Adventures in Machine Learning

Order Matters! How OrderedDict Can Transform Your Python Dictionaries

Introduction to OrderedDict

As Python developers, we often work with dictionaries to store and retrieve data. While dictionaries are great for their quick data access and manipulation, they have one limitation: a dictionary does not preserve the order in which items were added.

In situations where order preservation is important, we can use a subclass of the built-in “dict” class: OrderedDict. OrderedDict, as the name suggests, is a dictionary that preserves the order in which items were added.

This means that we can iterate over the keys and values of an OrderedDict in the exact order they were added. This can be useful in situations such as when you want to process data in the order in which it was received or when you need to maintain a specific order for data to be displayed.

Difference between OrderedDict and default dict

Another related class in Python is the defaultdict. While both OrderedDict and defaultdict are subclasses of “dict”, they differ in their purpose and functionality.

The defaultdict is designed to provide default values for keys that do not already exist, whereas the OrderedDict is designed to preserve the order in which items were added to the dictionary.

Importing and using OrderedDict

To use the OrderedDict class, we need to import it from the “collections” module. This can be done using the following code:


from collections import OrderedDict


Once we have imported the class, we can create an instance of the class and start using its functionalities.

OrderedDict Functionalities

Creating an OrderedDict object

To create an OrderedDict object, we simply call the class constructor without any arguments:


my_dict = OrderedDict()


This creates an empty OrderedDict instance that we can add items to.

Adding items to an OrderedDict

We can add items to an OrderedDict using the “update()” method or by using dictionary style assignment:


my_dict = OrderedDict([(‘key1’, ‘value1’), (‘key2’, ‘value2’)])

my_dict.update({‘key3’: ‘value3’, ‘key4’: ‘value4’})


This creates a new instance of an OrderedDict and adds four items to it.

Replacing items in an OrderedDict

If we need to replace an existing value in an OrderedDict, we can use dictionary style assignment:


my_dict[‘key2’] = ‘new_value’


This replaces the existing value for the “key2” key with a new value.

Removing items from an OrderedDict

We can remove items from an OrderedDict using the “del” keyword or the “popitem()” method:


del my_dict[‘key1’]



The first line removes the item with the ‘key1’ key from the OrderedDict, while the second line removes the last item added to the OrderedDict.

Key-Value changes in an OrderedDict

We can also modify the value of an existing item using dictionary style assignment:


my_dict[‘key1’] = ‘updated_value’


This modifies the value of an existing item in the OrderedDict.

Using the move_to_end() function

The move_to_end() method moves a key-value item in the OrderedDict to the end of the dictionary. This is useful when we want to prioritize specific items or to move an item to the end of a queue:




This moves the item with the ‘key1’ key to the end of the OrderedDict.

Using the popitem() function

The popitem() method removes and returns the last key-value item added to the OrderedDict. This can be useful when we want to remove the last item added or process items in reverse order:


last_item = my_dict.popitem()


This removes the last item from the OrderedDict and assigns it to the “last_item” variable.

Reverse iteration in an OrderedDict

We can also iterate over an OrderedDict in reverse order of item addition. This can be useful when we want to process items in the reverse order:


for key, value in reversed(my_dict.items()):

print(key, value)


This iterates over the items of the OrderedDict in reverse order.

Testing for equality in an OrderedDict

We can test two OrderedDicts for equality using the “== operator:


dict1 = OrderedDict([(‘key1’, ‘value1’), (‘key2’, ‘value2’)])

dict2 = OrderedDict([(‘key2’, ‘value2’), (‘key1’, ‘value1’)])

print(dict1 == dict2) # prints True


This returns True if both dictionaries have the same keys and values, in the same order.


OrderedDict is a powerful data structure that can help to preserve the order of data when working with Python dictionaries. It provides a range of functionalities to add, remove, and modify items in an ordered fashion.

By using OrderedDict, we can ensure that the order of data is maintained and processed as intended, making it an essential tool in many programming applications.

Uses of OrderedDict Class

We have learned that OrderedDict is a subclass of the built-in “dict” type in Python. It is a dictionary data structure that preserves the order of items.

The standard “dict” data type in Python does not guarantee the order of items in a dictionary, but this can be an important feature in some applications. OrderedDict allows us to preserve the order of items and is especially useful when working with data that needs to maintain a specific order, such as in a pipeline of data processing.

