Python Dictionaries: An Academic Guide

In Python, a dictionary is a fundamental data structure that stores data in key-value pairs. Each key is unique, and each key maps to a corresponding value. Dictionaries are highly versatile, allowing developers to store, retrieve, and manipulate data efficiently.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}

Understanding Python Dictionaries

Dictionaries in Python are:

Ordered (as of Python 3.7+)
Mutable (values can be changed or updated)
Non-duplicate (each key must be unique)

They are declared using curly braces {}, where data is structured as key: value pairs.

Example: Creating and Printing a Dictionary

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict)

Accessing Dictionary Items

To access a value, refer to its key directly:

print(thisdict["brand"])  # Output: Ford

Ordered vs Unordered Dictionaries

Before Python 3.7, dictionaries were unordered collections. From Python 3.7 onward, the insertion order is preserved, making dictionaries ordered by default. This change enables predictable item access and iteration based on order.

Mutability and Key Uniqueness

Dictionaries are mutable — items can be added, changed, or deleted. However, keys must remain unique. If a duplicate key is assigned, the latest value will overwrite the existing one.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964,
  "year": 2020  # Overwrites the previous 'year'
}
print(thisdict)

Determining Dictionary Length

To find out how many items a dictionary contains, use the built-in len() function:

print(len(thisdict))  # Output: 3

Data Types in Dictionary Items

Values within a dictionary can be of any data type including strings, integers, booleans, lists, and more:

thisdict = {
  "brand": "Ford",
  "electric": False,
  "year": 1964,
  "colors": ["red", "white", "blue"]
}

Type Identification

To check if a variable is a dictionary:

print(type(thisdict))  # Output: <class 'dict'>

Using the dict() Constructor

Python provides a constructor method dict() to create dictionaries:

thisdict = dict(name="John", age=36, country="Norway")
print(thisdict)

Overview of Python Collections

Python offers four primary collection data types:

List: Ordered and changeable. Allows duplicates.
Tuple: Ordered and immutable. Allows duplicates.
Set: Unordered, unchangeable*, and unindexed. No duplicates.
Dictionary: Ordered**, mutable. No duplicates.

*Set items are unchangeable, but elements can be added or removed.

**Dictionaries are ordered as of Python 3.7. Earlier versions treat them as unordered.

Choosing the Right Collection

Selecting the appropriate collection type is essential for ensuring both data integrity and performance. Dictionaries are particularly powerful when working with datasets that require fast lookups and structured key-value relationships.

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