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.

from collections import OrderedDict

od = OrderedDict()
od['apple'] = 1
od['banana'] = 2
od['cherry'] = 3

print(list(od.items()))

[('apple', 1), ('banana', 2), ('cherry', 3)]

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.

Why do we need OrderedDict

• Maintains the insertion order of keys reliably across all Python versions (useful pre-3.7).
• Enables reordering of keys using move_to_end() for LIFO/FIFO-style access.
• Supports popping items from either end (popitem(last=True/False)), unlike regular dict.
• Equality check is order-sensitive, making it useful when key order matters.

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.

Suggested Quiz

10 Questions

What is an OrderedDict in Python?

• A

  A dictionary that preserves the order of keys

• B

  A dictionary that stores data in random order

• C

  A list that behaves like a dictionary

• D

  A set that stores unique elements in order

Explanation:

What is a key feature of OrderedDict that differentiates it from a standard dict in terms of equality checks?

• A

  It ignores the order of keys during comparison

• B

  It only compares values for equality

• C

  It considers both the keys and their order for equality

• D

  It requires keys to be unique for comparison

Explanation:

What method would you use to move a specific key to the end of an OrderedDict?

• A

  shift_to_end(key)

• B

  move_to_end(key)

• C

  push(key)

• D

  push_to_back(key)

Explanation:

Which of the following statements is true regarding the performance of OrderedDict compared to standard dict?

• A

  OrderedDict is faster for all operations

• B

  OrderedDict is slower due to its underlying implementation

• C

  Both have the same performance characteristics

• D

  OrderedDict is faster only for key retrieval

Explanation:

When using popitem() on an OrderedDict, what does the parameter last=True indicate?

• A

  It removes the first item in the OrderedDict

• B

  It removes the last item in the OrderedDict

• C

  It removes a random item from the OrderedDict

• D

  It removes the oldest item based on insertion time

Explanation:

Which module in Python provides the OrderedDict class?

• A

  collections

• B

  dict

• C

  order

• D

  ordered_dict

Explanation:

How can you create an OrderedDict from an existing dictionary in Python?

• A

  OrderedDict(dict)

• B

  dict.OrderedDict()

• C

  dict.toOrdered()

• D

  OrderedDict.from_dict(dict)

Explanation:

You can directly pass a regular dictionary to OrderedDict() to create an ordered dictionary.

What is the primary data structure used by OrderedDict to maintain the order of keys?

• A

  Array

• B

  Hash table

• C

  Doubly linked list

• D

  Stack

Explanation:

What happens when you assign a new value to an existing key in an OrderedDict?

• A

  The order of the key is preserved, and its value is updated

• B

  The key is moved to the end

• C

  The key is removed and re-inserted at the end

• D

  The order of keys is reset

Explanation:

What will the following code output?

from collections import OrderedDict

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

d.move_to_end('b', last=False)

for key, value in d.items():

print(key, value)

• A

  b 2, a 1, c 3

  
  

• B

  a 1, c 3, b 2

• C

  a 1, b 2, c 3

• D

  c 3, b 2, a 1

Explanation:

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