Collections and Data Structures Flashcards
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Read the first 7 Collections and Data Structures flashcards as text
What does `collections.OrderedDict` guarantee that a regular `dict` does NOT guarantee in Python versions before 3.7?
Answer: Insertion order preservation
OrderedDict preserves the insertion order of keys, which regular dicts did not guarantee before Python 3.7.
Which method of `collections.deque` removes and returns an element from the LEFT end?
Answer: popleft()
`popleft()` removes and returns the leftmost element of a deque in O(1) time.
What is the output of: `from collections import Counter; c = Counter('aabbc'); print(c.most_common(2))`?
Answer: [('a', 2), ('b', 2)]
`most_common(2)` returns the two most frequent elements as a list of (element, count) tuples; ties are ordered arbitrarily.
What happens when you access a missing key in a `collections.defaultdict(list)`?
Answer: Creates an entry with an empty list
defaultdict calls its factory function (list) to create a default value for missing keys, inserting an empty list.
Which named tuple method returns a new instance replacing specified fields with new values?
Answer: _replace()
`_replace()` returns a new named tuple instance with specified fields replaced by new values.
What is the time complexity of `collections.deque.appendleft()`?
Answer: O(1)
deque is implemented as a doubly-linked list, so appendleft() is O(1) unlike list.insert(0, x) which is O(n).
How do you create a named tuple class called `Point` with fields `x` and `y`?
Answer: Point = namedtuple('Point', ['x', 'y'])
`collections.namedtuple('Point', ['x', 'y'])` creates a new tuple subclass with named fields.