🕵️ Stack Trace Lineup

class Meta(type):
    def __new__(mcs, name, bases, ns):
        ns["created_at"] = "2026"
        return super().__new__(mcs, name, bases, ns)

class Foo(metaclass=Meta):
    pass

print(Foo.created_at)

# (no error — metaclass __new__ injects the attribute)
2026

class A: pass
class B: pass

a = A()
a.__class__ = B
print(type(a).__name__)

Traceback (most recent call last):
  File "lineup.py", line 5, in <module>
    a.__class__ = B
    ^^^^^^^^^^^
TypeError: __class__ assignment: 'A' object layout differs from 'B'

import asyncio

async def main():
    async with asyncio.TaskGroup() as tg:
        tg.create_task(asyncio.sleep(0.1))
        tg.create_task(asyncio.sleep(0.05))
    print("done")

asyncio.run(main())

# (no error — TaskGroup is the 3.11+ way to spawn structured concurrency)
done

from typing import Protocol

class Drawable(Protocol):
    def draw(self) -> None: ...

class Circle:
    def draw(self) -> None: print("circle")

def render(d: Drawable):
    d.draw()

render(Circle())

# (no error — structural typing via Protocol; no isinstance check needed at runtime)
circle

class Sentinel:
    _instance = None
    def __new__(cls):
        if cls._instance is None:
            cls._instance = super().__new__(cls)
        return cls._instance

print(Sentinel() is Sentinel())

# (no error — classic singleton via __new__)
True