Dead Code Detection

The Hidden Cost of Dead Code

Dead code isn’t just clutter. It’s active technical debt that:

Slows Development

Developers read and reason about code that does nothing

Hides Bugs

Unused code paths can mask logic errors and security issues

Increases Attack Surface

Vulnerable code that’s “not used” can still be exploited

Bloats Bundles

Unused imports increase load times and memory usage

The problem? Removing code is scary. What if it’s actually used somewhere you didn’t check?

How Skylos Finds Dead Code

Skylos builds a complete reference graph of your codebase, then identifies definitions with zero references.

What Gets Detected

Category	Example	Detection Method
Unreachable functions	`def helper(): ...` never called	No call references found
Unused imports	`import json` but `json` never used	No name references found
Unused classes	`class OldModel: ...` never instantiated	No instantiation or inheritance
Unused variables	`result = compute()` but `result` never read	Assigned but never referenced
Unused parameters	`def fn(a, b): return a`	Parameter `b` never used in body

The Confidence System

Not all “unused” code is actually dead. A helper function might be:

Called dynamically via getattr()
Used by a framework implicitly
Part of a public API

Skylos assigns a confidence score (0-100) to each finding, so you can filter out uncertain results.

Confidence Penalties

Pattern	Penalty	Reason
Private name (`_foo`)	-80	Convention for internal use
Dunder (`__str__`)	-100	Called implicitly by Python
Underscore var (`_`)	-100	Intentionally unused
In `__init__.py`	-15	Often public API re-exports
Framework decorator	-40	Called by framework
Dynamic module	-40	May use `getattr()`
Test-related	-100	Called by test runner

Using Confidence Threshold

# Default: only findings with ≥60% confidence
skylos .

# Include more uncertain findings
skylos . --confidence 40

# Only high-confidence findings
skylos . --confidence 80

Framework Awareness

Skylos understands that framework code is called implicitly:

Django
Flask / FastAPI
Pydantic
Pytest

# Not flagged - Django calls this via URL routing
def user_detail(request, pk):
    return render(request, 'user.html', {'user': User.objects.get(pk=pk)})

# Not flagged - Signal receiver
@receiver(post_save, sender=User)
def create_profile(sender, instance, **kwargs):
    Profile.objects.create(user=instance)

# Not flagged - Class-based view methods
class UserView(View):
    def get(self, request):
        return HttpResponse("Hello")

# ✅ Not flagged - Route handler
@app.route('/users')
def get_users():
    return jsonify(users)

# ✅ Not flagged - Error handler
@app.errorhandler(404)
def not_found(e):
    return "Not found", 404

# ✅ Not flagged - FastAPI dependency
async def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

# ✅ Not flagged - Pydantic model
class UserCreate(BaseModel):
    name: str
    email: str

# ✅ Not flagged - Validator
@field_validator('email')
def validate_email(cls, v):
    if '@' not in v:
        raise ValueError('Invalid')
    return v

# ✅ Not flagged - Fixture
@pytest.fixture
def db_session():
    return create_session()

# ✅ Not flagged - Test function
def test_user_creation(db_session):
    user = User(name='Test')
    assert user.name == 'Test'

Smart Tracing (Runtime Analysis)

Static analysis can’t catch everything. When code is called dynamically via getattr(), visitor patterns, or reflection, Skylos may flag it as unused. The solution: Run your tests with call tracing enabled.

skylos . --trace

This:

Runs your test suite with sys.settrace() enabled
Records every function that was actually called
Uses that data to eliminate false positives

What Gets Captured

Pattern	Static Analysis	With `--trace`
`visitor.visit(node)` → `visit_FunctionDef()`	❌ Missed	✅ Caught
`getattr(obj, "method")()`	❌ Missed	✅ Caught
Plugin hooks (`pytest_configure`)	❌ Missed	✅ Caught
Reflection / dynamic imports	❌ Missed	✅ Caught

When to Use `--trace`

Use It
Skip It

Projects with visitor patterns (AST, CST)
Plugin architectures
Heavy use of getattr() / reflection
Many false positives from static analysis

How It Works

Tip: Commit .skylos_trace to your repo if your test suite is stable. Then skylos . will use it automatically without re-running tests.

