Python Dictionary and Set Basics

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1. Dictionary

Python's built‑in dictionary type provides associative array functionality using a hash table. The built‑in len() function returns the number of key‑value pairs. The del statement removes entries, and the dictionary offers methods such as clear, copy, get, items, keys, update, and values.

Example code:

x = {1: "one", 2: "two"}

x["first"] = "one"

x[("Delorme", "Hahashen", 2000)] = (1, 2, 3)

list(x.keys())

[1, 2, 'first', ('Delorme', 'Hahashen', 2000)]

x[1]

'one'

x[2]

'two'

x.get(1, "not available")

'one'

x.get(2, "not available")

'two'

x.get(3, "not available")

'not available'

Dictionary keys must be immutable types such as numbers, strings, or tuples, while values can be any object, including mutable types like lists or other dictionaries. Accessing a non‑existent key raises KeyError; using get() or providing a default value can avoid this exception.

2. Set

A set is defined as an unordered collection of distinct hashable elements, which can serve as dictionary keys.

Characteristics: (1) composed of different elements, (2) unordered, (3) elements must be immutable.

Python's set type stores unique objects without associated values, similar to the keys of a dictionary.

Example code:

x = set([1, 2, 3, 1, 3, 5])

x

{1, 2, 3, 5}

1 in x

True

4 in x

False

Calling set() on a sequence removes duplicate members; the in keyword checks membership.

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