Dictionaries #

Another type of value that can contain additional values within it is a dictionary. Imagine a translation dictionary, such as this Czech-English one:

• Jablko: Apple
• Knoflík: Button
• Myš: Mouse

The dictionary in Python contains records, and each record assigns some value to a key. In our example, the value Apple is assigned to the key Jablko, the value Button belongs to the key Knoflík and the key Myš points to Mouse.

In Python, such a dictionary would be written as follows:

dictionary = {'Jablko': 'Apple', 'Knoflík': 'Button', 'Myš': 'Mouse'}

These keys and values are words - short texts, i.e. strings, that need to be put in quotation marks. Each key is separated from its value by a colon, individual pairs are separated from each other by a comma, and the entire dictionary is enclosed in curly braces.

When you want to find something in such a dictionary, you need to know what to look for. You need a key. Using square brackets, you can find out the value that corresponds to a given key.

dictionary['Jablko']
'Apple'

It's similar to lists, only in square brackets there's no index (order of the element) or range with a colon, but rather a key.

Modifying dictionaries #

What happens when the key is not in the dictionary?

>>> slovnik['Pes']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'Pes'

Python complains about KeyError - a key error.

Similarly to lists, dictionaries can also be modified. You create a new entry like this:

>>> dictionary['Pes'] = 'Dog'
>>> dictionary
{'Jablko': 'Apple', 'Knoflík': 'Button', 'Myš': 'Mouse', 'Pes': 'Dog'}

If you need to change an existing record, use the same command. Only one value can belong to one key.

>>> slovnik['Pes'] = 'Extension cord'
>>> slovnik
{'Jablko': 'Apple', 'Knoflík': 'Button', 'Myš': 'Mouse', 'Pes': 'Extension cord'}

If you want to delete a record from the dictionary, it is done similarly to lists with the command 'del'.

>>> del slovnik['Pes']
>>> slovnik
{'Jablko': 'Apple', 'Knoflík': 'Button', 'Myš': 'Mouse'}

And if you want to find out how many records are in the dictionary, you will ask similarly as for the number of characters in a string or elements in a list. You will use the len() function.

>>> len(slovnik)
3

To think about #

To each key can only belong one value. How would you arrange it to have more values?

Try to save these contacts to a Python variable:

Katka:

• 4925219

Jirka:

• 7477058
• 3251156

Verča:

• 1019103

>>> contacts = {'Katka': ['4925219'], 'Jirka': ['7477058', '3251156'], 'Verča': ['1019103']}

Summary #

Great! What do you know about dictionaries?

• A record consists of a key and a value.
• In a dictionary, you search for an item using a key.
• Records can be overwritten, added, or deleted using del.

Are you ready for the next part?