cache_utils.py

"""
Utilities related to caching.
"""
import collections
import cPickle as pickle
import functools
import zlib
from xblock.core import XBlock


def memoize_in_request_cache(request_cache_attr_name=None):
    """
    Memoize a method call's results in the request_cache if there's one. Creates the cache key by
    joining the unicode of all the args with &; so, if your arg may use the default &, it may
    have false hits.

    Arguments:
        request_cache_attr_name - The name of the field or property in this method's containing
         class that stores the request_cache.
    """
    def _decorator(func):
        """Outer method decorator."""
        @functools.wraps(func)
        def _wrapper(self, *args, **kwargs):
            """
            Wraps a method to memoize results.
            """
            request_cache = getattr(self, request_cache_attr_name, None)
            if request_cache:
                cache_key = '&'.join([hashvalue(arg) for arg in args])
                if cache_key in request_cache.data.setdefault(func.__name__, {}):
                    return request_cache.data[func.__name__][cache_key]

                result = func(self, *args, **kwargs)

                request_cache.data[func.__name__][cache_key] = result
                return result
            else:
                return func(self, *args, **kwargs)
        return _wrapper
    return _decorator


class memoized(object):  # pylint: disable=invalid-name
    """
    Decorator. Caches a function's return value each time it is called.
    If called later with the same arguments, the cached value is returned
    (not reevaluated).
    https://wiki.python.org/moin/PythonDecoratorLibrary#Memoize

    WARNING: Only use this memoized decorator for caching data that
    is constant throughout the lifetime of a gunicorn worker process,
    is costly to compute, and is required often.  Otherwise, it can lead to
    unwanted memory leakage.
    """

    def __init__(self, func):
        self.func = func
        self.cache = {}

    def __call__(self, *args):
        if not isinstance(args, collections.Hashable):
            # uncacheable. a list, for instance.
            # better to not cache than blow up.
            return self.func(*args)
        if args in self.cache:
            return self.cache[args]
        else:
            value = self.func(*args)
            self.cache[args] = value
            return value

    def __repr__(self):
        """
        Return the function's docstring.
        """
        return self.func.__doc__

    def __get__(self, obj, objtype):
        """
        Support instance methods.
        """
        return functools.partial(self.__call__, obj)


def hashvalue(arg):
    """
    If arg is an xblock, use its location. otherwise just turn it into a string
    """
    if isinstance(arg, XBlock):
        return unicode(arg.location)
    else:
        return unicode(arg)


def zpickle(data):
    """Given any data structure, returns a zlib compressed pickled serialization."""
    return zlib.compress(pickle.dumps(data, pickle.HIGHEST_PROTOCOL))


def zunpickle(zdata):
    """Given a zlib compressed pickled serialization, returns the deserialized data."""
    return pickle.loads(zlib.decompress(zdata))