Python Dictionary

Page Contents

Basic Introduction

In Python, a dictionary is a collection of key-value pairs. It is a mutable, unordered collection that allows you to store data in the form of key-value pairs. Each key in a dictionary must be unique, and it is used to access its corresponding value. Keys must be immutable, meaning they cannot be changed after they are created. Values in a dictionary can be of any data type, including other dictionaries.

Starting from Python 3.7, dictionaries are guaranteed to maintain insertion order. This means that when you iterate over a dictionary, the order in which items were inserted will be preserved. This change was made to the Python language specification, and it’s also reflected in the built-in dict type.

Here is an example:

# Creating a dictionary
my_dict = {
    "name": "Shivaay",
    "age": 6,
    "city": "Delhi"
}

# Adding a new key-value pair
my_dict["gender"] = "Male"

# Iterating over key-value pairs
for key, value in my_dict.items():
    print(key, ":", value)

# Output
name : Shivaay
age : 6
city : Delhi
gender : Male

Before Python 3.7, dictionaries did not guarantee any specific order for their items. However, in Python 3.6, due to the implementation details of CPython, dictionaries were often observed to maintain insertion order in practice. But it was not guaranteed by the language specification. Starting from Python 3.7, this behavior became guaranteed and part of the language specification itself.

Read python Dictionary items

Reading from a Python dictionary involves accessing its values using keys. You can do this using square brackets [] with the key inside or using the get() method if you’re not sure whether the key exists or not. Here’s how you can read from a dictionary:

Accessing Values by Key:

You can access values from a dictionary directly by using the key inside square brackets []

person = {"name": "Shivaay", "location": "Delhi", "age": 6}

# Accessing values using keys
print("Name: ", person["name"])
print("Age: ", person["age"])
print("Location: ", person["location"])

# Output
Name:  Shivaay
Age:  6
Location:  Delhi

Using the `get()` Method:

The get() method allows you to specify a default value if the key is not found in the dictionary.

fruit = {
    "fruit": "Banana",
    "size": "Medium",
    "color": "Yellow"
}

fruit_name = fruit.get("fruit","NOT FOUND")
fruit_size = fruit.get("size","NOT FOUND")
fruit_color = fruit.get("color","NOT FOUND")
fruit_name_dummy = fruit.get("dummy","NOT FOUND")

print("fruit_name: ",fruit_name)
print("fruit_size: ",fruit_size)
print("fruit_color: ",fruit_color)
print("fruit_name_dummy: ",fruit_name_dummy)

# Output
fruit_name:  Banana
fruit_size:  Medium
fruit_color:  Yellow
fruit_name_dummy:  NOT FOUND

Iterating Over Keys:

You can iterate over the keys in the dictionary using a loop.

print("Iterating over Keys of dictionary.")
for key in fruit:
    print("Key: ", key)

# Output
Iterating over Keys of dictionary.
Key:  fruit
Key:  size
Key:  color

for key in fruit.keys():
    print("Key: ", key)

# Output
Key:  fruit
Key:  size
Key:  color

Iterating Over Key-Value Pairs:

You can iterate over the key-value pairs in the dictionary using the items() method:

person = {"name": "Proedu.co", "location": "Delhi", "age": 10}
print("Iterating Over Key-Value Pairs.")
for key, value in person.items():
    print("Key: ", key, ", Value: ", value)

# Output
Iterating Over Key-Value Pairs.
Key:  name , Value:  Proedu.co
Key:  location , Value:  Delhi
Key:  age , Value:  10

Checking for Key Existence:

You can check if a key exists in the dictionary using the in keyword:

fruit = {
    "fruit": "Banana",
    "size": "Medium",
    "color": "Yellow"
}

print("Checking for Key Existence.")
if "color" in fruit:
    print("Key 'Color' is present in fruit dictionary.")
else:
    print("Key 'Color' is NOT present in fruit dictionary.")

