JohannesEbke / app_json_file_cache

Python decorator to store results of expensive calculations in properly located cache files

app_json_file_cache

Provides a function decorator that caches the return value in a JSON file in the appropriate application cache directory.

It requires all function parameters and return values to be encodeable to JSON, so that the cache is human-readable.

It supports a "vary" guard value (e.g. a data model version) that protects against using old versions of cache.

Additionally, a cheaper default function (e.g. returning shipped, precalculated values) can be specified as a stand-in until an explicit recalculate function is called.

Usage

Example usage:

from app_json_file_cache import AppCache
cache = AppCache("myapp")

@cache("expensive")
def expensive_function():
    return calculator()

More Example usage:

from app_json_file_cache import AppCache
cache = AppCache("myapp")

@cache("expensive", vary=VERSION)
def expensive_function(param):
    return calculator(param)

expensive_function.recalculate(param1) # recalculate if external effects change

Using a default function until the next recalculate:

from app_json_file_cache import AppCache
cache = AppCache("myapp")

def cheap_standin(param):
    return estimate(param)

@cache("expensive", vary=VERSION, cheap_default_func=cheap_standin)
def expensive_function(param):
    return calculator(param)

expensive_function(param1) # cheap_standin is used
expensive_function.recalculate(param1) # cache is updated with expensive_function
expensive_function(param1) # cached value from expensive_function is used

Caveats

• Names must be unique per app. If you reuse names, chaos ensues.
• Each set of function parameter values creates a new file. This may lead to too many files in a directory on some systems.
• Mixing positional and keyword arguments is not supported
• It's your responsibility that return values are serializable to JSON.