Python 3 Dictionary

Python 3 Dictionary
A dictionary is another mutable container model that can store objects of any type.

Each key=>value pair in a dictionary is separated by a colon (:), and each pair is separated by a comma (,). The entire dictionary is enclosed in curly braces ({}) and has the following format:

d = {key1 : value1, key2 : value2 }

Keys must be unique, but values do not need to be.

Values can be of any data type, but keys must be immutable, such as strings, numbers, or tuples.

A simple dictionary example:

dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}

You can also create a dictionary like this:

dict1 = { 'abc': 456 }
dict2 = { 'abc': 123, 98.6: 37 }

Accessing dictionary values

Place the corresponding key in square brackets, as shown below:

Example

#!/usr/bin/python3

dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}

print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])

The above example outputs:

dict['Name']: Geekdoc
dict['Age']: 7

If you access data using a key that is not in the dictionary, you will get an error like the following:

Example

#!/usr/bin/python3

dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}

print ("dict['Alice']: ", dict['Alice'])

The above example outputs:

Traceback (most recent call last):
File "test.py", line 5, in <module>
print ("dict['Alice']: ", dict['Alice'])
KeyError: 'Alice'

Modifying a Dictionary

Adding new content to a dictionary involves adding new key/value pairs and modifying or deleting existing key/value pairs. The following example shows:

Example

#!/usr/bin/python3

dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}

dict['Age'] = 8 # Update Age
dict['School'] = "Geek Tutorial" # Add information

print ("dict['Age']: ", dict['Age'])
print ("dict['School']: ", dict['School'])

The above example outputs:

dict['Age']: 8
dict['School']: Geek Tutorial

Deleting Dictionary Elements

You can delete a single element or clear a dictionary; clearing a dictionary only requires one operation.

Use the del command to explicitly delete a dictionary, as shown in the following example:

Example

#!/usr/bin/python3

dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}

del dict['Name'] # Delete the key 'Name'
dict.clear() # Clear the dictionary
del dict # Delete the dictionary

print ("dict['Age']: ", dict['Age'])
print ("dict['School']: ", dict['School'])

However, this will raise an exception because the dictionary no longer exists after the del operation:

Traceback (most recent call last):
File "test.py", line 9, in <module>
print ("dict['Age']: ", dict['Age'])
TypeError: 'type' object is not subscriptable

Note: The del() method will be discussed later.

Characteristics of Dictionary Keys

Dictionary values can be any python object, both standard and user-defined, but keys cannot.

Two important points to remember:

1) The same key cannot appear twice. If the same key is assigned twice during creation, the last value will be remembered, as shown in the following example:

Example:

#!/usr/bin/python3

dict = {‘Name’: ‘Geekdoc’, ‘Age’: 7, ‘Name’: ‘小治客’}

print (“dict[‘Name’]: “, dict[‘Name’])

Output of the above example:

dict['Name']: 小治客


<p>2) Keys must be immutable, so numbers, strings, or tuples can be used, but lists cannot.

Example:

<pre><code class="language-python line-numbers">Example:

#!/usr/bin/python3

dict = {['Name']: 'Geekdoc', 'Age': 7}

print ("dict['Name']: ", dict['Name'])

The above example outputs:

Traceback (most recent call last):
File "test.py", line 3, in <module>
dict = {['Name']: 'Geekdoc', 'Age': 7}
TypeError: unhashable type: 'list'

Dictionary Built-in Functions & Methods

Python dictionaries include the following built-in functions:

>>> dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}
>>> len(dict)
3

>>> dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}
>>> str(dict)
"{'Name': 'Geekdoc', 'Class': 'First', 'Age': 7}"

>>> dict = {'Name': 'Geekdoc', 'Age': 7, 'Class': 'First'}
>>> type(dict)

Python dictionaries have the following built-in methods:

x = True
country_counter = {}

def addone(country):
    if country in country_counter:
        country_counter[country] += 1
    else:
        country_counter[country] = 1

addone('China')
addone('Japan')
addone('china')

print(len(country_counter))

confusion = {}
confusion[1] = 1
confusion['1'] = 2
confusion[1] += 1

sum = 0
for k in confusion:
    sum += confusion[k]

print(sum)

After-class exercises