Effect 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 .

What Is It? 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.

Example 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.

Thanks Thanks to Rackspace for allowing me to work on this project, and having an excellent open source employee contribution policy

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

lvh

Manish Tomar

Tom Prince

IRC There is a #python-effect IRC channel on irc.freenode.net.

See Also 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