Eff

Eff is a Python library to work with algebraic effects.

Algbraic effects are all side-effects of a piece of code, like reading a user input, writing a text on the screen, doing network requests, reading a file etc. Eff allows to easily handle such effects making the code cleaner and testing easier.

Features:

Easy to understand. You don't need to read long scientific papers to understand how to use the library and what it does.
Easy to integrate. If you decided to add a logger into a function, changes will be minimal, no need to pass the logger down through the whole call stack.
Fast. The classic approach for handling algebraic effects is using coroutines or exceptions. This library uses global shared state instead which doesn't require to unwrap the whole call stack at runtime.
Type-safe. Effect handlers container is just a class. Annotate effect handlers type to make their usage type-safe.

Installation

python3 -m pip install --user eff

Usage

from io import StringIO
from typing import Callable

import eff

class Eff(eff.ects):
    show: Callable

# Use global effects manager in a function.
def greet(username: str) -> None:
    Eff.show(f'Hello, {username}!')

# Provide actual side-effects handlers
# for the project prod entry point.
def main() -> None:
    with Eff(show=print):
        greet('World')

# Mock side-effects in tests.
def test_greet() -> None:
    stream = StringIO()
    with Eff(show=stream.write):
        greet('World')
    stream.seek(0)
    assert stream.read() == 'Hello, World!'

if __name__ == '__main__':
    main()