fates 🔮¶

A robust, fully-typed, and async-ready Result pattern implementation for Python.

fates brings expressive, functional error handling to Python, completely removing the need for defensive try/except blocks. It helps you build predictable pipelines with absolute type safety for both synchronous and asynchronous operations.

✨ Features¶

🛡️ 100% Type Safe: Full mypy / pyright compliance using generic protocols and TypeVar covariance.
⚡ Async Native: First-class support for asynchronous mapping and monadic binding.
🧩 Zero Dependencies: Lightweight and clean, relying only on standard library primitives (and typing_extensions for older Python versions).

🚀 Installation¶

pip install fates

📖 Quick Start¶

Synchronous Usage¶

from fates import Result, Ok, Err

def divide(a: int, b: int) -> Result[float, str]:
    if b == 0:
        return Err("Cannot divide by zero")
    return Ok(a / b)

# Monadic binding and mapping
result = (
    divide(10, 2)
    .map(lambda x: x * 2)
    .unwrap_or(0.0)
)
print(result)  # Output: 10.0

Asynchronous Pipelines¶

fates shines when working with async databases or HTTP clients. Use amap and abind to pipe async operations seamlessly.

import asyncio
from fates import Ok, Err, Result

async def fetch_user_id(username: str) -> Result[int, str]:
    # Simulating async DB call
    await asyncio.sleep(0.1)
    return Ok(42) if username == "admin" else Err("User not found")

async def get_user_role_async(user_id: int) -> Result[str, str]:
    await asyncio.sleep(0.1)
    return Ok("superuser")

async def main():
    # Chain async functions using abind
    pipeline = await fetch_user_id("admin").abind(get_user_role_async)
    
    print(pipeline)  # Output: Ok('superuser')
    print(pipeline.unwrap())  # Output: superuser

asyncio.run(main())

🛠️ API Reference¶

Extracting Values¶

.unwrap() — Returns the success value or raises an UnwrapError.
.unwrap_or(default) — Returns the success value or a fallback value.
.unwrap_err() — Returns the error value or raises an UnwrapError.
.expect(note) — Returns the success value or crashes with a custom message.

Transforming Results¶

.map(mapper) — Transforms the success value inside Ok.
.map_err(mapper) — Transforms the error value inside Err.
.bind(binder) — Monadic bind. Chains another operation that returns a Result.
.catch(binder) — Recovers from an error by returning an alternative Result.
.resolve(mapper) — Merges both paths into a single success-type value.

Async Operations¶

.amap(async_mapper) — Asynchronously transforms the success value.
.amap_err(async_mapper) — Asynchronously transforms the error value.
.abind(async_binder) — Asynchronously chains another operation returning a Result.

⚖️ License¶

This project is licensed under the MIT License.