Python Dictionary

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Python Dictionary

Summary: in this tutorial, you’ll learn about Python Dictionary that allows you to organize related information.

Introduction to the Python Dictionary type

A Python dictionary is a collection of key-value pairs where each key is associated with a value.

A value in the key-value pair can be a number, a string, a list, a tuple, or even another dictionary. In fact, you can use a value of any valid type in Python as the value in the key-value pair.

A key in the key-value pair must be immutable. In other words, the key cannot be changed, for example, a number, a string, a tuple, etc.

Python uses the curly braces {} to define a dictionary. Inside the curly braces, you can place zero, one, or many key-value pairs.

The following example defines an empty dictionary:

empty_dict = {}

Code language: Python (python)

Typically, you define an empty dictionary before a loop, either for loop or while loop. And inside the loop, you add key-value pairs to the dictionary.

To find the type of a dictionary, you use the type() function as follows:

empty_dict = {}

print(type(empty_dict))

Code language: Python (python)

Ouptut:

<class 'dict'>

Code language: Python (python)

The following example defines a dictionary with some key-value pairs:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

Code language: Python (python)

The person dictionary has five key-value pairs that represent the first name, last name, age, favorite colors, and active status.

Accessing values in a Dictionary

To access a value by key from a dictionary, you can use the square bracket notation or the get() method.

1) Using square bracket notation

To access a value associated with a key, you place the key inside square brackets:

dict[key]

Code language: Python (python)

The following shows how to get the values associated with the key first_name and last_name in the person dictionary:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}
print(person['first_name'])
print(person['last_name'])

Code language: Python (python)

Output:

John
Doe

Code language: Python (python)

2) Using the get() method

If you attempt to access a key that doesn’t exist, you’ll get an error. For example:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

ssn = person['ssn']

Code language: Python (python)

Error:

Traceback (most recent call last):
File "dictionary.py", line 15, in <module>
ssn = person['ssn']
KeyError: 'ssn'

Code language: Python (python)

To avoid this error, you can use the get() method of the dictionary:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

ssn = person.get('ssn')
print(ssn)

Code language: Python (python)

Ouput:

None

Code language: Python (python)

If the key doesn’t exist, the get() method returns None instead of throwing a KeyError. Note that None means no value exists.

The get() method also returns a default value when the key doesn’t exist by passing the default value to its second argument.

The following example returns the '000-00-0000' string if the ssn key doesn’t exist in the person dictionary:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

ssn = person.get('ssn', '000-00-0000')
print(ssn)

Code language: Python (python)

Output:

000-00-0000

Code language: Python (python)

Adding new key-value pairs

Since a dictionary has a dynamic structure, you can add new key-value pairs to it at any time.

To add a new key-value pair to a dictionary, you specify the name of the dictionary followed by the new key in square brackets along with the new value.

The following example adds a new key-value pair to the person dictionary:

person['gender'] = 'Famale'

Code language: Python (python)

Modifying values in a key-value pair

To modify a value associated with a key, you specify the dictionary name with the key in square brackets and the new value associated with the key:

dict[key] = new_value

Code language: Python (python)

The following example modifies the value associated with the age of the person dictionary:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

person['age'] = 26

print(person)

Code language: Python (python)

Output:

{'first_name': 'John', 'last_name': 'Doe', 'age': 26, 'favorite_colors': ['blue', 'green'], 'active': True}

Code language: Python (python)

Removing key-value pairs

To remove a key-value pair by a key, you use the del statement:

del dict[key]

Code language: Python (python)

In this syntax, you specify the dictionary name and the key that you want to remove.

The following example removes the key 'active' from the person dictionary:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

del person['active']
print(person)

Code language: Python (python)

Output:

{'first_name': 'John', 'last_name': 'Doe', 'age': 25, 'favorite_colors': ['blue', 'green']}

Code language: Python (python)

Looping through a dictionary

To examine a dictionary, you can use a for loop to iterate over its key-value pairs, or keys, or values.

Note that since Python 3.7, when you loop through a dictionary, you’ll get the key-value pairs in the same order that you insert them.

1) Looping all key-value pairs in a dictionary

Python dictionary provides a method called items() that returns an object which contains a list of key-value pairs as tuples in a list.

For example:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

print(person.items())

Code language: Python (python)

Output:

dict_items([('first_name', 'John'), ('last_name', 'Doe'), ('age', 25), ('favorite_colors', ['blue', 'green']), ('active', True)])

Code language: Python (python)

To iterate over all key-value pairs in a dictionary, you use a for loop with two variable key and value to unpack each tuple of the list:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

for key, value in person.items():
print(f"{key}: {value}")

Code language: Python (python)

Output:

first_name: John
last_name: Doe
age: 25
favorite_colors: ['blue', 'green']
active: True

Code language: Python (python)

Note that you can use any variable name in the for loop. They don’t have to be the key and value.

2) Looping through all the keys in a dictionary

Sometimes, you just want to loop through all keys in a dictionary. In this case, you can use a for loop with the keys() method.

The keys() method returns an object that contains a list of keys in the dictionary.

For example:


person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

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

Code language: Python (python)

Output:

first_name
last_name
age
favorite_colors
active

Code language: Python (python)

In fact, looping through all keys is the default behavior when looping through a dictionary. Therefore, you don’t need to use the keys() method.

The following code returns the same output like the one in the above example:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

for key in person:
print(key)

Code language: Python (python)

3) Looping through all the values in a dictionary

The values() method returns a list of values without any keys.

To loop through all the values in a dictionary, you use a for loop with the values() method:

person = {
'first_name': 'John',
'last_name': 'Doe',
'age': 25,
'favorite_colors': ['blue', 'green'],
'active': True
}

for value in person.values():
print(value)

Code language: Python (python)

Output:

John
Doe
25
['blue', 'green']
True

Code language: Python (python)

Summary

  • A Python dictionary is a collection of key-value pairs, where each key has an associated value.
  • Use square brackets or get() method to access a value by its key.
  • Use the del statement to remove a key-value pair by the key from the dictionary.
  • Use for loop to iterate over keys, values, key-value pairs in a dictionary.

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