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lazy_evaluation.py
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100 lines (78 loc) · 2.76 KB
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#!/usr/bin/env python
# -*- coding: utf-8 -*-
"""
Lazily-evaluated property pattern in Python.
https://en.wikipedia.org/wiki/Lazy_evaluation
*References:
bottle
https://github.com/bottlepy/bottle/blob/cafc15419cbb4a6cb748e6ecdccf92893bb25ce5/bottle.py#L270
django
https://github.com/django/django/blob/ffd18732f3ee9e6f0374aff9ccf350d85187fac2/django/utils/functional.py#L19
pip
https://github.com/pypa/pip/blob/cb75cca785629e15efb46c35903827b3eae13481/pip/utils/__init__.py#L821
pyramimd
https://github.com/Pylons/pyramid/blob/7909e9503cdfc6f6e84d2c7ace1d3c03ca1d8b73/pyramid/decorator.py#L4
werkzeug
https://github.com/pallets/werkzeug/blob/5a2bf35441006d832ab1ed5a31963cbc366c99ac/werkzeug/utils.py#L35
*TL;DR80
Delays the eval of an expr until its value is needed and avoids repeated evals.
"""
from __future__ import print_function
import functools
class lazy_property(object):
def __init__(self, function):
self.function = function
functools.update_wrapper(self, function)
def __get__(self, obj, type_):
if obj is None:
return self
val = self.function(obj)
obj.__dict__[self.function.__name__] = val
return val
def lazy_property2(fn):
attr = '_lazy__' + fn.__name__
@property
def _lazy_property(self):
if not hasattr(self, attr):
setattr(self, attr, fn(self))
return getattr(self, attr)
return _lazy_property
class Person(object):
def __init__(self, name, occupation):
self.name = name
self.occupation = occupation
self.call_count2 = 0
@lazy_property
def relatives(self):
# Get all relatives, let's assume that it costs much time.
relatives = "Many relatives."
return relatives
@lazy_property2
def parents(self):
self.call_count2 += 1
return "Father and mother"
def main():
Jhon = Person('Jhon', 'Coder')
print(u"Name: {0} Occupation: {1}".format(Jhon.name, Jhon.occupation))
print(u"Before we access `relatives`:")
print(Jhon.__dict__)
print(u"Jhon's relatives: {0}".format(Jhon.relatives))
print(u"After we've accessed `relatives`:")
print(Jhon.__dict__)
print(Jhon.parents)
print(Jhon.__dict__)
print(Jhon.parents)
print(Jhon.call_count2)
if __name__ == '__main__':
main()
### OUTPUT ###
# Name: Jhon Occupation: Coder
# Before we access `relatives`:
# {'call_count2': 0, 'name': 'Jhon', 'occupation': 'Coder'}
# Jhon's relatives: Many relatives.
# After we've accessed `relatives`:
# {'relatives': 'Many relatives.', 'call_count2': 0, 'name': 'Jhon', 'occupation': 'Coder'}
# Father and mother
# {'_lazy__parents': 'Father and mother', 'relatives': 'Many relatives.', 'call_count2': 1, 'name': 'Jhon', 'occupation': 'Coder'}
# Father and mother
# 1