endexai / once

Utility for initialization ensuring functions are called only once

Once

This library provides functionality to ensure a function is called exactly once in Python, heavily inspired by std::call_once.

During initialization, we often want to ensure code is run exactly once. But thinking about all the different ways this constraint can be violated can be time-consuming and complex. We don't want to have to reason about what other callers are doing and from which thread.

Introducing a simple solution - the once.once decorator! Simply decorate a function with this decorator, and this library will handle all the edge cases to ensure it is called exactly once! The first call will invoke the function, and all subsequent calls will return the same result. Enough talking, let's cut to an example:

import once

@once.once
def my_expensive_object():
    load_expensive_resource()
    load_more_expensive_resources()
    return ObjectSingletonUsingLotsOfMemory()

def caller_one():
    my_expensive_object().use_it()

def caller_two_from_a_separate_thread():
    my_expensive_object().use_it()

def optional_init_function_to_prewarm():
    my_expensive_object()

This module is extremely simple, with no external dependencies, and heavily tested for races.