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"""
Simple caching of expensive values.
"""
from typing import Callable
from typing import Dict
from typing import Generic
from typing import TypeVar
import weakref
Key = TypeVar('Key')
Value = TypeVar('Value')
class Cache(Generic[Key, Value]): # pylint: disable=too-few-public-methods
"""
Cache of expensive values.
"""
_reset_caches: weakref.WeakSet = weakref.WeakSet()
def __init__(self) -> None:
self._data: Dict[Key, Value] = {}
Cache._reset_caches.add(self)
def lookup(self, key: Key, compute_value: Callable[[], Value]) -> Value:
"""
Lookup a value by its key, computing it only if this is the first lookup.
"""
value = self._data.get(key)
if value is not None:
return value
value = compute_value()
self._data[key] = value
return value
def __contains__(self, key: Key) -> bool:
return key in self._data
@staticmethod
def reset() -> None:
"""
Clear all the caches (for tests).
"""
for cache in Cache._reset_caches:
cache._data.clear() # pylint: disable=protected-access
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