Python Dictionaries (Dictionary)

In this article, you will learn everything about Python dictionary; how to create, access, add, and delete elements, as well as various built-in methods.

What is a dictionary in Python?

Python dictionaries are an unordered collection of items. Other compound data types only have values as elements, while dictionaries have a key:value pair (key:value).

Dictionaries are optimized to retrieve values when the key is known.

How to create a dictionary?

Creating a dictionary is as simple as placing items in curly braces {} separated by commas.

An item has a key and a corresponding value, represented as a pair, key: value.

although the value can be any data type and can be repeated, the key must be an immutable type (with immutable elementsstring,numberortuple）and must be unique.

#　Empty dictionary
my_dict　=　{}
#　Dictionary with integer keys
my_dict　=　{1:　'apple',　2:　'ball'}
#　Mixed key dictionary
my_dict　=　{'name':　'John',　1:　[2,　4,　3}]
#　Using dict()
my_dict　=　dict({1:'apple',　2:'ball'})
#　Each item is a sequence of pairs
my_dict　=　dict([(1,'apple'),　(2,'ball')])

As you can see above, we can also use the built-in function dict() to create a dictionary.

How to access elements in the dictionary?

Although indexing is used with other container types to access values, dictionaries use keys. Keys can be used within brackets or with the get() method.

The difference in using get() is that if the key is not found, it will return None instead of KeyError.

When the program is run, the output is:

Jack
26

How to change or add elements in the dictionary?

Dictionaries are mutable. We can use the assignment operator to add new items or change the value of existing items.

If the key already exists, the value will be updated; otherwise, a new key:value pair will be added to the dictionary.

When the program is run, the output is:

{'name':　'Jack',　'age':　27}
{'name':　'Jack',　'age':　27,　'address':　'Downtown'}

How to remove or delete elements from the dictionary?

We can use the pop() method to remove a specific item from the dictionary. This method uses the provided key to remove the item and returns the value.

The method popitem() can be used to remove and return any item (key, value) from the dictionary. The clear() method can be used to delete all items at once.

We can also use the del keyword to delete a single item or the entire dictionary itself.

When the program is run, the output is:

16
{1:　1,　2:　4,　3:　9,　5:　25}
(1,　1)
{2:　4,　3:　9,　5:　25}
{2:　4,　3:　9}
{}

Python dictionary method

The following lists the methods available for the dictionary. Some of them have been used in the above examples.

Here are some examples of using these methods.

marks = {}.fromkeys(['Math','English','Science'], 0)
# Output: {'English': 0, 'Math': 0, 'Science': 0}
print(marks)
for item in marks.items():
　　　　print(item)
# Output: ['English', 'Math', 'Science']
list(sorted(marks.keys()))

Python Dictionary Comprehension

Dictionary comprehension is an elegant and concise way to create a new dictionary from an iterator in Python.

Dictionary comprehension includes an expression pair (key: value), followed by a for statement in curly braces {}.

This is an example of making a dictionary where each item is a pair of numbers and their squares.

squares = {x: x*x for x in range(6)}
# Output: {0: 0,　1:　1,　2:　4,　3:　9,　4:　16,　5:　25}
print(squares)

This code is equivalent to

squares = {}
for x in range(6):
　　　squares[x] = x*x

Dictionary comprehension can choose to include morefororif statement.

Optional if statements can filter items to form a new dictionary.

This is an example of a dictionary that only contains odd numbers.

odd_squares = {x: x*x for x in range(11) if x%2　==　1}
# Output: {1:　1,　3:　9,　5:　25,　7:　49,　9:　81}
print(odd_squares)

For more information, please visitPython dictionary comprehension.

Other Dictionary Operations

Dictionary Membership Testing

We can use the keyword to test if a key is in the dictionary with 'in'. Note that membership testing applies only to keys, not to values.

squares = {1:　1,　3:　9,　5:　25,　7:　49,　9:　81}
# Output: True
print(1　in squares)
# Output: True
print(2　not in squares)
# Membership test is performed on keys only, not on values
# Output: False
print(49　in squares)

Iterating over the dictionary

Using a for loop, we can iterate over each key in the dictionary.

squares = {1:　1,　3:　9,　5:　25,　7:　49,　9:　81}
for i in squares:
　　　　print(squares[i])

Built-in dictionary functions

Built-in functions such as all(), any(), len(), cmp(), sort(), etc. are usually used with dictionary to perform different tasks.

Here are some examples of using built-in functions to process dictionaries.

squares = {1:　1,　3:　9,　5:　25,　7:　49,　9:　81}
# Output:　5
print(len(squares))
# Output: [1,　3,　5,　7,　9]
print(sorted(squares))