3.8 Python Dictionaries: A Comprehensive Guide

Python dictionaries are a powerful and versatile built-in data structure used to store data as key-value pairs. They are ideal for situations where data is best represented through associations, such as:

• Names and phone numbers
• Product IDs and descriptions
• Employee IDs and their records

Key Features of Dictionaries

• Mutable: Dictionaries can be modified after creation (adding, updating, or removing items).
• Ordered (Python 3.7+): Items are stored in the order they were inserted. In versions prior to Python 3.7, dictionaries were unordered.
• Indexed using keys: Each value is associated with a unique key, which is used for accessing the value.
• Unique keys: Keys within a single dictionary must be unique. If a duplicate key is added, the latest value associated with that key will overwrite the previous one.
• Mapping type: Dictionaries establish a mapping between keys and their corresponding values.

Dictionary Syntax

A dictionary is defined using curly braces {} with key-value pairs separated by colons :. Each key-value pair is separated by a comma ,.

dictionary_name = {key1: value1, key2: value2, ...}

Example:

state_capitals = {
    "Andhra Pradesh": "Amaravati",
    "Punjab": "Chandigarh",
    "West Bengal": "Kolkata"
}

Creating Dictionaries

There are several ways to create dictionaries in Python:

1. Using Curly Braces {}

This is the most common and straightforward method.

my_dict = {"name": "Alice", "age": 25, "city": "New York"}

2. Using the dict() Constructor

The dict() constructor can be used with keyword arguments or an iterable of key-value pairs.

Using keyword arguments:

employee = dict(name="Ravi", age=30, department="HR")
print(employee)
# Output: {'name': 'Ravi', 'age': 30, 'department': 'HR'}

Using an iterable of key-value pairs (e.g., a list of tuples):

student = dict([("name", "Anita"), ("score", 89), ("subject", "Math")])
print(student)
# Output: {'name': 'Anita', 'score': 89, 'subject': 'Math'}

3. Using dict.fromkeys()

This method creates a new dictionary with keys from a sequence and a common value (which defaults to None).

keys = ["a", "b", "c"]
default_value = 0
new_dict = dict.fromkeys(keys, default_value)
print(new_dict)
# Output: {'a': 0, 'b': 0, 'c': 0}

Dictionary Key Rules

• Immutable Types: Dictionary keys must be of an immutable data type. This includes:
  • Strings (str)
  • Numbers (int, float, complex)
  • Tuples (tuple)
• No Mutable Keys: Mutable types like lists (list) or dictionaries (dict) cannot be used as keys because their contents can change, which would break the integrity of the key-value mapping.

Example of valid keys:

valid_dict = {
    "name": "Bob",
    101: "Employee ID",
    (1, 2): "Coordinates"
}

Example of invalid key (will raise TypeError):

# This will raise a TypeError because a list is mutable
# invalid_dict = {["apple", "banana"]: "fruits"}

• Duplicate Keys: If you attempt to assign a value to a key that already exists in the dictionary, the new value will overwrite the old one.

Example:

data = {"x": 10, "y": 20, "x": 30}
print(data)
# Output: {'x': 30, 'y': 20}

Accessing Dictionary Values

You can access the values associated with keys in two primary ways:

1. Using Square Brackets []

This method directly accesses the value using its key. If the key does not exist, it will raise a KeyError.

student = {"name": "Anita", "score": 89}
print(student["name"])
# Output: Anita

# print(student["grade"]) # This would raise a KeyError

2. Using the get() Method

The get() method is a safer way to access values as it returns None (or a specified default value) if the key is not found, instead of raising an error.

student = {"name": "Anita", "score": 89}

# Get the value for an existing key
print(student.get("score"))
# Output: 89

# Get the value for a non-existent key, with a default value
print(student.get("grade", "NA"))
# Output: NA

# Get the value for a non-existent key without a default (returns None)
print(student.get("age"))
# Output: None

Modifying a Dictionary

Dictionaries are mutable, meaning you can change their contents:

Update an Existing Value

Assign a new value to an existing key.

student = {"name": "Anita", "score": 89}
student["score"] = 95
print(student)
# Output: {'name': 'Anita', 'score': 95}

Add a New Key-Value Pair

Assign a value to a key that does not exist yet.

student = {"name": "Anita", "score": 95}
student["passed"] = True
print(student)
# Output: {'name': 'Anita', 'score': 95, 'passed': True}

Removing Elements from a Dictionary

You can remove items from a dictionary using several methods:

1. Using the del Keyword

The del keyword removes an item by its key. It raises a KeyError if the key doesn't exist.

student = {"name": "Anita", "score": 95, "passed": True}
del student["score"]
print(student)
# Output: {'name': 'Anita', 'passed': True}

