Python-Design-Patterns lazy_evaluation
lazy_evaluation Model
Python-Design-Patterns lazy_evaluation
# file: 'lazy_evaluation.py'
#!/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'} # noqa flake8
# Father and mother
# 1
lazy_evaluation.py
lazy_evaluation Test
# file: 'test_lazy.py'
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from __future__ import print_function
import unittest
from creational.lazy_evaluation import Person
class TestDynamicExpanding(unittest.TestCase):
def setUp(self):
self.John = Person('John', 'Coder')
def test_innate_properties(self):
self.assertDictEqual({'name': 'John', 'occupation': 'Coder', 'call_count2': 0}, self.John.__dict__)
def test_relatives_not_in_properties(self):
self.assertNotIn('relatives', self.John.__dict__)
def test_extended_properties(self):
print(u"John's relatives: {0}".format(self.John.relatives))
self.assertDictEqual(
{'name': 'John', 'occupation': 'Coder', 'relatives': 'Many relatives.', 'call_count2': 0},
self.John.__dict__,
)
def test_relatives_after_access(self):
print(u"John's relatives: {0}".format(self.John.relatives))
self.assertIn('relatives', self.John.__dict__)
def test_parents(self):
for _ in range(2):
self.assertEqual(self.John.parents, "Father and mother")
self.assertEqual(self.John.call_count2, 1)
test_lazy.py
