OrderedDict in Python

OrderedDict is a subclass of Python's built-in dictionary dict that remembers the order in which keys are inserted. Unlike older versions of Python where dictionaries did not guarantee order, OrderedDict preserves insertion order reliably.

Note: From Python 3.7 onwards, the built-in dict also preserves insertion order as an official language feature. So, OrderedDict is mainly useful if you:

• Work with Python versions before 3.7.
• Need extra features like reordering keys or popping items in LIFO/FIFO order.

OrderedDict vs dict in Python

Understanding OrderedDict and dict is important since both preserve insertion order (dict since Python 3.7), but OrderedDict offers extra features like key reordering and popping items from either end, giving more control. See the comparison table below for clarity.

Example:

from collections import OrderedDict

print("dict")
d = {}
d['a'] = 1
d['b'] = 2
d['c'] = 3
d['d'] = 4

for key, val in d.items():
    print(key, val)

print("ordered dict")
od = OrderedDict()
od['d'] = 4
od['b'] = 2
od['a'] = 1
od['c'] = 3

for key, val in od.items():
    print(key, val)

dict
a 1
b 2
c 3
d 4
ordered dict
d 4
b 2
a 1
c 3

Explanation:

• First part creates a regular dictionary d, adds keys in order and prints them, showing insertion order .
• Second part creates an OrderedDict od with keys added in a different order and prints them, preserving that order exactly.

Examples & key features

Let’s explore some examples to understand how OrderedDict works and see its key features in action.

1. Insertion order preservation

OrderedDict maintains the sequence exactly as elements were added. This is particularly useful in applications such as JSON serialization, form field processing or displaying logs, where the order of items carries semantic meaning.

from collections import OrderedDict

d = {'a': 1, 'b': 2, 'c': 3}
for k, v in d.items():
    print(k, v)

print("OrderedDict:")
od = OrderedDict([('d', 4), ('b', 2), ('a', 1), ('c', 3)])
for k, v in od.items():
    print(k, v)

a 1
b 2
c 3
OrderedDict:
d 4
b 2
a 1
c 3

Explanation: This code creates a regular dictionary d and prints its items in insertion order. Then, it creates an OrderedDict od with keys in a different order and prints them, showing how OrderedDict preserves the exact insertion sequence.

2. Changing value does not affect order

In an OrderedDict, modifying the value of an existing key does not change its position in the order. This means you can update the values without affecting the original key order.

from collections import OrderedDict

od = OrderedDict([('a', 1), ('b', 2), ('c', 3), ('d', 4)])
od['c'] = 5  

for k, v in od.items():
    print(k, v)

a 1
b 2
c 5
d 4

Explanation: This code creates an OrderedDict od with keys 'a', 'b', 'c', and 'd'. It then updates the value of the key 'c' to 5. When printing, the keys remain in their original order, showing that changing a value does not affect the key order in an OrderedDict.

3. Equality checks consider order

Unlike regular dicts, OrderedDict checks both content and order for equality, so differing orders make them unequal. This is useful when order matters.

from collections import OrderedDict

od1 = OrderedDict([('a', 1), ('b', 2), ('c', 3)])
od2 = OrderedDict([('c', 3), ('b', 2), ('a', 1)])

print(od1 == od2)

False

Explanation: This code creates two OrderedDicts od1 and od2, with the same keys and values but in different orders. When comparing them, the result is False because OrderedDicts consider both the content and the order of keys for equality.

4. Reversing an orderedDict

OrderedDict doesn’t have a built-in .reverse() method, but you can reverse its order by using Python’s reversed() function on list(od.items()). Creating a new OrderedDict from this reversed list preserves the reversed order.

from collections import OrderedDict

d1 = OrderedDict([('a', 1), ('b', 2), ('c', 3)])
d2 = OrderedDict(reversed(list(d1.items())))

for k, v in d2.items():
    print(k, v)

c 3
b 2
a 1

Explanation: This code creates an OrderedDict d1 with keys 'a', 'b', and 'c'. It then reverses the order of d1’s items using reversed() and creates a new OrderedDict d2 with this reversed order.

5. Pop last or first item

popitem() in OrderedDict removes the last item if last=True (default) or the first if last=False, making it useful for stacks and queues.

from collections import OrderedDict
d = OrderedDict([('a', 1), ('b', 2), ('c', 3)])

res = d.popitem(last=True)  # Remove last inserted item
print(res)

('c', 3)

Explanation: This code removes and returns the last item ('c', 3) from the OrderedDict using popitem(last=True).

6. Move keys to front or end

With the move_to_end() method, OrderedDict provides the flexibility to reposition keys. You can push a specific key to the beginning or end of the dictionary without deleting and re-inserting it.

from collections import OrderedDict
d = OrderedDict([('a', 1), ('b', 2), ('c', 3)])

d.move_to_end('a')         # Move 'a' to end
d.move_to_end('b', last=False)  # Move 'b' to front

for k, v in d.items():
    print(k, v)

b 2
c 3
a 1

Explanation: This code moves the key 'a' to the end of the OrderedDict and moves 'b' to the front. When printed, the keys appear in the order: 'b', 'c', 'a'.

7. Deleting and re-inserting keys

Deleting and re-inserting a key in an OrderedDict moves it to the end, preserving insertion order useful for tracking recent actions or updating featured items.

from collections import OrderedDict
od = OrderedDict([('a', 1), ('b', 2), ('c', 3), ('d', 4)])
od.pop('c')    # Delete 'c'

for k, v in od.items():
    print(k, v)

od['c'] = 3    # Re-insert 'c' at end
for k, v in od.items():
    print(k, v)

a 1
b 2
d 4
a 1
b 2
d 4
c 3

Explanation: This code deletes the key 'c' from the OrderedDict, then prints the remaining items. After that, it re-inserts 'c' with its value at the end and prints all items again.

Other considerations

• In Python 2.7, OrderedDict uses more memory than a normal dict due to its doubly linked list implementation.
• OrderedDict in Python 2.7 is not a subclass of dict but a specialized container from the collections module.
• From Python 3.7 onwards, regular dicts guarantee insertion order.
• popitem method in OrderedDict allows it to be used like a stack, enabling implementations like an LRU cache.