🕵️ Stack Trace Lineup

import contextvars

v = contextvars.ContextVar("v", default=0)
v.set(42)
print(v.get())

# (no error)
42

from concurrent.futures import ThreadPoolExecutor

def square(x): return x * x

with ThreadPoolExecutor(max_workers=3) as ex:
    results = list(ex.map(square, range(5)))
print(results)

# (no error)
[0, 1, 4, 9, 16]

import sys
class NoRepr:
    def __repr__(self):
        raise RuntimeError("nope")

n = NoRepr()
print(type(n).__name__)
# attempting to print n itself would fail

# (no error — type().__name__ doesn't call __repr__)
NoRepr

from dataclasses import dataclass

@dataclass(slots=True, frozen=True)
class Vec:
    x: float
    y: float

v = Vec(1.0, 2.0)
# attempted reassignment
try:
    v.x = 5.0
except Exception as e:
    print(type(e).__name__)

# (no error in main — the exception IS caught)
FrozenInstanceError

import weakref
class Node:
    pass

# No strong reference held to Node()
r = weakref.ref(Node())
print(r())

Traceback (most recent call last):
  File "lineup.py", line 6, in <module>
    r = weakref.ref(Node())
weakref.WeakReferenceError: target object has no strong references