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Cached function calls with expiration in python with shelve and decorator

Jun 08, 2011 | categories: python View Comments

Cacheing with decorators is nice. Sometimes you don't want to use something super heavyweight, but just a little something you rolled on your own.

import time
import random

@scached(cache_file='shelvecache.db', expiry=datetime.timedelta(seconds=5))
def f1(foo, bar='baz'):
    """ Example of using the cache decorator """

    print " ** starting in f1.. sleepy time"
    time.sleep(5)
    # The result of all my hard work
    result = random.random()
    print " ** woke up with", result
    return result

if __name__ == '__main__':
    print f1('hai') # slow
    print f1('hai') # fast
    print f1(foo='hai') # fast

    print "okay.. sleeping on the outside"
    time.sleep(5)

    print f1('hai') # slow again
    print f1('hai') # fast again

Here's the code that provides the @scached decorator.

import datetime
import decorator
import shelve
from hashlib import md5

def scached(cache_file, expiry):
    """ Decorator setup """

    def scached_closure(func, *args, **kw):
        """ The actual decorator """
        key = md5(':'.join([func.__name__, str(args), str(kw)])).hexdigest()
        d = shelve.open(cache_file)

        # Expire old data if we have to
        if key in d:
            if d[key]['expires_on'] < datetime.datetime.now():
                del d[key]

        # Get new data if we have to
        if key not in d:
            data = func(*args, **kw)
            d[key] = {
                'expires_on' : datetime.datetime.now() + expiry,
                'data': data,
            }

        # Return what we got
        result = d[key]['data']
        d.close()

        return result

    return decorator.decorator(scached_closure)

For extra cool points, combine the above with my post on shelve and context managers.

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