Builtins Hard

What does this code print?

print(round(0.5), round(1.5), round(2.5))

  • 1 2 3

  • 0 2 2

  • 0 1 2

  • 1 2 2

Answer

Banker's rounding (round half to even) applies to every .5 case: 0.5 -> 0, 1.5 -> 2, 2.5 -> 2. Each result is the nearest even integer.

What does this code print?

print(0.1 + 0.2)

  • 0.3

  • 0.30000000000000001

  • 0.29999999999999999

  • 0.30000000000000004

Answer

Binary floating point cannot represent 0.1 or 0.2 exactly, so the sum is slightly above 0.3 and repr shows the shortest string that round-trips: 0.30000000000000004.

What does this code print?

print(hash(True) == hash(1))

  • True

  • False

  • TypeError

  • 1

Answer

True == 1 and equal objects must hash equally, so hash(True) and hash(1) are both 1.

What does this code print?

print({True: 'a', 1: 'b'})

  • {True: 'a', 1: 'b'}

  • {1: 'b'}

  • {True: 'b'}

  • {True: 'a'}

Answer

Because True == 1 and they hash the same, the dict treats them as one key. The first key inserted (True) is kept, but the later assignment overwrites its value with 'b'.

What does this code print?

print(float('nan') == float('nan'))

  • True

  • False

  • nan

  • ValueError

Answer

IEEE-754 defines NaN as unequal to everything, including itself, so the comparison is False.

What does this code print?

n = float('nan')
print(n in [n])

  • True

  • False

  • ValueError

  • nan

Answer

Even though nan != nan, membership tests short-circuit on identity first (x is element or x == element). The list holds the same object, so the identity check succeeds and in returns True.

What does this code print?

print(round(-0.5), round(-1.5), round(-2.5))

  • -1 -2 -3

  • 0 -1 -2

  • -1 -1 -2

  • 0 -2 -2

Answer

Banker's rounding also works on negatives: -0.5 -> 0, -1.5 -> -2, -2.5 -> -2 — each snapped to the nearest even integer.

What does this code print?

print(round(2.675, 2))

  • 2.68

  • 2.7

  • 2.67

  • 2.675

Answer

2.675 cannot be stored exactly; the nearest float is slightly below 2.675, so rounding to two places gives 2.67 rather than the "textbook" 2.68. Banker's rounding is irrelevant here — the stored value simply is not a true half.

What does this code print?

print(hash(-1))

  • -1

  • -2

  • 1

  • 0

Answer

In CPython, -1 is reserved internally to signal an error from __hash__, so the hash of the integer -1 is remapped to -2.

What does this code print?

print(sorted([True, False, 0, 1]))

  • [False, 0, True, 1]

  • [False, 0, 1, True]

  • [0, False, 1, True]

  • [True, 1, False, 0]

Answer

Since False == 0 and True == 1, sorting compares equal values and Python's sort is stable: it preserves the input order within each equal group. False precedes 0 and True precedes 1 exactly as they appeared.

What does this code print?

print({1, True, 1.0})

  • {1, True, 1.0}

  • {True}

  • {1.0}

  • {1}

Answer

1, True and 1.0 are all equal and hash the same, so the set keeps only the first one inserted — the integer 1.

What does this code print?

print(all([]))

  • True

  • False

  • None

  • ValueError

Answer

all is vacuously true on an empty iterable: there is no element that fails, so it returns True. (any([]) is the mirror image and returns False.)

What does this code print?

print(bin(10))

  • 1010

  • 0b1010

  • '0b1010'

  • b'1010'

Answer

bin returns a string with a 0b prefix. print shows the string content without quotes, so the output is 0b1010.

What does this code print?

print(len("café".encode("utf-8")))

  • 3

  • 6

  • 4

  • 5

Answer

len on the string is 4 code points, but once encoded to UTF-8 the é takes two bytes, so the bytes object has length 5.

What does this code print?

print(int("0x1f", 0))

  • ValueError

  • 31

  • 0

  • 0x1f

Answer

Base 0 tells int to auto-detect the base from the literal prefix. The 0x marks hexadecimal, so 1f parses to 31.

What does this code print?

print(bytes([104, 105]))

  • [104, 105]

  • 'hi'

  • b'hi'

  • b'104105'

Answer

bytes from a list of integers treats each as a byte value; 104 and 105 are the ASCII codes for h and i, giving b'hi'.

What does this code print?

a = 256
b = 256
print(a is b)

  • True

  • False

  • 256

  • None

Answer

CPython caches small integers from -5 to 256, so both names point at the same cached object and is is True. This is a CPython implementation detail — never rely on is for value equality.

What does this code print?

print([float('nan')].index(float('nan')))

  • 0

  • -1

  • None

  • ValueError

Answer

Unlike in on a stored object, list.index builds a fresh NaN to search for. The two NaN objects are different, so the identity shortcut fails, == is False, and no match is found — index raises ValueError.

What does this code print?

print(max([(1, "a"), (1, "b")], key=lambda t: t[0]))

  • (1, 'b')

  • (1, 'a')

  • 1

  • (1, 'ab')

Answer

Both items have the same key (1). On ties, max returns the first maximal element encountered, so (1, 'a') wins. (min follows the same first-wins rule.)

What does this code print?

print(ord("😀"))

  • TypeError

  • 2

  • 128512

  • 55357

Answer

ord accepts any single Unicode code point, including astral-plane emoji. 😀 is U+1F600, whose decimal value is 128512 — Python strings are not UTF-16, so there is no surrogate pair to trip over.