Cached properties

Works just like a normal property, but returned values are cached:

from objecttools import CachedProperty

class ExpensiveOperations(object):
    @CachedProperty
    def expensive_attribute(self):
        return self.calculate()

    # To make it settable

    @expensive_attribute.setter
    def expensive_attribute(self, value):
        pass

    # To make it deletable

    @expensive_attribute.deleter
    def expensive_attribute(self):
        pass

e = ExpensiveOperations()
e.other_attribute = 1
print(e.expensive_attribute)  # Takes a long time.
print(e.expensive_attribute)  # Very quick; just retrieve from cache
v = e.expensive_attribute

e.other_attribute = 2  # expensive_attribute should be different now!
print(e.expensive_attribute)  # Old value that is wrong.
del e.expensive_attribute
print(e.expensive_attribute)  # Takes a long time, but returns new value.
e.other_attribute = 1
# Reset to known value
e.expensive_attribute = v
print(e.expensive_attribute)  # Correct value!

Singletons

from objecttools import Singleton

Sentinel = Singleton.create('Sentinel')

Sentinel() is Sentinel()  # True

d.get('missing_value', Sentinel()) is Sentinel()  # True

class GlobalState(dict, metaclass=Singleton):
    attr = 0


gs = GlobalState()
gs['value'] = 7
gs.attr = 1

print(GlobalState()['value'] + GlobalState().attr)  # 8

For Python 2 and 3 compatibility, use it as a decorator:

@Singleton.as_decorator
class Class(object):
    pass

Installing

$ pip install objecttools

$ git clone 'https://github.com/MitalAshok/objecttools.git'
$ python ./objecttools/setup.py install