Full Example

In order for an article to be completed it must have the following rules

1. stock is > 0.
2. image url is present.
3. price exists.

from collections import namedtuple
from typing import Union

from rules_engine import Otherwise, Rule, RulesEngine, then

Article = namedtuple("Article", "title price image_url stock")
article = Article(title="Iphone Case", price=1000, image_url="http://localhost/image", stock=None)


def produce_article_completed_event():
    return None


def article_controller(article: Article):
    if not article.stock:
        return False
    if not article.price:
        raise ValueError("Article price missing")
    if not article.image_url:
        raise ValueError("Article image_url missing")
    return produce_article_completed_event()

To be able to change to rules engine, we need to split the conditions and actions.

Rules engine is pretty simple if the condition is True, its corresponding action will be executed.

### Conditions
def no_article_stock(article):
    return not article.stock


def no_article_price(article):
    return not article.price


def no_article_image_url(article):
    return not article.image_url

### Actions
def article_price_missing_error(article):
    raise ValueError("Article price missing")


def article_image_missing_error(article):
    raise ValueError("Article image_url missing")


### Rules Engine
def article_complete_rules(article):
    RulesEngine(
        Rule(no_article_stock, then(False)),
        Rule(no_article_price, article_price_missing_error),
        Rule(no_article_image_url, article_image_missing_error),
        Otherwise(produce_article_completed_event()),
    ).run(article)