Library Medium

What does this code print?

from collections import defaultdict
d = defaultdict(list)
for i in range(5):
    d[i % 2].append(i)
print(dict(d))

  • {0: [0, 2, 4], 1: [1, 3]}

  • {0: [0, 1, 2, 3, 4], 1: []}

  • {0: 3, 1: 2}

  • raises KeyError

Answer

defaultdict(list) auto-creates an empty list the first time each key is touched, so .append never raises. Even numbers group under key 0 and odd numbers under key 1.

What does this code print?

from itertools import chain
print(list(chain.from_iterable([[1, 2], [3], [4, 5]])))

  • [1, 2, 3, 4, 5]

  • [[1, 2], [3], [4, 5]]

  • [1, 3, 4]

  • [[1, 2, 3, 4, 5]]

Answer

chain.from_iterable takes a single iterable of iterables and concatenates them, flattening one level to [1, 2, 3, 4, 5].

What does this code print?

import json
data = json.loads(json.dumps((1, 2, 3)))
print(type(data).__name__, data)

  • list [1, 2, 3]

  • tuple (1, 2, 3)

  • str (1, 2, 3)

  • list [1, 2, 3, 3]

Answer

JSON has no tuple type: json.dumps serializes a tuple as an array, and json.loads reads any array back as a list. The round trip loses the tuple type.

What does this code print?

import re
print(re.findall(r"(\w)(\d)", "a1b2c3"))

  • [('a', '1'), ('b', '2'), ('c', '3')]

  • ['a1', 'b2', 'c3']

  • ['a', '1', 'b', '2', 'c', '3']

  • [('a1'), ('b2'), ('c3')]

Answer

When a pattern has more than one capturing group, re.findall returns a list of tuples, one per match, with each group's text. Here each match is a (letter, digit) pair.

What does this code print?

from functools import reduce
print(reduce(lambda a, b: a + b, [], 10))

  • 10

  • raises TypeError

  • 0

  • None

Answer

With an explicit initializer, reduce returns that initializer unchanged when the iterable is empty; the function is never called. Without an initializer, an empty iterable would raise TypeError.

What does this code print?

from collections import Counter
c = Counter(a=3, b=1)
c.subtract(Counter(a=1, b=2))
print(dict(c))

  • {'a': 2, 'b': -1}

  • {'a': 2}

  • {'a': 2, 'b': 0}

  • {'a': 4, 'b': 3}

Answer

The subtract method decrements counts in place and, unlike the - operator, keeps zero and negative results. b becomes 1 - 2 = -1.

What does this code print?

from itertools import product
print(list(product([0, 1], repeat=2)))

  • [(0, 0), (0, 1), (1, 0), (1, 1)]

  • [(0, 1), (1, 0)]

  • [(0, 0), (1, 1)]

  • [0, 1, 0, 1]

Answer

product([0, 1], repeat=2) is the Cartesian product of the set with itself, enumerating all four ordered pairs with the rightmost element varying fastest.

What does this code print?

from itertools import combinations
print(list(combinations("ABC", 2)))

  • [('A', 'B'), ('A', 'C'), ('B', 'C')]

  • [('A', 'B'), ('B', 'A'), ('A', 'C')]

  • [('A', 'A'), ('B', 'B'), ('C', 'C')]

  • [('A', 'B'), ('A', 'C'), ('B', 'C'), ('C', 'B')]

Answer

combinations yields unordered selections without repetition, in input order, so pairs like ('B', 'A') never appear. There are three 2-element combinations of ABC.

What does this code print?

from functools import partial
f = partial(int, base=2)
print(f("101"))

  • 5

  • 101

  • 3

  • raises ValueError

Answer

partial(int, base=2) pre-binds the base keyword. Calling f("101") parses "101" as binary, which equals 5.

What does this code print?

from datetime import date
print(date(2020, 1, 1).weekday())

  • 2

  • 3

  • 0

  • 1

Answer

weekday() numbers days with Monday as 0 through Sunday as 6. Jan 1, 2020 was a Wednesday, so it returns 2. (isoweekday() would give 3.)

