GeezerVison

Effect is a library for helping you write purely functional code by isolating the effects (that is, IO or state manipulation) in your code. Documentation is available at https://effect.readthedocs.org/, and its PyPI page is https://pypi.python.org/pypi/effect.

It supports 3.6 and above.

You can install it by running pip install effect.

Effect lets you isolate your IO and state-manipulation code.

The benefits of this are many: first, the majority of your code can become purely functional, leading to easier testing and ability to reason about behavior. Also, because it separates the specification of an effect from the performance of the effect, there are two more benefits: testing becomes easier still, and it's easy to provide alternative implementations of effects.

Effect is somewhat similar to "algebraic effects", as implemented in various typed functional programming languages. It also has similarities to Twisted's Deferred objects.

A very quick example of using Effects:

from effect import sync_perform, sync_performer, Effect, TypeDispatcher

class ReadLine(object):
    def __init__(self, prompt):
        self.prompt = prompt

def get_user_name():
    return Effect(ReadLine("Enter a candy> "))

@sync_performer
def perform_read_line(dispatcher, readline):
    return raw_input(readline.prompt)

def main():
    effect = get_user_name()
    effect = effect.on(
        success=lambda result: print("I like {} too!".format(result)),
        error=lambda e: print("sorry, there was an error. {}".format(e)))

    dispatcher = TypeDispatcher({ReadLine: perform_read_line})
    sync_perform(dispatcher, effect)

if __name__ == '__main__':
    main()

Effect takes what we call an intent, which is any object. The dispatcher argument to sync_perform must have a performer function for your intent.

This has a number of advantages. First, your unit tests for get_user_name become simpler. You don't need to mock out or parameterize the raw_input function - you just call get_user_name and assert that it returns a ReadLine object with the correct 'prompt' value.

Second, you can implement ReadLine in a number of different ways - it's possible to override the way an intent is performed to do whatever you want.

For more information on how to implement the actual effect-performing code, and other details, see the documentation. There is also a full example of interacting with the user and using an HTTP client to talk to the GitHub API in the effect-examples repository.

Some talks have been given about Effect.

• Side Effects are a Public API by Chris Armstrong at Strange Loop (2015-09-26)
• Functionalish Programming in Python with Effect by Robert Collins at Kiwi PyCon (2015-09-05)

Effect was originally written by Christopher Armstrong, but now has contributions from the following people:

• cyli
• lvh
• Manish Tomar
• Tom Prince

For integrating Effect with Twisted's Deferreds, see the txEffect package (pypi, github).

Over the past few years, the ecosystem of libraries to help with functional programming in Python has exploded. Here are some libraries I recommend:

• pyrsistent - persistent (optimized immutable) data structures in Python
• toolz - a general library of pure FP functions
• fn.py - a Scala-inspired set of tools, including a weird lambda syntax, option type, and monads

Effect is licensed under the MIT license:

Copyright (C) 2014 Christopher Armstrong

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.