# Output
Checking for Key Existence.
Key 'Color' is present in fruit dictionary.

Iterating over dictionary values():

The values() method will return a list of all the values in the dictionary.

fruit = {
    "fruit": "Banana",
    "size": "Medium",
    "color": "Yellow"
}

x = fruit.values()
print("Values: ", x)

# Adding a new key-value pair in fruit dictionary.
fruit["price"] = 100
print("Values: ", x)

# Output
Values:  dict_values(['Banana', 'Medium', 'Yellow'])
Values:  dict_values(['Banana', 'Medium', 'Yellow', 100])

print("Looping Over dictionary values()")
for value in x:
    print("Value: ", value)

# Output
Looping Over dictionary values()
Value:  Banana
Value:  Medium
Value:  Yellow
Value:  100

Change python Dictionary values

To change the items in a Python dictionary, you can simply assign new values to the existing keys or by using update method.

Assign new values to existing key

person = {"name": "Shivaay", "location": "Delhi", "age": 6}
print("Dictionary Before: ",person)

# Changing value of an existing key "name".
person["name"] = "Shivaay Singh Chauhan"
print("Dictionary After: ",person)

# Output
Dictionary Before:  {'name': 'Shivaay', 'location': 'Delhi', 'age': 6}
Dictionary After:  {'name': 'Shivaay Singh Chauhan', 'location': 'Delhi', 'age': 6}

Using `update` method

You can also use the update() method to update multiple items in the dictionary at once by providing another dictionary containing the items to be updated or added:

person = {"name": "Prakash", "age": 30, "city": "Delhi"}
print("Dictionary Before: ",person)

# Changing value of an existing key "name".
person.update({"age": 35, "occupation": "Engineer"})
print("Dictionary After: ",person)

# Output
Dictionary Before:  {'name': 'Prakash', 'age': 30, 'city': 'Delhi'}
Dictionary After:  {'name': 'Prakash', 'age': 35, 'city': 'Delhi', 'occupation': 'Engineer'}

In the above example, we first change the value associated with the key “age” to 35. Then, we add a new key-value pair, “occupation”: “Engineer”, to the dictionary.

Adding items in Python dictionary

To add items to a Python dictionary, you can simply assign a value to a new key or use the update() method to add multiple items at once. Here’s how you can do it:

Using assignment

You can add a new key-value pair to a dictionary by assigning a value to a new key:

### Adding item in dictionary Using assignment.
# Original dictionary
my_dict = {"name": "John", "age": 30}

# Adding a new key-value pair
my_dict["city"] = "New York"
print(my_dict) 

# Output
{'name': 'John', 'age': 30, 'city': 'New York'}

Using `update()` method

You can use the update() method to add multiple items at once by providing another dictionary containing the items to be added:

my_dict = {"name": "John"}
print("my_dict before: ",my_dict)

# Add multiple items at once.
my_dict.update({"age": 30, "city": "New York"})
print("my_dict after update: ",my_dict)

# Output
my_dict before:  {'name': 'John'}
my_dict after update:  {'name': 'John', 'age': 30, 'city': 'New York'}

Remove items from Python dictionary

To remove items from a Python dictionary, you can use the del statement to delete a specific key-value pair, or you can use the pop() method to remove an item by its key and get its value. Here’s how you can do it:

Using the `del` Statement:

You can remove a specific key-value pair from a dictionary using the del statement:

person = {"name": "Shivaay", "location": "Delhi", "age": 6}
print("person before: ", person)

del person["age"]
print("person after: ", person)

# Output
person before:  {'name': 'Shivaay', 'location': 'Delhi', 'age': 6}
person after:  {'name': 'Shivaay', 'location': 'Delhi'}

In the above example, the key "age" and its corresponding value 6 are removed from the dictionary.

Using the `del` Statement to delete entire dictionary

The del keyword can also delete the dictionary completely:

person = {"name": "Shivaay", "location": "Delhi", "age": 6}
print("person before: ", person)

del person
print("person after: ", person)

# Output
NameError: name 'person' is not defined

The above example will throw ERROR because person dictionary does not exist.

Using `clear()` method

You can also clear the dictionary using clear() method. This deletes all elements in the dictionary.

person = {"name": "Shivaay", "location": "Delhi", "age": 6}
print("person before: ", person)

person.clear()
print("person after: ", person)

# Output
person before:  {'name': 'Shivaay', 'location': 'Delhi', 'age': 6}
person after:  {}

Using the `pop()` Method:

You can use the pop() method to remove an item by its key and get its value at the same time:

my_dict = {"name": "John", "age": 30, "city": "New York"}
print("my_dict before: ", my_dict)

# Removing a key-value pair using pop
removed_value = my_dict.pop("age")
print("Removed Value:", removed_value)  # Output: Removed Value: 30
print("my_dict after: ", my_dict)

# Output
my_dict before:  {'name': 'John', 'age': 30, 'city': 'New York'}
Removed Value: 30
my_dict after:  {'name': 'John', 'city': 'New York'}

In this example, the key "age" and its corresponding value 30 are removed from the dictionary, and the value 30 is assigned to the variable removed_value.

