🕵️ Stack Trace Lineup

import threading

lock = threading.Lock()
lock.acquire()
lock.acquire()  # second time, same thread

# (no error — but this BLOCKS forever; the demo is the bug)
# threading.Lock is NOT reentrant; use threading.RLock for that.

import struct
b = struct.pack("i", 2**40)
print(b)

Traceback (most recent call last):
  File "lineup.py", line 2, in <module>
    b = struct.pack("i", 2**40)
        ~~~~~~~~~~~^^^^^^^^^^^^^
struct.error: 'i' format requires -2147483648 <= number <= 2147483647

import weakref

class Tree:
    def __init__(self, val): self.val = val; self.children = []

root = Tree(1)
ref = weakref.ref(root)
root = None
print(ref())

# (no error — but the output is the lesson)
None

import asyncio

async def coro():
    await asyncio.sleep(0)
    return 42

result = coro()
print(result)

# (no error — but the printed value is NOT 42)
<coroutine object coro at 0x7f3e2b1d8e40>
sys:1: RuntimeWarning: coroutine 'coro' was never awaited

import pickle

x = lambda: 1
pickle.dumps(x)

Traceback (most recent call last):
  File "lineup.py", line 4, in <module>
    pickle.dumps(x)
    ~~~~~~~~~~~~^^^
  File "pickle.py", line 245, in dumps
    Pickler(file, protocol).dump(obj)
  [Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded