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Mastering OrderedDict in Python


OrderedDict vs dict in Python: The Right Tool for the Job

Choosing Between OrderedDict and dict

In Python, dictionaries were traditionally unordered data structures. However, there was a need for a new type of dictionary that could keep its items ordered. This led to the introduction of the OrderedDict class in the collections module.

Getting Started With Python’s OrderedDict

  1. Creating OrderedDict Objects: You can create an OrderedDict by passing in key-value pairs or by using the OrderedDict() function.
  2. Managing Items in an OrderedDict: You can add, modify, or delete items in an OrderedDict using the usual dictionary methods.
  3. Iterating Over an OrderedDict: The items in an OrderedDict are ordered based on their insertion order, so you can iterate over them in the order they were added.
  4. Iterating in Reversed Order: You can also iterate over the items in an OrderedDict in reverse order using the reversed() function.

Exploring Unique Features of Python’s OrderedDict

  1. Reordering Items: You can change the order of items in an OrderedDict using the move_to_end() method.
  2. Removing Items: You can remove items from an OrderedDict using the popitem() method, which removes the last item by default.
  3. Testing for Equality: You can test for equality between two dictionaries using the == operator, but the order of items is also taken into account for OrderedDicts.
  4. Appending New Attributes: You can append new attributes to a dictionary instance by directly assigning them.

Merging and Updating Dictionaries With Operators

You can merge two dictionaries using the update() method or the | operator. The order of items is preserved in OrderedDicts.

Considering Performance

Using an OrderedDict instead of a regular dictionary may have a slight impact on performance, as OrderedDicts require more memory to store the order of items.

Selecting the Right Dictionary for the Job

If you need to preserve the order of items, use an OrderedDict. If the order of items is not important, use a regular dictionary for better performance.

Building a Dictionary-Based Queue

You can implement a dictionary-based queue using an OrderedDict. The order of items in the queue represents their priority.


In conclusion, Python’s OrderedDict class provides a valuable solution when you need to preserve the order of items in a dictionary. Although the built-in dict class now keeps its items ordered, OrderedDict still offers unique features and can be used to solve specific problems. By understanding the differences and considering the specific requirements of your code, you can choose the right dictionary class for the job.