Both methods allow you to remove specific items from a dictionary. Use the del statement when you don’t need the value of the removed item, and use the pop() method when you want to get the value of the removed item.

Loop over Python dictionary

Looping over a Python dictionary allows you to iterate through its key-value pairs. There are several ways to iterate over a dictionary in Python, depending on whether you want to iterate over keys, values, or both. Here are the common methods:

Looping Over Keys:

You can iterate over the keys of a dictionary using a for loop:

person = {
    "name": "Prakash",
    "age": 35,
    "city": "New Delhi"
}

for key in person:
    print("key: ", key)

# Output
key:  name
key:  age
key:  city

for key in person.keys():
    print("key: ", key)

# Output
key:  name
key:  age
key:  city

Looping Over Values:

You can iterate over the values of a dictionary using the values() method:

person = {
    "name": "Prakash",
    "age": 35,
    "city": "New Delhi"
}

for value in person.values():
    print("Value: ", value)

# Output
Value:  Prakash
Value:  35
Value:  New Delhi

Looping Over Key-Value Pairs:

You can iterate over the key-value pairs of a dictionary using the items() method:

person = {
    "name": "Prakash",
    "age": 35,
    "city": "New Delhi"
}

for key, value in person.items():
    print("Key: ", key, " Value: ", value)

# Output
Key:  name  Value:  Prakash
Key:  age  Value:  35
Key:  city  Value:  New Delhi

Copy a dictionary in Python

In Python, you can create a copy of a dictionary using various methods. Here are some common approaches to copy a dictionary:

Using the `copy()` Method:

You can use the copy() method to create a shallow copy of the dictionary:

# Original dictionary
my_dict = {"name": "Proedu.co", "age": 6, "city": "New Delhi"}

# Creating a shallow copy using the copy() method
copied_dict = my_dict.copy()

# Modifying the original dictionary
my_dict["name"] = "Shivaay"

print("Original Dictionary:", my_dict)
print("Copied Dictionary:", copied_dict)

# Output
Original Dictionary: {'name': 'Shivaay', 'age': 6, 'city': 'New Delhi'}
Copied Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}

Note that copy() creates a shallow copy, so changes to nested objects within the dictionary will affect both the original and copied dictionaries.

Using the `dict()` Constructor:

You can use the dict() constructor with the original dictionary as an argument to create a copy. This approach also creates a shallow copy of the dictionary.

my_dict = {"name": "Proedu.co", "age": 6, "city": "New Delhi"}
copied_dict = dict(my_dict)
print("Original Dictionary:", my_dict)
print("Copied Dictionary:", copied_dict)

# Output
Original Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}
Copied Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}

Using Dictionary Comprehension:

You can use dictionary comprehension to create a copy of the dictionary. This approach is similar to using the dict() constructor.

my_dict = {"name": "Proedu.co", "age": 6, "city": "New Delhi"}
copied_dict = {key: value for key, value in my_dict.items()}
print("Original Dictionary:", my_dict)
print("Copied Dictionary:", copied_dict)

# Output
Original Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}
Copied Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}

Using `copy.deepcopy()` for Deep Copy:

If your dictionary contains nested objects and you need to create a deep copy (i.e., copy the nested objects as well), you can use the copy.deepcopy() function from the copy module:

import copy
my_dict = {"name": "Proedu.co", "age": 6, "city": "New Delhi"}
copied_dict = copy.deepcopy(my_dict)
print("Original Dictionary:", my_dict)
print("Copied Dictionary:", copied_dict)

# Output
Original Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}
Copied Dictionary: {'name': 'Proedu.co', 'age': 6, 'city': 'New Delhi'}

Nested dictionary in Python

In Python, a nested dictionary is a dictionary that contains another dictionary (or dictionaries) as its values. This allows you to represent hierarchical data structures where each level of the hierarchy is represented by a separate dictionary. Here’s an example of a nested dictionary.

nested_dict = {
    "person1": {
        "name": "Shivaay",
        "age": 6
    },
    "person2": {
        "name": "Proedu.co",
        "age": 5
    }
}

# Accessing nested dictionary values
print("Person1 Name: ", nested_dict["person1"]["name"])
print("Person2 Age: ", nested_dict["person2"]["age"])

# Output
Person1 Name:  Shivaay
Person2 Age:  5

In this example, nested_dict contains two key-value pairs where each value is another dictionary representing information about a person. You can access the nested dictionaries and their values using multiple levels of indexing.

