Source code for zict.cache

from __future__ import annotations

import weakref
from import Iterator, MutableMapping

from zict.common import KT, VT, ZictBase, close, discard, flush, locked

[docs]class Cache(ZictBase[KT, VT]): """Transparent write-through cache around a MutableMapping with an expensive __getitem__ method. Parameters ---------- data: MutableMapping Persistent, slow to read mapping to be cached cache: MutableMapping Fast cache for reads from data. This mapping may lose keys on its own; e.g. it could be a LRU. update_on_set: bool, optional If True (default), the cache will be updated both when writing and reading. If False, update the cache when reading, but just invalidate it when writing. Notes ----- If you call methods of this class from multiple threads, access will be fast as long as all methods of ``cache``, plus ``data.__delitem__``, are fast. Other methods of ``data`` are not protected by locks. Examples -------- Keep the latest 100 accessed values in memory >>> from zict import Cache, File, LRU, WeakValueMapping >>> d = Cache(File('myfile'), LRU(100, {})) # doctest: +SKIP Read data from disk every time, unless it was previously accessed and it's still in use somewhere else in the application >>> d = Cache(File('myfile'), WeakValueMapping()) # doctest: +SKIP """ data: MutableMapping[KT, VT] cache: MutableMapping[KT, VT] update_on_set: bool _gen: int _last_updated: dict[KT, int] def __init__( self, data: MutableMapping[KT, VT], cache: MutableMapping[KT, VT], update_on_set: bool = True, ): super().__init__() = data self.cache = cache self.update_on_set = update_on_set self._gen = 0 self._last_updated = {} @locked def __getitem__(self, key: KT) -> VT: try: return self.cache[key] except KeyError: pass gen = self._last_updated[key] with self.unlock(): value =[key] # Could another thread have called __setitem__ or __delitem__ on the # same key in the meantime? If not, update the cache if gen == self._last_updated.get(key): self.cache[key] = value self._last_updated[key] += 1 return value @locked def __setitem__(self, key: KT, value: VT) -> None: # If the item was already in cache and data.__setitem__ fails, e.g. because # it's a File and the disk is full, make sure that the cache is invalidated. discard(self.cache, key) gen = self._gen gen += 1 self._last_updated[key] = self._gen = gen with self.unlock():[key] = value if key not in self._last_updated: # Another thread called __delitem__ in the meantime discard(, key) elif gen != self._last_updated[key]: # Another thread called __setitem__ in the meantime. We have no idea which # of the two ended up actually setting # Case 1: the other thread did not enter this locked code block yet. # Prevent it from setting the cache. self._last_updated[key] += 1 # Case 2: the other thread already exited this locked code block and set the # cache. Invalidate it. discard(self.cache, key) else: # No race condition self._last_updated[key] += 1 if self.update_on_set: self.cache[key] = value @locked def __delitem__(self, key: KT) -> None: del[key] del self._last_updated[key] discard(self.cache, key) def __len__(self) -> int: return len( def __iter__(self) -> Iterator[KT]: return iter( def __contains__(self, key: object) -> bool: # Do not let MutableMapping call[key] return key in def flush(self) -> None: flush(self.cache,
[docs] def close(self) -> None: close(self.cache,
[docs]class WeakValueMapping(weakref.WeakValueDictionary[KT, VT]): """Variant of weakref.WeakValueDictionary which silently ignores objects that can't be referenced by a weakref.ref """ def __setitem__(self, key: KT, value: VT) -> None: try: super().__setitem__(key, value) except TypeError: pass