Merge pull request faif#80 from lord63-forks/patch-3

Add pattern: lazy_evaluation

Author: faif

README.md

@@ -19,13 +19,15 @@ Current Patterns:
 |[builder](builder.py)| call many little discrete methods rather than having a huge number of constructor parameters |
 |[catalog](catalog.py)| general methods will call different specialized methods based on construction parameter |
 |[chain](chain.py)| apply a chain of successive handlers to try and process the data |
+|[chaining_method](chaining_method.py)| continue callback next object method |
 |[command](command.py)| bundle a command and arguments to call later |
 |[composite](composite.py)| encapsulate and provide access to a number of different objects |
 |[decorator](decorator.py)| wrap functionality with other functionality in order to affect outputs |
 |[facade](facade.py)| use one class as an API to a number of others |
 |[factory_method](factory_method.py)| delegate a specialized function/method to create instances |
 |[flyweight](flyweight.py)| transparently reuse existing instances of objects with similar/identical state |
 |[graph_search](graph_search.py)| (graphing algorithms, not design patterns) |
+|[lazy_evaluation](lazy_evaluation.py)| lazily-evaluated property pattern in Python |
 |[mediator](mediator.py)| an object that knows how to connect other objects and act as a proxy |
 |[memento](memento.py)| generate an opaque token that can be used to go back to a previous state |
 |[mvc](mvc.py)| model<->view<->controller (non-strict relationships) |
@@ -38,4 +40,3 @@ Current Patterns:
 |[strategy](strategy.py)| selectable operations over the same data |
 |[template](template.py)| an object imposes a structure but takes pluggable components |
 |[visitor](visitor.py)| invoke a callback for all items of a collection |
-|[chaining_method](chaining_method.py)| continue callback next object method |

lazy_evaluation.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+"""
+Lazily-evaluated property pattern in Python.
+
+https://en.wikipedia.org/wiki/Lazy_evaluation
+http://stevenloria.com/lazy-evaluated-properties-in-python/
+"""
+
+
+deflazy_property(fn):
+"""Decorator that makes a property lazy-evaluated."""
+attr_name='_lazy_'+fn.__name__
+
+@property
+def_lazy_property(self):
+ifnothasattr(self, attr_name):
+setattr(self, attr_name, fn(self))
+returngetattr(self, attr_name)
+return_lazy_property
+
+
+classPerson(object):
+def__init__(self, name, occupation):
+self.name=name
+self.occupation=occupation
+
+@lazy_property
+defrelatives(self):
+# Get all relatives, let's assume that it costs much time.
+relatives="Many relatives."
+returnrelatives
+
+
+defmain():
+Jhon=Person('Jhon', 'Coder')
+print("Name:{0} Occupation:{1}".format(Jhon.name, Jhon.occupation))
+print("Before we access `relatives`:")
+print(Jhon.__dict__)
+print("Jhon's relatives:{0}".format(Jhon.relatives))
+print("After we've accessed `relatives`:")
+print(Jhon.__dict__)
+
+
+if__name__=='__main__':
+main()
+
+### OUTPUT ###
+# Name: Jhon Occupation: Coder
+# Before we access `relatives`:
+#{'name': 'Jhon', 'occupation': 'Coder'}
+# Jhon's relatives: Many relatives.
+# After we've accessed `relatives`:
+#{'_lazy_relatives': 'Many relatives.', 'name': 'Jhon', 'occupation': 'Coder'}