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- from __future__ import annotations
- import collections.abc as c
- import inspect
- import typing as t
- from weakref import ref
- from weakref import WeakMethod
- T = t.TypeVar("T")
- class Symbol:
- """A constant symbol, nicer than ``object()``. Repeated calls return the
- same instance.
- >>> Symbol('foo') is Symbol('foo')
- True
- >>> Symbol('foo')
- foo
- """
- symbols: t.ClassVar[dict[str, Symbol]] = {}
- def __new__(cls, name: str) -> Symbol:
- if name in cls.symbols:
- return cls.symbols[name]
- obj = super().__new__(cls)
- cls.symbols[name] = obj
- return obj
- def __init__(self, name: str) -> None:
- self.name = name
- def __repr__(self) -> str:
- return self.name
- def __getnewargs__(self) -> tuple[t.Any, ...]:
- return (self.name,)
- def make_id(obj: object) -> c.Hashable:
- """Get a stable identifier for a receiver or sender, to be used as a dict
- key or in a set.
- """
- if inspect.ismethod(obj):
- # The id of a bound method is not stable, but the id of the unbound
- # function and instance are.
- return id(obj.__func__), id(obj.__self__)
- if isinstance(obj, (str, int)):
- # Instances with the same value always compare equal and have the same
- # hash, even if the id may change.
- return obj
- # Assume other types are not hashable but will always be the same instance.
- return id(obj)
- def make_ref(obj: T, callback: c.Callable[[ref[T]], None] | None = None) -> ref[T]:
- if inspect.ismethod(obj):
- return WeakMethod(obj, callback) # type: ignore[arg-type, return-value]
- return ref(obj, callback)
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