Creating Cached Instances

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

When creating instances of a class, you want to return a cached reference to a previous instance created with the same arguments (if any).

Solution

The problem being addressed in this Shortcut sometimes arises when you want to ensure that there is only one instance of a class created for a set of input arguments. Practical examples include the behavior of libraries, such as the logging module, that only want to associate a single logger instance with a given name. For example:

>>> import logging
>>> a = logging.getLogger('foo')
>>> b = logging.getLogger('bar')
>>> a is b
False
>>> c = logging.getLogger('foo')
>>> a is c
True
>>>

To implement this behavior, you should make use of a factory function that’s separate from the class itself. For example:

# The class in question
class Spam(object):
    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

If you use this implementation, you’ll find that it behaves in the manner shown earlier:

>>> a = get_spam('foo')
>>> b = get_spam('bar')
>>> a is b
False
>>> c = get_spam('foo')
>>> a is c
True
>>>

Discussion

Writing a special factory ...