How to Fix Specification Pattern Errors

In this tutorial, you'll learn about How to Fix Specification Pattern Errors. We cover key concepts, practical examples, and best practices.

Fix specification pattern errors when validation logic duplicated across codebase or complex if-chains.

Quick Fix

Wrong

def can_discount(order):
    return order.total>100 and order.loyalty>2 and not order.is_holiday
# Same logic duplicated in multiple services and views

Business rule in one-off function. Can't compose with other rules. Duplication across layers.

Right

from abc import ABC,abstractmethod
class Spec(ABC):
    @abstractmethod
    def is_satisfied(self,candidate): pass
    def __and__(self,other): return AndSpec(self,other)
    def __or__(self,other): return OrSpec(self,other)
    def __not__(self): return NotSpec(self)
class LoyalCustomer(Spec):
    def is_satisfied(self,o): return o.loyalty>2
class HighValue(Spec):
    def is_satisfied(self,o): return o.total>100
class NotHoliday(Spec):
    def is_satisfied(self,o): return not o.is_holiday
discount = LoyalCustomer() & HighValue() & NotHoliday()
if discount.is_satisfied(order): print('Discount')

Composable business rules. New specification = new class. Reusable across layers.

Prevention

Specification pattern encapsulates business rules into composable objects.

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