1from typing import Any 2from typing import cast 3from typing import Dict 4from typing import Generic 5from typing import TypeVar 6from typing import Union 7 8 9__all__ = ["Store", "StoreKey"] 10 11 12T = TypeVar("T") 13D = TypeVar("D") 14 15 16class StoreKey(Generic[T]): 17 """StoreKey is an object used as a key to a Store. 18 19 A StoreKey is associated with the type T of the value of the key. 20 21 A StoreKey is unique and cannot conflict with another key. 22 """ 23 24 __slots__ = () 25 26 27class Store: 28 """Store is a type-safe heterogenous mutable mapping that 29 allows keys and value types to be defined separately from 30 where it (the Store) is created. 31 32 Usually you will be given an object which has a ``Store``: 33 34 .. code-block:: python 35 36 store: Store = some_object.store 37 38 If a module wants to store data in this Store, it creates StoreKeys 39 for its keys (at the module level): 40 41 .. code-block:: python 42 43 some_str_key = StoreKey[str]() 44 some_bool_key = StoreKey[bool]() 45 46 To store information: 47 48 .. code-block:: python 49 50 # Value type must match the key. 51 store[some_str_key] = "value" 52 store[some_bool_key] = True 53 54 To retrieve the information: 55 56 .. code-block:: python 57 58 # The static type of some_str is str. 59 some_str = store[some_str_key] 60 # The static type of some_bool is bool. 61 some_bool = store[some_bool_key] 62 63 Why use this? 64 ------------- 65 66 Problem: module Internal defines an object. Module External, which 67 module Internal doesn't know about, receives the object and wants to 68 attach information to it, to be retrieved later given the object. 69 70 Bad solution 1: Module External assigns private attributes directly on 71 the object. This doesn't work well because the type checker doesn't 72 know about these attributes and it complains about undefined attributes. 73 74 Bad solution 2: module Internal adds a ``Dict[str, Any]`` attribute to 75 the object. Module External stores its data in private keys of this dict. 76 This doesn't work well because retrieved values are untyped. 77 78 Good solution: module Internal adds a ``Store`` to the object. Module 79 External mints StoreKeys for its own keys. Module External stores and 80 retrieves its data using these keys. 81 """ 82 83 __slots__ = ("_store",) 84 85 def __init__(self) -> None: 86 self._store = {} # type: Dict[StoreKey[Any], object] 87 88 def __setitem__(self, key: StoreKey[T], value: T) -> None: 89 """Set a value for key.""" 90 self._store[key] = value 91 92 def __getitem__(self, key: StoreKey[T]) -> T: 93 """Get the value for key. 94 95 Raises ``KeyError`` if the key wasn't set before. 96 """ 97 return cast(T, self._store[key]) 98 99 def get(self, key: StoreKey[T], default: D) -> Union[T, D]: 100 """Get the value for key, or return default if the key wasn't set 101 before.""" 102 try: 103 return self[key] 104 except KeyError: 105 return default 106 107 def setdefault(self, key: StoreKey[T], default: T) -> T: 108 """Return the value of key if already set, otherwise set the value 109 of key to default and return default.""" 110 try: 111 return self[key] 112 except KeyError: 113 self[key] = default 114 return default 115 116 def __delitem__(self, key: StoreKey[T]) -> None: 117 """Delete the value for key. 118 119 Raises ``KeyError`` if the key wasn't set before. 120 """ 121 del self._store[key] 122 123 def __contains__(self, key: StoreKey[T]) -> bool: 124 """Return whether key was set.""" 125 return key in self._store 126