CODEWITHSUNDEEP
pythonpython-3.xclasscachingproperties
callmeGuy
callmeGuy

Reputation: 1034

How to cache a property at class level in python?

So I am working on a Customer class that is supposed to be wrapper for some other classes that retrieve information about a specific customer from a server and from online, as below. 

class Customer:
    def __init__(self, name):
        self.name = name

    @property
    @lru_cache()
    def online_info(self):
       print('retrieving customer online info')
       return Online().search_result(for=self)

    @property
    @lru_cache()
    def server_info(self):
      print('retrieving customer server info')
      return Server().search_result(for=self)

The online and server calls have to be @property decorated. The problem I am facing is when trying to cache the online_info and server_info calls. The cache would somehow have to be at a class level so that even if a news customer is instantiated, the lru_cache wold remember previous calls from other instantiations for the same name call. Note my print statements. This is the behavious I am trying to achieve:

>>> cutomer1 = Customer('John')
>>> customer1.online_info
retrieving customer online info
John Smith has the following info bla bla bla ....

>>> cutomer2 = Customer('John')
>>> customer2.online_info # this one will not be calling the function, lru_cache will return the value saved from customer1.online_info
John Smith has the following info bla bla bla ....

Can someone explain how I achieve this behaviour? Is this possible?

Upvotes: 2

Views: 3367

Answers (2)

Dale
Dale

Reputation: 229

Assuming that you want to use the calling pattern you present: You want "property" attributes of instances which, when fetched, return information for the customer identified by self.name. But you want that customer information to be cached on a class-wide basis, so if you create two instances with the same self.name, work isn't duplicated between the two instances.

It seems to me that what you want is a "property" attribute whose fget function calls a class method, and the class method caches its results.

This code implements that pattern

import functools

class Customer:
    def __init__(self, name):
        self.name = name

    @property
    def info(self):
       print('retrieving info 1 for', self.name)
       return Customer.info_implement(self.name)

    @classmethod
    @functools.cache
    def info_implement(cls, name):
       print('retrieving info 2 for', name)
       return "info for " + name

customer1 = Customer('John')
print("customer1 info", customer1.info)
print("customer1 info", customer1.info)

customer2 = Customer('John')
print("customer2 info", customer2.info)
print("customer2 info", customer2.info)

and produces this output

retrieving info 1 for John
retrieving info 2 for John
customer1 info info for John
retrieving info 1 for John
customer1 info info for John
retrieving info 1 for John
customer2 info info for John
retrieving info 1 for John
customer2 info info for John

Upvotes: 0

Aran-Fey
Aran-Fey

Reputation: 43166

Instead of caching the property values on the class, I would recommend re-using the same Customer instance for each "John", so that

>>> Customer('John') is Customer('John')
True

This would make Customer a singleton of sorts. Singleton implementations can be found aplenty in this question: Creating a singleton in Python. Borrowing one of those implementations gives us a pseudo-singleton metaclass like this:

class NameSingleton(type):
    def __init__(cls, *args, **kwargs):
        cls._instances = {}

    def __call__(cls, name, *args, **kwargs):
        try:
            return cls._instances[name]
        except KeyError:
            instance = super().__call__(name, *args, **kwargs)
            cls._instances[name] = instance
            return instance

Use this as the metaclass for Customer and you're done:

class Customer(metaclass=NameSingleton):
    def __init__(self, name):
        self.name = name

    ...

Demo:

>>> Customer('John') is Customer('John')
True
>>> Customer('John') is Customer('not John')
False

Upvotes: 2

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