python3-cookbook/cookbook/c08/p25_cached_objects.py

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
"""
Topic: 创建缓存实例
Desc :
"""

import logging

a = logging.getLogger('foo')
b = logging.getLogger('bar')
print(a is b)
c = logging.getLogger('foo')
print(a is c)

# The class in question
class Spam:
    def __init__(self, name):
        self.name = name

# Caching support
import weakref

_spam_cache = weakref.WeakValueDictionary()


def get_spam(name):
    if name not in _spam_cache:
        s = Spam(name)
        _spam_cache[name] = s
    else:
        s = _spam_cache[name]
    return s


# Note: This code doesn't quite work
class Spam1:
    _spam_cache = weakref.WeakValueDictionary()

    def __new__(cls, name):
        print('Spam1__new__')
        if name in cls._spam_cache:
            return cls._spam_cache[name]
        else:
            self = super().__new__(cls)
            cls._spam_cache[name] = self
            return self

    def __init__(self, name):
        print('Initializing Spam')
        self.name = name


s = Spam1('Dave')
t = Spam1('Dave')
print(s is t)


class CachedSpamManager:
    def __init__(self):
        self._cache = weakref.WeakValueDictionary()

    def get_spam(self, name):
        if name not in self._cache:
            s = Spam(name)
            self._cache[name] = s
        else:
            s = self._cache[name]
        return s

    def clear(self):
        self._cache.clear()


class Spam2:
    manager = CachedSpamManager()

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

def get_spam(name):
    return Spam2.manager.get_spam(name)


# ------------------------最后的修正方案------------------------
class CachedSpamManager2:
    def __init__(self):
        self._cache = weakref.WeakValueDictionary()

    def get_spam(self, name):
        if name not in self._cache:
            temp = Spam3._new(name)  # Modified creation
            self._cache[name] = temp
        else:
            temp = self._cache[name]
        return temp

    def clear(self):
        self._cache.clear()


class Spam3:
    def __init__(self, *args, **kwargs):
        raise RuntimeError("Can't instantiate directly")

    # Alternate constructor
    @classmethod
    def _new(cls, name):
        self = cls.__new__(cls)
        self.name = name
        return self

print('------------------------------')
cachedSpamManager = CachedSpamManager2()
s = cachedSpamManager.get_spam('Dave')
t = cachedSpamManager.get_spam('Dave')
print(s is t)