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Understanding Dictionaries in Python: Key-Value Pairs and Operations

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This document explores the concept of dictionaries in Python, which are optimized for efficient key-based searching through key-value pairs. It outlines important characteristics such as the immutability of keys, legal constraints, and the properties of mutable dictionaries. The document also discusses common dictionary operations, including insertion, deletion, and how to view keys, values, and items. Additionally, it highlights the behavior of view objects that reflect changes in the dictionary and provides examples of manipulating dictionaries in practice.

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Understanding Dictionaries in Python: Key-Value Pairs and Operations

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  1. Dictionaries Alexandra Stefan CSE1310 – University of Texas at Arlington

  2. Dictionary • A sequence of key-value pairs: {key:value, key:value,… key: value} • Optimized for efficient search based on the key • May not always print the elements in the same order • As you make modifications (insert/delete elements) the order of the pairs may change to enable fast searching

  3. Legal Keys • No duplicate keys • Cannot have multiple entries for a key • Keys must be immutable types • Strings • Integers • Tuples • Mix and match is okay Slide by Chris Conly

  4. Working with dictionaries • {key:value, key:value,… key: value} • What operations would you like? What operations do you know from the other collections?

  5. Working with dictionaries • They are mutable • Shallow copies • d = {key:value,key:value,… key: value} • Functions: • d[new_key] = new_value • del d[key] • d.items(), d.keys(), d.values() • list(d.keys()), sorted(d.keys()) • len(d): number of key-value pairs • in: verifies that a key is in the dictionary • min(__), max(__)

  6. keys(), values(), items() • They are called view objects • They reflect changes made to the dictionary • If you add or delete items in the dictionary, the views will reflect that • “If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond.” – Python Standard Library • To get a list of them use the list constructor: • list(d.keys()), list(d.values()), list(d.items()) • How connected are the lists and the views they are based on? • d = {1:[1,2]}, lv = list(d.values()), d[1][0] = 10, d, lv, lv[0][1] = 200, lv, d

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