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When keeping track of many different dictionaries with the built-in Python containers, we could try storing dictionaries in a list, or even a dictionary of dictionaries. This may work in some cases, but there are a few problems which might come up.

When storing dictionaries in a list, the order is preserved, but we have to access the elements by their index before we can access the dictionary:

first_order = {'order_2905': {'type': 'shoes', 'size': 12, 'price': 22.50}}
second_order = {'order_6184': {'type': 'pants', 'size': 'medium', 'price': 14.99}}
third_order = {'order_4829': {'type': 't-shirt', 'size': 'large', 'price': 9.99}}

list_of_dicts = [first_order, second_order, third_order]

In order to get the price of a specific order, we must know the index of it already before we can access the dictionary data stored inside:

print(list_of_dicts[1]['order_6184']['price'])

# Output
# 14.99

On the other hand, depending on the Python version, the dict container can preserve the order, but it is difficult to move elements around:

dict_of_dicts = {}
dict_of_dicts.update(first_order)
dict_of_dicts.update(second_order)
dict_of_dicts.update(third_order)

print(dict_of_dicts['order_6184']['price'])

# Output
# 14.99

Note: The dict class is unordered in earlier versions of python, so implementing it this way must have version 3.6 or greater.

To solve these issues, we can use an OrderedDict!

The OrderedDict container allows us to access values using keys, but it also preserves the order of the elements inside of it. Let’s take a closer look at the example of processing customer orders from earlier in the lesson:

Import and create an OrderedDict.

from collections import OrderedDict

orders = OrderedDict()

The order of the data is preserved when adding it to the OrderedDict:

orders.update({'order_2905': {'type': 'shoes', 'size': 12, 'price': 22.50}})
orders.update({'order_6184': {'type': 'pants', 'size': 'medium', 'price': 14.99}})
orders.update({'order_4829': {'type': 't-shirt', 'size': 'large', 'price': 9.99}})

Data can be accessed using keys like a normal dictionary:

# Get a specific order
find_order = orders['order_2905']

The order can be retrieved by converting it to a list then accessing by index:

# Get the data in a list format
orders_list = list(orders.items())
third_order = orders_list[2]

When using an OrderedDict, we are able to use its methods for moving the data around. We can move an element to the back or front and pop the data from the back or front of the OrderedDict:

# Move an item to the end of the OrderedDict
orders.move_to_end('order_4829')

# Pop the last item in the dictionary
last_order = orders.popitem()

Note: These two methods also accept boolean arguments which determine if the element is moved / popped from the front or back of the OrderedDict.

For more information about the OrderedDict container, take a look at the Python Documentation.

Now let’s try using an OrderedDict in our clothes app!

Instructions

We want to add some logic to our application which will organize orders by their status. A list of orders is provided which includes the order number and the status. The status of an order can be purchased, returned, or canceled. To make things more organized, we want to remove the canceled orders and push the returned orders to the end. In order to do this, we can use an OrderedDict!

For this first checkpoint, import the OrderedDict class and create a new object from that class called orders. Use the constructor to automatically convert the order_data into an OrderedDict.

We need to keep track of which orders to remove and which ones to push back. To do this, create two new lists called to_move and to_remove. Iterate through each item in orders and check what the status is. If the status is 'returned' then add the key (order number string) to the to_move list. Otherwise, if the status is 'canceled' then add it to the to_remove list.

Now that we have the list of items to remove from orders, for every item in the to_remove list, .pop() the element from orders.

Now that all of the canceled orders have been removed, use another loop to push back any of the 'returned' orders from to_move to the end of orders. This will be similar to the last step, but this time we are using the .move_to_end() method.

Finally, use print to output the orders to the console!

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