2. Using pop()

The pop(key) method removes the item with the specified key and returns its value. It also raises a KeyError if the key is not found, unless a default value is provided.

student = {"name": "Anita", "passed": True, "subject": "Math"}
removed_value = student.pop("passed")
print(removed_value) # Output: True
print(student)       # Output: {'name': 'Anita', 'subject': 'Math'}

# Get value for a non-existent key with a default
default_removed = student.pop("grade", "Not found")
print(default_removed) # Output: Not found

3. Using popitem()

The popitem() method removes and returns the last inserted key-value pair as a tuple. In Python versions prior to 3.7, it removed and returned an arbitrary key-value pair. It raises a KeyError if the dictionary is empty.

student = {"name": "Anita", "subject": "Math", "grade": "A"}
last_item = student.popitem()
print(last_item) # Output: ('grade', 'A')
print(student)   # Output: {'name': 'Anita', 'subject': 'Math'}

4. Using clear()

The clear() method removes all items from the dictionary, making it empty.

student = {"name": "Anita", "subject": "Math"}
student.clear()
print(student)
# Output: {}

Iterating Through a Dictionary

You can iterate through a dictionary in various ways:

Iterating Through Keys (Default)

By default, iterating through a dictionary yields its keys.

employee = {"id": 101, "name": "Ravi", "role": "Manager"}
print("Iterating through keys:")
for key in employee:
    print(key)
# Output:
# id
# name
# role

Iterating Through Values

Use the .values() method to iterate only through the values.

employee = {"id": 101, "name": "Ravi", "role": "Manager"}
print("\nIterating through values:")
for value in employee.values():
    print(value)
# Output:
# 101
# Ravi
# Manager

Iterating Through Key-Value Pairs

Use the .items() method to iterate through both keys and values simultaneously as tuples.

employee = {"id": 101, "name": "Ravi", "role": "Manager"}
print("\nIterating through key-value pairs:")
for key, value in employee.items():
    print(f"{key} => {value}")
# Output:
# id => 101
# name => Ravi
# role => Manager

Dictionary Operators in Python

Python offers operators for dictionary manipulation, especially for combining dictionaries.

Accessing Items

The square bracket [] operator is used to access values.

d1 = {"a": 1, "b": 2}
print(d1["b"])  # Output: 2

Union (|) Operator (Python 3.9+)

The union operator combines two dictionaries. If keys overlap, the value from the right-hand dictionary is used.

d1 = {"a": 1, "b": 2}
d2 = {"c": 3, "d": 4}
combined = d1 | d2
print(combined)  # Output: {'a': 1, 'b': 2, 'c': 3, 'd': 4}

d3 = {"b": 20, "e": 5}
combined_overlap = d1 | d3
print(combined_overlap) # Output: {'a': 1, 'b': 20, 'e': 5}

Update (|=) Operator (Python 3.9+)

The update operator updates a dictionary with items from another dictionary. It's similar to the update() method but uses operator syntax.

d1 = {"a": 1, "b": 2}
d2 = {"c": 3, "d": 4}
d1 |= d2
print(d1)  # Output: {'a': 1, 'b': 2, 'c': 3, 'd': 4}

d1 = {"a": 1, "b": 2}
d3 = {"b": 20, "e": 5}
d1 |= d3
print(d1) # Output: {'a': 1, 'b': 20, 'e': 5}

Common Dictionary Methods

Here's a summary of frequently used dictionary methods:

Example using setdefault():

my_dict = {"name": "Alice"}
# Key "name" exists, returns its value
print(my_dict.setdefault("name", "Bob"))  # Output: Alice

# Key "age" does not exist, inserts it with default value and returns it
print(my_dict.setdefault("age", 25))      # Output: 25
print(my_dict)                           # Output: {'name': 'Alice', 'age': 25}

Built-in Functions for Dictionaries

Several built-in functions work directly with dictionaries:

Example:

info = {"brand": "Toyota", "year": 2020}

print(len(info))   # Output: 2
print(type(info))  # Output: <class 'dict'>
print(str(info))   # Output: {'brand': 'Toyota', 'year': 2020}

Interview Questions on Python Dictionaries

• What is a Python dictionary and how does it differ from a list?
• How do you create a dictionary in Python?
• What types can be used as dictionary keys? Why can’t lists be keys?
• How do you access values in a dictionary? Explain the difference between using [] and .get().
• How can you add or update items in a dictionary?
• What methods are used to remove items from a dictionary? How do pop() and popitem() differ?
• How can you iterate through keys, values, and key-value pairs in a dictionary?
• How do dictionary union (|) and update (|=) operators work in Python 3.9+?
• What does the setdefault() method do in a dictionary?
• How can you create a new dictionary from a sequence of keys with a common value?

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