Persistent caching decorator

pip install filecache==0.81


filecache is a decorator which saves the return value of functions even after the interpreter dies. For example this is useful on functions that download and parse webpages. You can specify how long the return values should be cached (use seconds, like time.sleep) or just cache forever. The cache is in a file right next to the calling .py file.


pip install filecache

# or for developing and debugging after cloning this package

pip install -e .


from filecache import filecache

def time_consuming_function(args):
    # cache results forever

def another_function(args):
    # etc

@filecache(24 * 60 * 60)
def yet_another_function(args):
    # etc

How it works

Each time a decorated function is called, filecache checks in a shelve [1] to see if the arguments and function name have a recorded return value. If the recorded return value is still valid, it's returned, otherwise the original function is called and its return value is recorded with a timestamp.

[1] A shelve is a dictionary in a file with string keys and pickle values. The shelve file is named according to the module name that called filecache.


  • All arguments of the decorated function and the return value need to be picklable for this to work.

  • The cache isn't automatically cleaned, it is only overwritten. If your function can receive many different arguments that rarely repeat, your cache may forever grow. One day I might add a feature that once in every 100 calls scans the db for outdated stuff and erases.

  • This is less useful on methods of a class because the instance (self) is cached, and if the instance isn't the same, the cache isn't used. This makes sense because class methods are affected by changes in whatever is attached to self.

  • Tested on python 2.7, 3.3, 3.8

  • License: BSD, do what you wish with this. Could be awesome to hear if you found it useful and/or you have suggestions. ubershmekel at gmail

Here's a trick to invalidate a single value:

def somefunc(x, y, z):
    return x * y * z

del somefunc._db[filecache._args_key(somefunc, (1,2,3), {})]
# or just iterate through `somefunc._db` (it's a shelve, which is like a dict) to find the right key.