Find names of live instances of a Python class

find.py

import gc
from itertools import chain
from weakref import WeakKeyDictionary

from signals import Signal
import signals.other_file  # noqa


def find_instance_names(cls: type):
    results = WeakKeyDictionary()
    __marker = object()
    for instance in gc.get_referrers(cls):
        if isinstance(instance, cls):
            results[instance] = set(chain.from_iterable(
                get_instance_names(r, instance) for r in gc.get_referrers(instance)
                if isinstance(r, dict) and r.get('__marker') is not __marker))
    return results


def get_instance_names(module_dict, instance):
    base = module_dict.get('__name__') or '(unknown)'
    for name in keys_by_value(module_dict, instance):
        yield f"{base}.{name}"


def keys_by_value(d, needle):
    for k, v in d.items():
        if v is needle:
            yield k


print(f"done: {dict(find_instance_names(Signal))}!")

signals.__init__.py

# signals/__init__.py
class Signal:
    def __init__(self):
        pass


sig_a = Signal()

signals.other_file.py

# signals/other_file.py
from signals import Signal

sig_b = Signal()