Library Easy

What does this code print?

from collections import Counter
print(Counter("mississippi").most_common(1))

  • [('i', 4)]

  • [('s', 4)]

  • [('i', 5)]

  • [('m', 1)]

Answer

Counter.most_common(1) returns the single most frequent element as a (value, count) pair. Both "i" and "s" appear 4 times, and ties are broken by first insertion order, so "i" wins.

What does this code print?

from collections import deque
d = deque([1, 2, 3])
d.appendleft(0)
print(d)

  • deque([0, 1, 2, 3])

  • deque([1, 2, 3, 0])

  • deque([0, 3, 2, 1])

  • [0, 1, 2, 3]

Answer

deque.appendleft adds an item at the front in O(1), giving deque([0, 1, 2, 3]). The repr of a deque includes the deque(...) wrapper.

What does this code print?

import json
print(json.dumps({"a": 1, "b": 2}))

  • {"a": 1, "b": 2}

  • {'a': 1, 'b': 2}

  • {"a":1,"b":2}

  • {a: 1, b: 2}

Answer

json.dumps always uses double quotes (valid JSON) and, by default, a space after each : and ,. Python's single-quoted repr is not JSON.

What does this code print?

import math
print(math.floor(3.7))

  • 3

  • 4

  • 3.0

  • 3.7

Answer

math.floor rounds toward negative infinity and returns an int, so math.floor(3.7) is 3 (not 3.0).

What does this code print?

import os.path
print(os.path.join("home", "user", "file.txt"))

  • home/user/file.txt

  • home\user\file.txt

  • home user file.txt

  • ['home', 'user', 'file.txt']

Answer

os.path.join inserts the platform separator between parts; on POSIX systems that is /, producing home/user/file.txt.

What does this code print?

from collections import namedtuple
Point = namedtuple("Point", ["x", "y"])
p = Point(1, 2)
print(p.x + p.y)

  • 3

  • 12

  • (1, 2)

  • Point(x=1, y=2)

Answer

A namedtuple gives fields named access. p.x is 1 and p.y is 2, so p.x + p.y is 3.

What does this code print?

from collections import defaultdict
d = defaultdict(int)
d["a"] += 1
d["a"] += 1
print(d["a"])

  • 2

  • 1

  • 0

  • raises KeyError

Answer

defaultdict(int) supplies 0 for a missing key, so the first += 1 starts from 0. After two increments d["a"] is 2 and no KeyError is raised.

What does this code print?

from itertools import chain
print(list(chain([1, 2], [3, 4])))

  • [1, 2, 3, 4]

  • [[1, 2], [3, 4]]

  • [1, 3, 2, 4]

  • [(1, 3), (2, 4)]

Answer

itertools.chain flattens one level: it yields the items of each iterable in turn, giving [1, 2, 3, 4].

What does this code print?

from functools import reduce
print(reduce(lambda a, b: a + b, [1, 2, 3, 4]))

  • 10

  • 24

  • [1, 2, 3, 4]

  • 1234

Answer

reduce folds the function across the list left to right: ((1+2)+3)+4 equals 10.

What does this code print?

from datetime import date, timedelta
print(date(2020, 1, 1) + timedelta(days=31))

  • 2020-02-01

  • 2020-01-32

  • 2020-02-31

  • 2020-01-31

Answer

January has 31 days, so adding a 31-day timedelta to Jan 1 lands on Feb 1, printed in ISO format as 2020-02-01.

What does this code print?

import re
print(re.findall(r"\d+", "a1b22c333"))

  • ['1', '22', '333']

  • ['1', '2', '3']

  • [1, 22, 333]

  • ['123']

Answer

\d+ matches maximal runs of digits, and re.findall returns them as a list of strings: ['1', '22', '333'].

What does this code print?

import string
print(string.ascii_lowercase[:5])

  • abcde

  • ABCDE

  • ['a', 'b', 'c', 'd', 'e']

  • abcdef

Answer

string.ascii_lowercase is the string "abcdefghijklmnopqrstuvwxyz"; slicing [:5] returns its first five characters, "abcde".

What does this code print?

import math
print(math.factorial(5))

  • 120

  • 25

  • 15

  • 720

Answer

math.factorial(5) is 5 * 4 * 3 * 2 * 1 which equals 120.

What does this code print?

from collections import Counter
c = Counter(a=2, b=1)
print(sorted(c.elements()))

  • ['a', 'a', 'b']

  • ['a', 'b']

  • ['a', 'b', 'b']

  • [('a', 2), ('b', 1)]

Answer

Counter.elements() yields each element as many times as its count, so it produces a, a, b; sorted, that is ['a', 'a', 'b'].

What does this code print?

from fractions import Fraction
print(Fraction(1, 2) + Fraction(1, 3))

  • 5/6

  • 2/5

  • 1/6

  • 0.8333333333333334

Answer

Fraction does exact rational arithmetic: 1/2 + 1/3 = 3/6 + 2/6 = 5/6, automatically reduced to lowest terms.

What does this code print?

from decimal import Decimal
print(Decimal("0.1") + Decimal("0.2"))

  • 0.3

  • 0.30000000000000004

  • 0.30

  • 0.6

Answer

Decimal built from strings avoids binary floating-point error, so Decimal("0.1") + Decimal("0.2") is exactly 0.3 (unlike 0.1 + 0.2 with floats).

What does this code print?

from enum import Enum
class Color(Enum):
    RED = 1
    GREEN = 2
print(Color.RED.value)

  • 1

  • Color.RED

  • RED

  • 0

Answer

.value returns the value assigned to an enum member, which is 1 for Color.RED. The member's .name would be 'RED'.

What does this code print?

from dataclasses import dataclass
@dataclass
class P:
    x: int
    y: int
print(P(1, 2))

  • P(x=1, y=2)

  • P(1, 2)

  • <__main__.P object at 0x...>

  • {'x': 1, 'y': 2}

Answer

@dataclass auto-generates a __repr__ showing the class name and each field as name=value, producing P(x=1, y=2).

What does this code print?

from pathlib import Path
print(Path("archive.tar.gz").suffix)

  • .gz

  • .tar.gz

  • .tar

  • gz

Answer

.suffix returns only the last extension, including the dot: .gz. Use .suffixes to get ['.tar', '.gz'].

What does this code print?

a = [1, 2, 3]
b = a.copy()
b.append(4)
print(a)

  • [1, 2, 3]

  • [1, 2, 3, 4]

  • [4, 1, 2, 3]

  • [1, 2, 3, 4, 4]

Answer

list.copy() makes a new (shallow) list, so appending to b does not affect a. a stays [1, 2, 3].