Here are some examples of the use cases where OrderedDict can be beneficial:

1. Storing data – When data needs to be stored with a specific order, OrderedDict can be used to ensure the order of items is preserved.


# Defining an empty ordered dictionary

en_sp_dict = OrderedDict()

# Adding items to the ordered dictionary

en_sp_dict [‘one’] = ‘uno’

en_sp_dict [‘two’] = ‘Dos’

en_sp_dict [‘three’] = ‘tres’

en_sp_dict [‘four’] = ‘cuatro’

# Iterating over the ordered dictionary

for key, value in en_sp_dict.items():

print(key, value)




one uno

two Dos

three tres

four cuatro


2. Web scraping – When scraping a web page to extract data, using OrderedDict can help to ensure that the data is extracted in the order in which it appears on the page.


# Importing Required Libraries

from bs4 import BeautifulSoup

import requests

# Fetching Web Page Data

url = “”

resp = requests.get(url)

# Parsing the HTML Page

soup = BeautifulSoup(resp.text, ‘html.parser’)

# Extracting Data with an Ordered Dictionary

stock_data = OrderedDict()

for row in‘tbody tr’):

ticker = row.select_one(‘.symbol-cell a’).text.strip()

last_price = row.select_one(‘.last-cell’).text.strip()

change = row.select_one(‘.change-cell div’).text.strip()

pct_change = row.select_one(‘.pct-change-cell div’).text.strip()

stock_data[ticker] = (last_price, change, pct_change)

# Printing the Extracted Data

for ticker, data in stock_data.items():

print(ticker, data)




BIOL (‘1.2700’, ‘Change: 0.1800 (16.47%)’, ‘% Change: +16.47%’)

CYBL (‘3.3200’, ‘Change: -0.3700 (-10.01%)’, ‘% Change: -10.01%’)

… “`


Stream Processing – When processing streams of data, preserving the order of items is critical. OrderedDict can be used to ensure the correct order in which the items are processed.


# Defining an empty ordered dictionary

items_dict = OrderedDict()

# Adding items to the ordered dictionary in a stream

items_dict[‘item_id1’] = {‘name’: ‘item1’, ‘price’: 10.0, ‘qty’: 2}

items_dict[‘item_id2’] = {‘name’: ‘item2’, ‘price’: 5.0, ‘qty’: 1}

items_dict[‘item_id3’] = {‘name’: ‘item3’, ‘price’: 2.0, ‘qty’: 3}

# Applying discount to the items in the ordered dictionary

for item_id, item in items_dict.items():

item[‘price’] = item[‘price’] * 0.9 if item[‘qty’] >= 2 else item[‘price’]

# Printing the final items in the ordered dictionary

for item_id, item in items_dict.items():

print(item_id, item)




item_id1 {‘name’: ‘item1’, ‘price’: 9.0, ‘qty’: 2}

item_id2 {‘name’: ‘item2’, ‘price’: 5.0, ‘qty’: 1}

item_id3 {‘name’: ‘item3’, ‘price’: 1.8, ‘qty’: 3}


OrderedDict provides several methods for adding, removing, and manipulating items ordered in a dictionary. These methods include insert(), update(), pop(), popitem(), move_to_end() and clear().

The syntax of these methods is easy to use and is somewhat like dictionary data type methods; the only difference being that they contain an additional insert index or the order in which the method needs to preserve the sequence of items. In conclusion, the OrderedDict class in Python is a powerful tool for preserving the order of data in dictionaries.

This article has gone through the main features of this class, covering the creation of an ordered dictionary object, adding and replacing items, removing items, and testing the equality of two OrderedDicts. We also explored several use cases that highlight the different applications of OrderedDict, such as storing data, web scraping, and stream processing.

By using OrderedDict, Python developers can maintain the order of data which is a crucial element in many programs. In summary, the use of OrderedDict is crucial for preserving the order of data in dictionaries in Python.

This article has discussed the definition and the functionalities of OrderedDict, along with the differences between OrderedDict and default dict. We have also covered the most important methods of OrderedDict, including adding, replacing, and removing items, and testing for equality.

Furthermore, we have looked at some common use cases where OrderedDict is beneficial, such as web scraping, stream processing, and data storage. By maintaining the order of data, we ensure the correct processing of data as intended.

Therefore, developers should understand the importance of OrderedDict and can benefit greatly by using it in situations that require order preservation.

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