A Python dictionary is a built-in mapping type that stores data in an unordered collection of key:value pairs. The keys must be unique and of an immutable type. Dictionaries are not sequence types like lists or tuples; therefore, their elements cannot be accessed via a numerical index. Instead, values are retrieved using their associated keys. As of Python 3.7, dictionaries preserve the order of insertion, but they are still fundamentally a mapping type, not an indexed sequence. Dictionaries are also mutable, meaning their contents (the key-value pairs) can be changed after creation.

A. Dictionaries are mutable. Items can be added, modified, or removed using methods like pop() or the del statement.

B. Dictionaries are mapping types, not sequence types. They are accessed by keys, not by numerical integer indices.

1. The Python Language Reference

Section 3.2. The standard type hierarchy: "Mapping types represent finite sets of objects indexed by arbitrary index keys. ... The only standard mapping type is the dictionary..." This source confirms dictionaries are mapping types indexed by keys

not numerical indices.

2. The Python Tutorial

Section 5.5. Dictionaries: "It is best to think of a dictionary as a set of key: value pairs

with the requirement that the keys are unique (within one dictionary). ... The del statement removes a key:value pair from a dictionary..." This source confirms the key/value pair structure (Option C) and their mutability (disproving Option A).

3. Python Glossary

"sequence" entry: "An iterable which supports efficient element access using integer indices... Built-in sequence types are list

str

tuple

and bytes." This definition explicitly excludes dictionaries

confirming they are not a sequence type (disproving Option B and supporting Option D).

4. MIT OpenCourseWare

6.0001 Introduction to Computer Science and Programming in Python

Fall 2016

Lecture 5 Notes

Section 5.2 Dictionaries (dicts): "A dictionary is a collection of key-value pairs. ... Dictionaries are mutable." This university courseware corroborates that dictionaries are composed of key-value pairs and are mutable.