Comparison: Why Not Just Use Your IDE?

Feature	IDE “Unused” Warning	Skylos
Cross-file analysis	❌ Single file	✅ Entire codebase
Framework awareness	❌	✅ Django, Flask, FastAPI, Pydantic
Confidence scoring	❌	✅ Filter uncertain findings
CI/CD integration	❌	✅ Block PRs, generate reports
Batch removal	❌	✅ Interactive selection
Import tracking	Basic	✅ Resolves re-exports

Safe Removal Workflow

Scan with high confidence

Start with findings you can trust:

skylos . --confidence 80

Review in interactive mode

Select what to remove:

skylos . -i --dry-run

Remove or comment out

# Delete selected items
skylos . -i

# Or comment out (safer)
skylos . -i --comment-out

Run tests

Verify nothing broke:

pytest

Comment-Out Mode

Instead of deleting, Skylos can comment out code with a marker:

# Before
def unused_helper():
    return "I'm not used"

# After --comment-out
# SKYLOS DEADCODE: def unused_helper():
# SKYLOS DEADCODE:     return "I'm not used"

Search for SKYLOS DEADCODE later to permanently remove or restore.

Output Formats

Table (Default)
Tree
JSON

─────────────────── Unreachable Functions ───────────────────
 #   Name                    Location
 1   unused_helper           utils.py:42
 2   legacy_processor        core/processing.py:128

────────────────────── Unused Imports ───────────────────────
 #   Name                    Location
 1   json                    api/views.py:3
 2   Optional                models.py:1

skylos . --tree

my-project/
├── api/
│   └── views.py
│       ├── L3 Unused import: json
│       └── L45 Unused function: old_endpoint
└── utils.py
    └── L42 Unused function: unused_helper

skylos . --json -o report.json

{
  "unused_functions": [
    {
      "name": "unused_helper",
      "file": "utils.py",
      "line": 42,
      "confidence": 85
    }
  ],
  "unused_imports": [...]
}

Real-World Impact

Case Study: E-commerce Platform

A 200K LOC Python codebase ran Skylos and found:

47 unused functions (3,200 lines of dead code)
156 unused imports (faster startup, smaller bundles)
12 unused classes (legacy models never migrated)

Result: 15% reduction in codebase size, faster CI builds, easier onboarding for new developers.

Overview

Concepts

Product: Scan

Product: Gate

Product: Fix

Tutorial

Reference

Dead Code Detection

The Hidden Cost of Dead Code

Slows Development

Hides Bugs

Increases Attack Surface

Bloats Bundles

How Skylos Finds Dead Code

What Gets Detected

The Confidence System

Confidence Penalties

Using Confidence Threshold

Framework Awareness

Smart Tracing (Runtime Analysis)

What Gets Captured

When to Use `--trace`

How It Works

Comparison: Why Not Just Use Your IDE?

Safe Removal Workflow

Comment-Out Mode

Output Formats

Real-World Impact

Case Study: E-commerce Platform

Next Steps

Framework Awareness

CI/CD Integration

Overview

Concepts

Product: Scan

Product: Gate

Product: Fix

Tutorial

Reference

​The Hidden Cost of Dead Code

Slows Development

Hides Bugs

Increases Attack Surface

Bloats Bundles

​How Skylos Finds Dead Code

​What Gets Detected

​The Confidence System

​Confidence Penalties

​Using Confidence Threshold

​Framework Awareness

​Smart Tracing (Runtime Analysis)

​What Gets Captured

​When to Use --trace

​How It Works

​Comparison: Why Not Just Use Your IDE?

​Safe Removal Workflow

​Comment-Out Mode

​Output Formats

​Real-World Impact

Case Study: E-commerce Platform

​Next Steps

Framework Awareness

CI/CD Integration

The Hidden Cost of Dead Code

How Skylos Finds Dead Code

What Gets Detected

The Confidence System

Confidence Penalties

Using Confidence Threshold

Framework Awareness

Smart Tracing (Runtime Analysis)

What Gets Captured

When to Use `--trace`

How It Works

Comparison: Why Not Just Use Your IDE?

Safe Removal Workflow

Comment-Out Mode

Output Formats

Real-World Impact

Next Steps