Introduction to Dictionarie

Dictionary is similar to list but more general. In a list, the position have to be integers but in dictionary the indices can be almost any type. Dictionary can be defined using dict built-in command.

>>> d = dict()
>>> type(d)

You can think of a dictionary as a mapping between a set of indices and they are called keys and a set of values. Each key maps to a value. The association of a key and a value is called key-value or sometimes an item.

A good  example to show a simple use of dictionary and key-value pair would be that dictionary maps from English to Italian words. So the keys and the values in this case are all strings.

>>>translator = dict() 
>>>print trnaslator
{}

The curly-brackets or braces represent an empty dictionary. To add items into an empty dictionary, you can use square brackets.

>>>translator['one'] = 'uno'
>>>print translator
{'one': 'uno'}

This line creates an item that maps from the key ‘one’ to the value ‘uno’. Using print built-in function we’ve printed out dictionary again, and we see a key-value pair with a colon between the key and value.

This output format is equal to the input format because you can create new dictionary in the same way. In our case we will extend translator dictionary.

translator = {'one': 'uno', 'two': 'due', 'three': 'tre', 'four': 'quattro' , 'five':'cinque', 'six':'sei', 'seven':'sette', 'eight':'otto', 'nine':'nove', 'ten':'dieci'}
print translator
{'four': 'quattro', 'seven': 'sette', 'five': 'cinque', 'three': 'tre', 'ten': 'dieci', 'eight': 'otto', 'nine': 'nove', 'six': 'sei', 'two': 'due', 'one': 'uno'}

As you can see after we’ve printed out the translator dictionary the key-value pairs are not in the same order as we’ve defined in declaration of dictionary. In general the order of items in the dictionary is very unpredictable. The reason for that is because the elements of a dictionary are never indexed with integer indices. Instead, you use the keys to look up the corresponding values. So let’s look for number five.

>>>print translator['five']
cinque

The key ‘five’ always maps to the value ‘five’ as defined in dictionary declaration so order of items doesn’t matter. If key isn’t defined in the dictionary Python interpreter will return a KeyError for example let’s look for number twenty three.

>>> print translator[‘twenty three’] 
KeyError: 'twenty three'