🕵️ Stack Trace Lineup

mod_2 = lambda x: x % 2
from operator import attrgetter

class Item:
    def __init__(self, p): self.p = p

items = [Item(3), Item(1), Item(4)]
sorted_items = sorted(items, key=attrgetter("p"))
print([i.p for i in sorted_items])

# (no error)
[1, 3, 4]

import json
import math

data = {"value": math.nan}
print(json.dumps(data))

# (no error in Python's default — but the OUTPUT is non-standard JSON.
# Most parsers will reject. Strict mode: json.dumps(data, allow_nan=False) raises.)
{"value": NaN}

class Counter:
    count = 0
    def __init__(self):
        Counter.count += 1

a = Counter(); b = Counter(); c = Counter()
print(Counter.count, a.count, b.count, c.count)

# (no error — but the bug is conceptual: class var vs instance var)
3 3 3 3

def outer():
    x = "outer"
    def inner():
        x = "inner"
        return x
    inner()
    return x

print(outer())

# (no error — closures don't pollute the parent scope without nonlocal)
outer

import csv, io

buf = io.StringIO()
writer = csv.writer(buf)
writer.writerow(["name", "age", None])
print(buf.getvalue())

Traceback (most recent call last):
  File "lineup.py", line 5, in <module>
    writer.writerow(["name", "age", None])
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
csv.Error: cannot write None to CSV row