Definition and Creating Dictionaries
Dictionary is a data structure that stores key-value pairs. Think of it like a language dictionary where each word (key) has a meaning (value) associated with it.
Ways to Create Dictionaries
Python provides several ways to create dictionaries, from empty ones to those already containing data.
# Creating empty dictionariesa = {}b = dict()print("Empty dictionary a:", a)print("Empty dictionary b:", b)# Output: Empty dictionary a: {}# Output: Empty dictionary b: {}# Creating dictionary with dataphone = {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}print("Dictionary phone:", phone)# Output: Dictionary phone: {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}# Creating nested dictionaryperson = {1: {'name': 'ann', 'age': 23}, 2: {'name': 'bob', 'age': 21}}print("Nested dictionary:", person)# Output: Nested dictionary: {1: {'name': 'ann', 'age': 23}, 2: {'name': 'bob', 'age': 21}}# Creating dictionary with constructormonth = dict([(1, 'Jan'), (2, 'Feb'), (3, 'Mar')])mass = dict(Mercury=3.3e23, Venus=4.9e24, Earth=6.0e24)print("Dictionary month:", month)print("Dictionary mass:", mass)# Output: Dictionary month: {1: 'Jan', 2: 'Feb', 3: 'Mar'}# Output: Dictionary mass: {'Mercury': 3.3e23, 'Venus': 4.9e24, 'Earth': 6.0e24}Dictionary Syntax
Dictionaries are written with curly braces {} and contain a list of key-value pairs separated by commas. Each key and value pair is separated by a colon.
# Basic dictionary syntaxphone = {'ann': 110, 'bob': 991, 'cat': 112, 'don': 999}print(phone)# Output: {'ann': 110, 'bob': 991, 'cat': 112, 'don': 999}Dictionaries are enclosed in curly braces, contain a list of key-value pairs separated by commas, and each key-value pair is separated by a colon.
Dictionary as Container
Dictionary belongs to the container category, which are objects that can store collections of other items. Containers differ in how they organize and manipulate the stored items.
Dictionary has special characteristics as a container. Dictionary stores collections of items in the form of key-value pairs. Containers differ in how items are organized and manipulated.
Dictionary Characteristics
Dictionary has several important characteristics that distinguish it from other data structures in Python.
Insertion Order and Unique Keys
Since Python 3.6, dictionaries maintain the insertion order of elements. When you redefine the value of an existing key, only the latest value is stored without changing the key's insertion order.
# Insertion order is preservedphone = {'ann': 110, 'bob': 991, 'cat': 112, 'bob': 999}print(phone)# Output: {'ann': 110, 'bob': 999, 'cat': 112}# Key 'bob' stays in second position even though its value changedKeys in dictionaries are unique, only the most recently assigned value will persist. Redefining values does not change the insertion order of keys.
Accessing Dictionaries
Dictionary uses a key-based indexing system to access stored values.
Indexing with Keys
# Accessing values with keysphone = {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}# Retrieving valuesprint(phone['cat'])# Output: 112# Using variable as keyname = 'bob'print(phone[name])# Output: 991# Adding or changing valuesphone['bob'] = 999 # Change existing key valuephone['eve'] = 111 # Add new keyprint(phone)# Output: {'ann': 110, 'bob': 999, 'cat': 112, 'dan': 999, 'eve': 111}Dictionary uses keys to index values. You can retrieve values by specifying the key in square brackets, change values of existing keys, or add new key-value pairs.
Dictionary Iteration
Dictionary can be iterated in various ways depending on the information you want to access.
Simple For Loop
# Iterating dictionary keysphone = {'ann': 110, 'bob': 999, 'cat': 112, 'dan': 999}for key in phone: print(key, phone[key])# Output:# ann 110# bob 999# cat 112# dan 999When you iterate a dictionary directly, Python retrieves keys in their insertion order.
For Loop with items() Method
# Iterating key-value pairs with items()phone = {'ann': 110, 'bob': 999, 'cat': 112, 'dan': 999}for key, val in phone.items(): print(key, val)# Output:# ann 110# bob 999# cat 112# dan 999The items() method retrieves key-value pairs simultaneously, allowing you to access both in a single iteration.
Dictionary Methods
Dictionary provides several methods to access keys, values, and key-value pairs.
keys(), values(), and items() Methods
# Using dictionary methodsphone = {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}# Getting all keyskeys = phone.keys()print("Keys:", list(keys))# Output: Keys: ['ann', 'bob', 'cat', 'dan']# Getting all valuesvalues = phone.values()print("Values:", list(values))# Output: Values: [110, 991, 112, 999]# Getting key-value pairsitems = phone.items()print("Items:", list(items))# Output: Items: [('ann', 110), ('bob', 991), ('cat', 112), ('dan', 999)]These methods return dynamic view objects. View objects are not lists and cannot be indexed or assigned. Lists can be obtained through the list() constructor. View objects maintain reference to the parent dictionary and are efficient for iteration.
get() Method
The get() method provides a safe way to access dictionary values without the risk of errors when keys are not found.
Regular Indexing Problems
# Problem accessing non-existing keysphone = {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}try: print(phone['pat'])except KeyError as e: print("KeyError:", e)# Output: KeyError: 'pat'Indexing a dictionary with a non-existing key will result in a KeyError.
Solution with get() Method
# Using get() method for safe accessphone = {'ann': 110, 'bob': 991, 'cat': 112, 'dan': 999}# get() returns None if key doesn't existresult = phone.get('pat')print("Result get('pat'):", result)# Output: Result get('pat'): None# get() with default valueresult = phone.get('pat', -1)print("Result get('pat', -1):", result)# Output: Result get('pat', -1): -1# get() for existing keyresult = phone.get('ann')print("Result get('ann'):", result)# Output: Result get('ann'): 110The get() method retrieves the key's value if it exists, or returns a default value if the key is not found. This prevents the program from stopping due to errors and provides better control in handling incomplete data.