You can also modify, add, or delete values within the nested dictionaries just like you would with any other dictionary.

nested_dict = {
    "person1": {
        "name": "Shivaay",
        "age": 6
    },
    "person2": {
        "name": "Proedu.co",
        "age": 5
    }
}

# Modifying age of Person1.
nested_dict["person1"]["age"] = 7

# Adding a new key-value pair to a nested dictionary
nested_dict["person2"]["gender"] = "Male"

# Deleting a key-value pair from a nested dictionary
del nested_dict["person1"]["age"]

print(nested_dict)

# Output
{'person1': {'name': 'Shivaay'}, 'person2': {'name': 'Proedu.co', 'age': 5, 'gender': 'Male'}}

Python Dictionary `fromkeys()` method

The fromkeys() method in Python dictionary is used to create a new dictionary with specified keys and optional default values. This method generates a dictionary where each key is taken from an iterable (like a list or tuple) and each key is associated with the same default value, which can be optionally specified.

Here’s the syntax of the fromkeys() method:

dictionary.fromkeys(iterable, value)

iterable: The iterable (e.g., list, tuple, etc.) containing keys that will be used to create the dictionary.

value (optional): The default value to associate with each key. If not provided, the default value will be None.

# Example1: Creating a dictionary with default values using fromkeys()
keys = ["name", "age", "city"]
default_value = "Unknown"
person = dict.fromkeys(keys, default_value)
print(person)

# Example2: Creating a dictionary with default values using fromkeys()
keys = ["name", "age", "city"]
person = dict.fromkeys(keys)
print(person)

# Output
{'name': 'Unknown', 'age': 'Unknown', 'city': 'Unknown'}
{'name': None, 'age': None, 'city': None}

In this example, the fromkeys() method creates a new dictionary my_dict with keys taken from the keys list and each key is associated with the default value "Unknown". If you don’t specify the default value, it defaults to None.

This method is useful when you want to initialize a dictionary with default values for a given set of keys. It’s particularly handy when you’re setting up a dictionary where you expect to fill in values later, or when you want to initialize a dictionary for use as a counter or lookup table.

Python Dictionary `setdefault()` Method

The setdefault() method in Python dictionary is used to retrieve the value of a specified key in the dictionary. If the key does not exist, it inserts the key with a specified value (defaulting to None if not provided) into the dictionary.

Here’s the syntax of the setdefault() method:

dictionary.setdefault(key, default_value)

key: The key to search for in the dictionary.

default_value (optional): The value to be inserted into the dictionary if the key is not found. If not provided, the default value will be None.

person = {"age": "6"}

# Using setdefault to add a new key-value pair.
name = person.setdefault("name", "Shivaay")
print("Person: ", person)
print("name: ", name)

# Using setdefault to retrieve value for an existing key.
age = person.setdefault("age",7)
print("age: ", age)

# Using setdefault() to add a key with a default value.
city = person.setdefault("city")
print("city: ", city)
print("Person: ", person)

# Output
Person:  {'age': '6', 'name': 'Shivaay'}
name:  Shivaay
age:  6
city:  None
Person:  {'age': '6', 'name': 'Shivaay', 'city': None}

Basic Introduction

Read python Dictionary items

Accessing Values by Key:

Using the get() Method:

Iterating Over Keys:

Iterating Over Key-Value Pairs:

Checking for Key Existence:

Iterating over dictionary values():

Change python Dictionary values

Assign new values to existing key

Using update method

Adding items in Python dictionary

Using assignment

Using update() method

Remove items from Python dictionary

Using the del Statement:

Using the del Statement to delete entire dictionary

Using clear() method

Using the pop() Method: