All about OrderedDict in Python

An OrderedDict is a type of dictionary that preserves the insertion order of elements, which was not guaranteed in Python versions prior to 3.7.

Before Python 3.7, standard dictionaries (dict) did not guarantee the order of keys. It was in this context that OrderedDict was born, via the collections module.

Since Python 3.7, order is preserved even with dict, but OrderedDict remains useful in certain specific cases.

It allows for example:

• to sort elements, then store them in a precise order;
• to make strict comparisons on the order of pairs;
• to use specific operations like move_to_end().

Basic Syntax

Here's how to create an OrderedDict:

from collections import OrderedDict

od = OrderedDict()
od["a"] = 1
od["b"] = 2
od["c"] = 3

print(od)  # OrderedDict([('a', 1), ('b', 2), ('c', 3)])

The insertion order is respected, unlike the old dictionaries before Python 3.7.

OrderedDict-Specific Methods

Even though an OrderedDict behaves like a standard dictionary, it offers some unique methods related to order management: move_to_end() and popitem().

move_to_end()

This method allows you to move a key either to the end (last=True by default), or to the beginning (last=False).

od = OrderedDict([('a', 1), ('b', 2), ('c', 3)])
od.move_to_end('a')
print(od)  # OrderedDict([('b', 2), ('c', 3), ('a', 1)])

popitem()

For its part, popitem() removes and returns an element according to its order:

• last=True: removes the last one;
• last=False: removes the first one.

od.popitem(last=False)  # Removes ('a', 1)

OrderedDict vs dict: What are the differences?

Since Python 3.7, standard dictionaries preserve insertion order, which has largely reduced the need to use OrderedDict.

Let's still make a comparison between the two elements.

Ultimately, OrderedDict remains useful for order-sensitive algorithms, such as LRU cache, HTML/XML parsing, or priority structures.

Frequently Asked Questions about OrderedDict

Here are the most frequently asked questions when you start with OrderedDict! 😋

Yes, especially if you're using Python < 3.7 or if you need specific methods like move_to_end() or popitem(last=False), which are absent from classic dictionaries.

Slightly, but it's really negligible.

Thanks to a Python course (this one is worth testing!).