What does this code print?

from functools import lru_cache
@lru_cache
def sq(n):
    return n * n
sq(3); sq(3); sq(4)
print(sq.cache_info().hits, sq.cache_info().misses)

  • 1 2

  • 0 3

  • 2 1

  • 3 0

Answer

The first sq(3) and sq(4) are cache misses; the repeated sq(3) is a hit. So cache_info() reports 1 hit and 2 misses.

What does this code print?

from collections import OrderedDict
d = OrderedDict([("a", 1), ("b", 2), ("c", 3)])
d.move_to_end("a")
print(list(d))

  • ['b', 'c', 'a']

  • ['a', 'b', 'c']

  • ['c', 'b', 'a']

  • ['a', 'c', 'b']

Answer

move_to_end("a") moves key "a" to the last position (the default end), leaving b and c in their relative order, so iteration yields ['b', 'c', 'a'].

What does this code print?

import re
print(re.sub(r"\s+", "_", "hello   world  foo"))

  • hello_world_foo

  • hello___world__foo

  • hello world foo

  • hello_ world_ foo

Answer

\s+ matches each maximal run of whitespace as one match, and re.sub replaces each with a single _, collapsing the gaps to hello_world_foo.

What does this code print?

import math
print(math.isclose(0.1 + 0.2, 0.3))

  • True

  • False

  • 0.30000000000000004

  • raises ValueError

Answer

0.1 + 0.2 is 0.30000000000000004 due to binary floating point, but math.isclose compares within a relative tolerance and reports True. Plain == would give False.

What does this code print?

from collections import deque
d = deque(maxlen=3)
for i in range(5):
    d.append(i)
print(d)

  • deque([2, 3, 4], maxlen=3)

  • deque([0, 1, 2], maxlen=3)

  • deque([0, 1, 2, 3, 4], maxlen=3)

  • raises IndexError

Answer

A bounded deque silently drops items from the opposite end when it is full. Appending on the right past maxlen=3 evicts from the left, keeping the last three values [2, 3, 4].

What does this code print?

import copy
a = [[1, 2], [3, 4]]
b = copy.copy(a)
b[0].append(99)
print(a)

  • [[1, 2, 99], [3, 4]]

  • [[1, 2], [3, 4]]

  • [[1, 2, 99], [3, 4, 99]]

  • [[1, 2], [3, 4, 99]]

Answer

copy.copy is shallow: b is a new outer list but its elements are the same inner lists as a. Mutating b[0] mutates the shared inner list, so a changes too.

What does this code print?

from itertools import count, islice
print(list(islice(count(10, 2), 4)))

  • [10, 12, 14, 16]

  • [10, 11, 12, 13]

  • [10, 12, 14, 16, 18]

  • [2, 4, 6, 8]

Answer

count(10, 2) is an infinite counter starting at 10 with step 2, and islice(..., 4) takes its first four values: [10, 12, 14, 16].

What does this code print?

from datetime import timedelta
print(timedelta(hours=1, minutes=30).total_seconds())

  • 5400.0

  • 90.0

  • 5400

  • 1.5

Answer

total_seconds returns the whole duration as a float number of seconds: 1 h 30 min is 5400.0 seconds.

What does this code print?

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

  • {"a": 2, "b": 1}

  • {"b": 1, "a": 2}

  • {"a": 1, "b": 2}

  • {'a': 2, 'b': 1}

Answer

sort_keys=True orders keys alphabetically in the output regardless of insertion order, but the values stay attached to their own keys: a keeps 2 and b keeps 1.

What does this code print?

from enum import Enum, auto
class Color(Enum):
    RED = auto()
    GREEN = auto()
    BLUE = auto()
print(Color.BLUE.value)

  • 3

  • 0

  • 2

  • BLUE

Answer

auto() assigns increasing integers starting at 1 by default, so RED=1, GREEN=2, BLUE=3. Color.BLUE.value is 3.