Gaddis Python 6e Chapter 09

Chapter Overview

• Title: R Python Sixth Edition Chapter 9

• Author: Tony Gaddis

• Focus: This chapter focuses on Dictionaries and Sets in Python.

Topics Covered

• Dictionaries

• Sets

• Serializing Objects

Dictionaries

Definition

• A dictionary is an object that stores a collection of data in key-value pairs.

• Keys must be immutable objects (e.g., strings, numbers, tuples).

• Example Format: dictionary = {key1:val1, key2:val2}.

Retrieving Values

• Dictionary elements are unsorted.

• Use the format dictionary[key] to retrieve values.

• If the key does not exist, a KeyError exception is raised.

• Use in operator to check for key existence.

Adding Elements

• Dictionaries are mutable.

• To add a new key-value pair, use dictionary[key] = value.

• If the key exists, its value is updated.

Deleting Elements

• Use del dictionary[key] to remove a key-value pair.

• If the key does not exist, a KeyError is raised.

Other Dictionary Features

Counting Elements

• Use len(dictionary) to count the number of elements in a dictionary.

Mixing Data Types

• Keys must be immutable, but values can be of any data type.

• Supports multiple immutable keys.

Creating Dictionaries

• For an empty dictionary: dict1 = {} or dict().

• Elements can be added during program execution.

Dictionary Methods

Common Methods

• clear(): Deletes all elements.

• get(key, default): Retrieves value without raising KeyError.

• items(): Returns all key-value pairs as tuples.

• keys(): Returns all keys.

• pop(key, default): Removes specified key and returns its value.

• popitem(): Returns and removes the last added key-value pair.

• values(): Returns all values as a sequence.

Merging and Updating Dictionaries

• Merge Operator (|): Combines two dictionaries, keeping the right-hand value for duplicate keys.

  • Example: dict3 = dict1 | dict2.

• Update Operator (|=): Merges another dictionary into the existing one.

  • Example: dict1 |= dict2.

Dictionary Comprehensions

• A concise way to create dictionaries from iterable elements.

• Example:

  squares = {item: item**2 for item in [1, 2, 3, 4]}

• Allows use of if-conditions to filter elements.

Sets

Definition

• A set is an unordered collection of unique items.

• Elements can be of various data types; duplicates are not stored.

Creating Sets

• Use set() for an empty set or set(iterable) for a filled one.

  • Example: Strings treated character-wise if passed as strings.

Operations on Sets

Basic Functions

• len(set): Counts elements in the set.

• add(value): Adds an element to the set.

• update(iterable): Adds multiple elements.

Deleting Elements

• remove(value): Removes a specified item. Raises KeyError if not found.

• discard(value): Removes the item without raising an error.

• clear(): Removes all elements.

Loops and Membership Testing

• Use for item in set: to iterate.

• Use in and not in operators to check membership.

Set Operations

Union, Intersection, and Difference

• Union: Combines two sets.

  • Method: set1.union(set2) or set1 | set2.

• Intersection: Common elements between two sets.

  • Method: set1.intersection(set2) or set1 & set2.

• Difference: Elements in one set but not the other.

  • Method: set1.difference(set2) or set1 - set2.

• Symmetric Difference: Unshared elements.

  • Method: set1.symmetric_difference(set2) or set1 ^ set2.

Subset and Superset Methods

• issubset(): Checks if all elements of A are in B.

• issuperset(): Checks if all elements of B are in A.

• Syntax: setA.issubset(setB) or setA >= setB.

Set Comprehensions

• Similar to list comprehensions, but use curly braces.

  • Example:

  set2 = {item for item in set1 if item < 10}

Serializing Objects

Definition

• Serialization: Converting an object into bytes for storage.

• Pickling: The process of serializing an object in Python.

Pickling an Object

• Steps:

  1. Import pickle module.

  2. Open a file in binary write mode.

  3. Use pickle.dump(object, file).

Unpickling an Object

• Steps:

  1. Import pickle module.

  2. Open a file in binary read mode.

  3. Use pickle.load(file) to retrieve the object.

Summary

• This chapter covered:

  • Dictionaries:

    • Creation, insertion, retrieval, deletion, and associated methods.

  • Sets:

    • Creation, addition, removal, operations, and comprehension.

  • Serializing Objects:

    • Pickling and unpickling objects.