🕵️ Stack Trace Lineup

nums = [1, 2, 3]
for i, n in enumerate(nums):
    nums[i] = n * 2
print(nums)

# (no error — modifying a list mid-enumerate IS safe for index-based assignment)
[2, 4, 6]

def greet(name="World"):
    return f"Hello, {name}!"

print(greet())
print(greet("Ada"))

# (no error)
Hello, World!
Hello, Ada!

x = 5
x += "abc"
print(x)

Traceback (most recent call last):
  File "lineup.py", line 2, in <module>
    x += "abc"
TypeError: unsupported operand type(s) for +=: 'int' and 'str'

def append_to(item, target=[]):
    target.append(item)
    return target

append_to(1)
append_to(2)

Traceback (most recent call last):
  File "lineup.py", line 6, in <module>
    append_to(2)
    ~~~~~~~~~^^^
MutableDefaultError: default argument 'target' was modified across calls

d = {1: "a", 2: "b", 3: "c"}
print(list(d.keys()))

# (no error)
[1, 2, 3]