130 lines
3.7 KiB
Python
130 lines
3.7 KiB
Python
from typing import Any
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from typing import cast
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from typing import Dict
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from typing import Generic
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from typing import TypeVar
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from typing import Union
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__all__ = ["Stash", "StashKey"]
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T = TypeVar("T")
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D = TypeVar("D")
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class StashKey(Generic[T]):
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"""``StashKey`` is an object used as a key to a ``Stash``.
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A ``StashKey`` is associated with the type ``T`` of the value of the key.
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A ``StashKey`` is unique and cannot conflict with another key.
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"""
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__slots__ = ()
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class Stash:
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r"""``Stash`` is a type-safe heterogeneous mutable mapping that
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allows keys and value types to be defined separately from
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where it (the ``Stash``) is created.
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Usually you will be given an object which has a ``Stash``:
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.. code-block:: python
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stash: Stash = some_object.stash
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If a module wants to store data in this Stash, it creates ``StashKey``\s
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for its keys (at the module level):
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.. code-block:: python
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some_str_key = StashKey[str]()
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some_bool_key = StashKey[bool]()
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To store information:
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.. code-block:: python
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# Value type must match the key.
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stash[some_str_key] = "value"
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stash[some_bool_key] = True
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To retrieve the information:
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.. code-block:: python
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# The static type of some_str is str.
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some_str = stash[some_str_key]
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# The static type of some_bool is bool.
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some_bool = stash[some_bool_key]
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Why use this?
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-------------
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Problem: module Internal defines an object. Module External, which
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module Internal doesn't know about, receives the object and wants to
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attach information to it, to be retrieved later given the object.
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Bad solution 1: Module External assigns private attributes directly on
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the object. This doesn't work well because the type checker doesn't
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know about these attributes and it complains about undefined attributes.
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Bad solution 2: module Internal adds a ``Dict[str, Any]`` attribute to
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the object. Module External stores its data in private keys of this dict.
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This doesn't work well because retrieved values are untyped.
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Good solution: module Internal adds a ``Stash`` to the object. Module
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External mints StashKeys for its own keys. Module External stores and
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retrieves its data using these keys.
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"""
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__slots__ = ("_storage",)
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def __init__(self) -> None:
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self._storage: Dict[StashKey[Any], object] = {}
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def __setitem__(self, key: StashKey[T], value: T) -> None:
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"""Set a value for key."""
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self._storage[key] = value
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def __getitem__(self, key: StashKey[T]) -> T:
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"""Get the value for key.
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Raises ``KeyError`` if the key wasn't set before.
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"""
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return cast(T, self._storage[key])
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def get(self, key: StashKey[T], default: D) -> Union[T, D]:
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"""Get the value for key, or return default if the key wasn't set
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before."""
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try:
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return self[key]
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except KeyError:
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return default
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def setdefault(self, key: StashKey[T], default: T) -> T:
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"""Return the value of key if already set, otherwise set the value
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of key to default and return default."""
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try:
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return self[key]
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except KeyError:
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self[key] = default
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return default
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def __delitem__(self, key: StashKey[T]) -> None:
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"""Delete the value for key.
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Raises ``KeyError`` if the key wasn't set before.
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"""
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del self._storage[key]
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def __contains__(self, key: StashKey[T]) -> bool:
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"""Return whether key was set."""
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return key in self._storage
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def __len__(self) -> int:
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"""Return how many items exist in the stash."""
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return len(self._storage)
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