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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.


  • 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.


python3 -m pip install --user eff


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:'Hello, {username}!')

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

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

if __name__ == '__main__':

Further reading

You don't have to read it but it's a good reading for better understanding of the motivation behind the library.