Dictionaries

Python has a dictionary data type.

The dictionary data type stores items as KEY: VALUE pairs. Unlike a list that stores items by their position in the array:

a[0], a[1], a[2], a[3]... 

A dictionary has no concept of order. The programmer determines what the key is, and assigns a value to it.

Consider the following list:

We create an empty list:

l = list()

We then append elements to the list. Recall that the list is ordered from zero.

for x in range(10):
    l.append(x)

The range() function returns a list of integers. In our example, range(10) will return the list:
[0,1,2,3,4,5,6,7,8,9]

Now let's remove the third element (l[2] - since the list is ordered from zero).

Notice how the item at position 2 has gone.

Actually the pop(x) method removes the element which has the value x. In the example above, if there was no "2" in the list, then an error would have been returned. The way to remove the 2-nd element is to use the del method.

In the example above, the element with value "3" is in the index position [2].

The statement:

del l[2]

deletes the "3"

[0, 1, 3, 4, 5, 6, 7, 8, 9]

becomes

[0, 1, 4, 5, 6, 7, 8, 9]

Now let's insert the "3" and the "2" back.

The insert(Pos, Value) function takes two arguments. Pos = the position (zero-based) where we will insert the element, and Value = the value of the element that we're inserting.

And now for the "2"

The function call, l.insert(2,2) will insert the value = 2 as position = 2.

DICTIONARIES are different. They are not based on indexed positions. Each entry in a dictionary has a key that marks where it is. The value is the actual element.

We can create an empty dictionary the following way:

Just like the list() function creates an empty list, the dict() function creates an empty dictionary. We can also create an empty list the following way:

l = []

And we can create an empty dictionary the following way:

d = {}

A dictionary uses curly braces (also known as brackets) to indicate that it's a dictionary.

Now let's add some elements to our empty dictionary. Remember, we need to add a key and an element. Unlike a list that only requires that you specify the element that you want to insert or append. A dictionary has no concept of ordering. We will see later how to sort dictionaries.

Notice a couple of things:

• The key can be anything. A string or a number.
• The value can be anything. A string or a number.
• The key does not have to be related to the value.

A dictionary has an update() method if you want to add more than one key:value pair at once.

Let's see how this works:

Note the statement:

d.update({4: 'four', 5: 'five', 'holiday': 'Kwanzaa', 'lunari': 'X'})

This adds four elements. The keys are:

• 4
• 5
• holiday
• lunari

The values of each of those keys are:

• four
• five
• Kwanzaa
• X

It's probably best practice to use the update() method for all inserts so that you get used to it for single or multiple inserts.

Since dictionaries are unordered, to get a list of all the keys, the dictionary provides a keys() method.

This is how it works:

And the dictionary object also has a values() method that retrieves all the values. It works like this:

The keys() method is typically used in a loop to print out all the values. Like this:

The loop iterates through the list. The list is a list of keys. And inside the loop, the following statement runs:

print(key, ': ', d[key])

The print() statement will print a comma-separated list of items and add a newline at the end.