Prescrypt

A Python to JavaScript transpiler for a well-defined subset of Python.

Status: Alpha. Use at your own risks.

Overview

Prescrypt converts Python 3.9+ code to ES6+ JavaScript. It prioritizes correctness over completeness, targeting common use cases rather than full Python compatibility.

Features

• Modern Python support - Python 3.10+ with pattern matching
• Modern JavaScript output - Targets ES6+ with const/let, arrow functions, classes
• Comprehensive test suite - 2400+ tests covering expressions, statements, and full programs
• Optimized output - Tree-shaking, constant folding, function inlining
• Source locations in errors - Clear error messages with file:line:column

Installation

pip install prescrypt
# or
uv add prescrypt

Quick Start

As a library

from prescrypt import py2js

code = """
def greet(name: str) -> str:
    return f"Hello, {name}!"

print(greet("World"))
"""

js = py2js(code)
print(js)

Output (stdlib helpers omitted):

function greet(name) {
    return ('Hello, ' + name + '!');
}
console.log(greet('World'));

Note: Type annotations (name: str) enable optimized output. Without them, the compiler uses runtime helpers for Python-compatible behavior.

From the command line

py2js input.py           # Creates input.js
py2js input.py output.js # Explicit output path

Supported Python Features

Fully Supported

• Data types: int, float, bool, str, None, list, dict, tuple
• Control flow: if/elif/else, for, while, break, continue
• Exception handling: try/except/finally, raise
• Functions: def, lambda, *args, default arguments, closures
• Classes: class, __init__, inheritance, super(), @property
• Decorators: @staticmethod, @classmethod, @property
• Comprehensions: list, dict, set, generator expressions
• Generators: yield, yield from, send(), throw(), close()
• Operators: arithmetic, comparison, logical, membership (in)
• Builtins: print, len, range, enumerate, zip, min, max, sorted, etc.

Partially Supported

• **kwargs - in function calls only (not in function definitions)
• with statement - single context manager only
• async def - generates async functions (await not yet implemented)

Not Supported

• Metaclasses
• Multiple inheritance
• exec(), eval()
• Most of the standard library

Architecture

Python Source → Parse → Desugar → Bind → Optimize → CodeGen → JavaScript

• Parse: Python's ast module with custom extensions
• Desugar: Simplifies syntax (e.g., a += b → a = a + b)
• Bind: Scope analysis, determines const vs let
• Optimize: Constant folding, dead code elimination
• CodeGen: Produces JavaScript output

Development

# Install dependencies
uv sync

# Run tests
make test

# Run linter
make lint

# Format code
make format

License

BSD 2-Clause License. See LICENSE for details.

Acknowledgments

Prescrypt was inspired by PScript, originally developed as part of Flexx. While the codebase has been largely rewritten, we acknowledge PScript's pioneering work in Python-to-JavaScript transpilation.