revalidate 0.1.1

A package to manage interdependent fields

Revalidate

A package to manage interdependent fields.

Installation

Install the package with pip:

pip install revalidate

What revalidate does

revalidate is useful when some values in a class are derived from other values. For example, in a shopping cart:

• price, quantity, and discount_percent are user inputs,
• subtotal, discount_amount, and total are derived values.

You only want to recompute derived values when needed, and only if one of their dependencies changed. That is exactly what revalidate handles.

Define your class

Here's a complete CartItem class that uses revalidate:

# coding=utf-8
from revalidate import variable, VariableBase, compute

class CartItem(VariableBase):
    # Inputs users can set
    price: float = variable()
    quantity: int = variable()
    discount_percent: float = variable()

    # Derived values with their dependencies
    subtotal = variable(depends_on=["price", "quantity"], read_only=True)
    discount_amount = variable(depends_on=["subtotal", "discount_percent"], read_only=True)
    total = variable(depends_on=["subtotal", "discount_amount"], read_only=True)

    def __init__(self, price: float, quantity: int, discount_percent: float = 0.0):
        self.price = price
        self.quantity = quantity
        self.discount_percent = discount_percent

    @compute("subtotal")
    def _compute_subtotal(self):
        print("Computing subtotal")
        return self.price * self.quantity

    @compute("discount_amount")
    def _compute_discount_amount(self):
        print("Computing discount_amount")
        return self.subtotal * (self.discount_percent / 100.0)

    @compute("total")
    def _compute_total(self):
        print("Computing total")
        return self.subtotal - self.discount_amount

CartItem inherits from VariableBase, and each field is declared with variable(). Dependencies tell revalidate what to invalidate when an input changes. For example, if discount_percent changes, discount_amount and total are invalidated, but subtotal remains valid.

Usage

Here's how it works in practice. First access computes all dependencies:

>>> item = CartItem(price=50.0, quantity=2, discount_percent=10.0)
>>> print(item.total)
Computing total
Computing subtotal
Computing discount_amount
90.0

Access the value again—no recomputation, everything is cached:

>>> print(item.total)
90.0

Change only the discount. Only discount_amount and total are recomputed. subtotal is unaffected:

>>> item.discount_percent = 20.0
>>> print(item.total)
Computing total
Computing discount_amount
80.0

Change the price. This invalidates subtotal, which cascades to all dependent values:

>>> item.price = 60.0
>>> print(item.total)
Computing total
Computing subtotal
Computing discount_amount
96.0

Further reading

See the Github repo for further information.