#!/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'}