Add pattern: lazy_evaluation

* add pattern. * update readme, sorted by alphabet.
faif · faif · Apr 2, 2015 · Apr 1, 2015 · Apr 1, 2015 · babfa91722721884b8c9769452c5e9e07fc31972
commit babfa91722721884b8c9769452c5e9e07fc31972
diff --git a/README.md b/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 |
diff --git a/lazy_evaluation.py b/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/
+"""
+
+
+def lazy_property(fn):
+    """Decorator that makes a property lazy-evaluated."""
+    attr_name = '_lazy_' + fn.__name__
+
+    @property
+    def _lazy_property(self):
+        if not hasattr(self, attr_name):
+            setattr(self, attr_name, fn(self))
+        return getattr(self, attr_name)
+    return _lazy_property
+
+
+class Person(object):
+    def __init__(self, name, occupation):
+        self.name = name
+        self.occupation = occupation
+
+    @lazy_property
+    def relatives(self):
+        # Get all relatives, let's assume that it costs much time.
+        relatives = "Many relatives."
+        return relatives
+
+
+def main():
+    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'}