product.cost
    fee = result * factor
    return result + fee

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3. Shingle

The normalized AST node sequence is broken into overlapping k-shingles (default k=3):

(FunctionDef, arguments, arg) → (arguments, arg, Add) → ...

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4. Fingerprint + Match

Each shingle set becomes a structural fingerprint. MinHash signatures compress these into compact 256-bit signatures. Locality Sensitive Hashing (LSH) indexes them — only units sharing a bucket are compared.

This reduces candidate pairs from O(n²) to nearly O(n). For 10,000 functions, that’s 50 million comparisons avoided.

5. Cluster + Report

Connected-component clustering groups related matches. Results render as text, JSON, CSV, or an interactive HTML report with collapsible groups and syntax-highlighted code previews.

Quick Start

pip install pychase

# Scan your project
pychase .

# Generate HTML report
pychase --format html --output duplicates.html .

# JSON output for CI
pychase --json --threshold 0.85 ./src

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Why MinHash + LSH Matters

Tools like dry4python compare every function against every other function — O(n²). With 10,000 functions, that’s 50 million comparisons. PyChase’s MinHash + LSH approach reduces this to near-linear time, making it viable for large monorepos and CI pipelines.

Comparison at a Glance

Tool Algorithm Clone Types Setup Output PyChase MinHash+LSH (O(n)) Type-1,2,3 pip install Text, JSON, CSV, HTML dry4python Brute-force (O(n²)) Type-2 only pip install Text, JSON jscpd Token hash Type-1,2 Node.js Text, JSON, HTML SonarQube Custom AST Type-1,2 DB + server + scanner Web dashboard

Try It

pip install pychase
pychase --threshold 0.55 --min-lines 2 --min-nodes 10 ./src

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