3.10 Difference Between Lists and Dictionaries in Python

Python offers robust data structures for managing collections of data. Among the most fundamental and frequently used are Lists and Dictionaries, each possessing distinct characteristics and catering to different programming needs.

Core Difference

• List: An ordered collection of items, where each item is accessed by its numerical index (position), starting from 0. Lists are mutable, meaning their contents can be changed after creation.

• Dictionary: A collection of key-value pairs. Items are accessed not by their position, but by their associated unique keys. Dictionaries are also mutable.

Feature-wise Comparison

Examples

List Example

# A list of marks
marks = [85, 90, 78, 90]

# Accessing an element by index
print(marks[1])    # Output: 90

# Adding an element
marks.append(95)
print(marks)       # Output: [85, 90, 78, 90, 95]

# Lists can contain duplicate values
print(marks[1] == marks[3]) # Output: True

Dictionary Example

# A dictionary representing student information
student = {'name': 'Alice', 'age': 20, 'major': 'Computer Science'}

# Accessing a value by key
print(student['name'])     # Output: Alice

# Adding a new key-value pair
student['grade'] = 'A'
print(student)             # Output: {'name': 'Alice', 'age': 20, 'major': 'Computer Science', 'grade': 'A'}

# Updating an existing value
student['age'] = 21
print(student)             # Output: {'name': 'Alice', 'age': 21, 'major': 'Computer Science', 'grade': 'A'}

# Dictionary keys must be unique
# student['name'] = 'Bob' # This would update the existing 'name' entry, not add a new one.

Summary

• Use a list when:

  • The order and position of items are important.
  • You need to store a sequence of items.
  • Duplicates are acceptable.
  • You frequently access items by their numerical position.

• Use a dictionary when:

  • You need to associate labels (keys) with specific values.
  • Data needs to be accessed quickly and directly using its identifier.
  • You are representing structured records or configurations.
  • The uniqueness of identifiers (keys) is crucial.

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Possible Interview Questions

• What are the main differences between a list and a dictionary in Python?
• How does indexing work differently in lists and dictionaries?
• Can dictionary keys be mutable types? Why or why not?
• When would you prefer to use a list over a dictionary, and vice versa?
• How does Python maintain order in lists and dictionaries (especially considering historical versions)?
• Can lists contain duplicate values? What about dictionary keys?
• How do you iterate over elements in a list compared to iterating over keys or values in a dictionary?
• What are typical use cases for lists and dictionaries in real-world programming?
• How does the performance of accessing elements differ between lists and dictionaries, and why?
• Explain the syntax difference between lists and dictionaries in Python.