flow-analysis 0.1.0

Static analysis tool for tracing exception flow through Python codebases

flow

Static analysis tool for tracing exception flow through Python codebases.

What can escape from my API endpoints? Flow answers this by parsing your code, building a call graph, and computing which exceptions propagate to each entrypoint.

Quick Start

pip install flow-analysis

# Check all endpoints for uncaught exceptions
flow audit -d /path/to/project

# Deep dive into one endpoint
flow escapes create_user -d /path/to/project

# Visualize the call tree
flow trace create_user -d /path/to/project

What It Does

Flow finds your HTTP routes and CLI scripts, traces the call graph, and reports which exceptions can escape:

$ flow audit

Scanning 23 entrypoints...

✗ 3 entrypoints have uncaught exceptions:

  POST /users/import
    └─ FileNotFoundError (importers.py:45)
    └─ ValidationError (validators.py:12)

  GET /reports/{id}
    └─ PermissionError (auth.py:89)

✓ 20 entrypoints fully covered by exception handlers

For a specific endpoint, see the full picture:

$ flow escapes create_user

Exceptions that can escape from POST /users:

  FRAMEWORK-HANDLED (converted to HTTP response):
    HTTPException
      └─ becomes: HTTP 404
      └─ raised in: routes/users.py:45 (get_user) [high confidence]

  CAUGHT BY GLOBAL HANDLER:
    ValidationError (@errorhandler(AppError))
      └─ raised in: validators.py:27 (validate_input) [high confidence]

  UNCAUGHT (will propagate to caller):
    ConnectionError
      └─ raised in: db/client.py:45 (execute) [medium confidence]
      └─ call path: create_user → save_user → db.execute

Visualize as a tree:

$ flow trace create_user

POST /users  → escapes: ValidationError, ConnectionError
├── validate_input()  → ValidationError
│   └── raises ValidationError (validators.py:27)
└── save_user()  → ConnectionError
    └── db.execute()  → ConnectionError
        └── raises ConnectionError (db/client.py:45)

Features

• Entrypoint detection: Flask routes, FastAPI routes, CLI scripts (if __name__ == "__main__")
• Global handler awareness: Understands @errorhandler, add_exception_handler
• Exception hierarchy: Knows that catching AppError also catches ValidationError if it's a subclass
• Polymorphism: Expands abstract method calls to all concrete implementations
• Framework-handled exceptions: Detects HTTPException, ValidationError → HTTP responses
• Confidence levels: Shows high/medium/low confidence based on resolution quality
• Resolution modes: --strict for precision, --aggressive for recall
• Exception stubs: Declare what external libraries can raise (requests, sqlalchemy, etc.)
• JSON output: All commands support -f json for CI/automation
• Caching: SQLite-based caching for fast repeated analysis

Commands

Command	Description
flow audit	Check all entrypoints for escaping exceptions
flow escapes <function>	Show what can escape from a specific function
flow trace <function>	Visualize exception flow as a call tree
flow entrypoints	List all HTTP routes and CLI scripts
flow raises <exception>	Find all places an exception is raised
flow catches <exception>	Find all places an exception is caught
flow exceptions	Show the exception class hierarchy
flow callers <function>	Find all callers of a function
flow subclasses <class>	Show class inheritance tree
flow stubs <action>	Manage exception stubs (list, init, validate)
flow stats	Show codebase statistics

All commands accept:

• -d, --directory: Directory to analyze (default: current)
• -f, --format: Output format (text or json)
• --no-cache: Disable caching

The escapes command accepts additional flags:

• --strict: High precision mode - only includes precisely resolved calls
• --aggressive: High recall mode - includes fuzzy matches

Supported Frameworks

Detected automatically:

• Flask (@app.route, @blueprint.route, @app.errorhandler)
• FastAPI (@router.get/post/put/delete, add_exception_handler)
• CLI scripts (if __name__ == "__main__")

Not yet supported:

• Django
• Celery tasks
• Scheduled jobs (APScheduler, etc.)

Custom patterns can be added via .flow/detectors/ (run flow init to set up).

Configuration

Flow can be configured via .flow/config.yaml:

resolution_mode: default  # "strict", "default", or "aggressive"
exclude:
  - vendor
  - migrations

Exception Stubs

Flow includes built-in stubs for common libraries (requests, sqlalchemy, httpx, redis, boto3). These declare what exceptions external library functions can raise.

Add custom stubs in .flow/stubs/:

# .flow/stubs/mylib.yaml
mylib:
  do_thing:
    - MyLibError
    - TimeoutError

Manage stubs with flow stubs list and flow stubs validate.

How It Works

1. Parse: LibCST parses all Python files
2. Extract: Find functions, classes, raise/catch sites, calls, entrypoints
3. Build call graph: Track who calls whom, resolve method calls
4. Propagate: Fixed-point iteration computes which exceptions escape each function
5. Report: For each entrypoint, show caught vs uncaught exceptions

Limitations

• Over-approximation: May report more exceptions than actually possible (e.g., all implementations of an abstract method)
• Under-approximation: Dynamic dispatch, eval(), and external libraries can't be fully traced
• No runtime info: Analysis is purely static

Development

git clone https://github.com/yourusername/flow-analysis
cd flow-analysis
pip install -e ".[dev]"
pytest

License

MIT