added specification design pattern.

washwash · washwash · commit ab9de6150655 · 2015-06-06T22:40:27.000+03:00
diff --git a/README.md b/README.md
@@ -37,6 +37,7 @@ Current Patterns:
 | [prototype](prototype.py) | use a factory and clones of a prototype for new instances (if instantiation is expensive) |
 | [proxy](proxy.py) | an object funnels operations to something else |
 | [publish_subscribe](publish_subscribe.py) | a source syndicates events/data to 0+ registered listeners |
+| [specification](specification.py) |  business rules can be recombined by chaining the business rules together using boolean logic |
 | [state](state.py) | logic is org'd into a discrete number of potential states and the next state that can be transitioned to |
 | [strategy](strategy.py) | selectable operations over the same data |
 | [template](template.py) | an object imposes a structure but takes pluggable components |
diff --git a/specification.py b/specification.py
@@ -0,0 +1,110 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+"""
+@author: Gordeev Andrey <gordeev.and.and@gmail.com>
+
+Specification provide recombination business logic by
+chaining together using boolean logic
+"""
+
+from abc import ABCMeta, abstractmethod
+
+
+class Specification:
+    __metaclass__ = ABCMeta
+
+    def and_specification(self, candidate):
+        raise NotImplementedError
+
+    def or_specification(self, candidate):
+        raise NotImplementedError
+
+    def not_specification(self):
+        raise NotImplementedError
+
+    @abstractmethod
+    def is_satisfied_by(self, candidate):
+        pass
+
+
+class CompositeSpecification(Specification):
+    @abstractmethod
+    def is_satisfied_by(self, candidate):
+        pass
+
+    def and_specification(self, candidate):
+        return AndSpecification(self, candidate)
+
+    def or_specification(self, candidate):
+        return OrSpecification(self, candidate)
+
+    def not_specification(self):
+        return NotSpecification(self)
+
+
+class AndSpecification(CompositeSpecification):
+    _one = Specification()
+    _other = Specification()
+
+    def __init__(self, one, other):
+        self._one = one
+        self._other = other
+
+    def is_satisfied_by(self, candidate):
+        return bool(self._one.is_satisfied_by(candidate) and
+                    self._other.is_satisfied_by(candidate))
+
+
+class OrSpecification(CompositeSpecification):
+    _one = Specification()
+    _other = Specification()
+
+    def __init__(self, one, other):
+        self._one = one
+        self._other = other
+
+    def is_satisfied_by(self, candidate):
+        return bool(self._one.is_satisfied_by(candidate) or
+                    self._other.is_satisfied_by(candidate))
+
+
+class NotSpecification(CompositeSpecification):
+    _wrapped = Specification()
+
+    def __init__(self, wrapped):
+        self._wrapped = wrapped
+
+    def is_satisfied_by(self, candidate):
+        return bool(not self._wrapped.is_satisfied_by(candidate))
+
+
+class User(object):
+
+    def __init__(self, super_user=False):
+        self.super_user = super_user
+
+
+class UserSpecification(CompositeSpecification):
+
+    def is_satisfied_by(self, candidate):
+        return isinstance(candidate, User)
+
+
+class SuperUserSpecification(CompositeSpecification):
+
+    def is_satisfied_by(self, candidate):
+        return getattr(candidate, 'super_user', False)
+
+
+if __name__ == '__main__':
+    print('Specification')
+    andrey = User()
+    ivan = User(super_user=True)
+
+    root_specification = UserSpecification().\
+        and_specification(SuperUserSpecification())
+
+    print(root_specification.is_satisfied_by(andrey))
+    print(root_specification.is_satisfied_by(ivan))
+
