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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)