Python OrderedDict

• OrderedDict is part of python collections module.
• We can create an empty OrderedDict and add items to it. If we create an OrderedDict by passing a dict argument, then the ordering may be lost because dict doesn’t maintain the insertion order.
• If an item is overwritten in the OrderedDict, it’s position is maintained.
• If an item is deleted and added again, then it moves to the last.
• OrderedDict popitem removes the items in FIFO order. It accepts a boolean argument last, if it’s set to True then items are returned in LIFO order.
• We can move an item to the beginning or end of the OrderedDict using move_to_end function. It accepts a boolean argument last, if it’s set to False then item is moved to the start of the ordered dict.
• From python 3.6 onwards, order is retained for keyword arguments passed to the OrderedDict constructor, refer PEP-468.
• We can use reversed() function with OrderedDict to iterate elements in the reverse order.
• Equality tests between OrderedDict objects are order-sensitive and are implemented as list(od1.items())==list(od2.items()).
• Equality tests between OrderedDict and other Mapping objects are order-insensitive like regular dictionaries. This allows OrderedDict objects to be substituted anywhere a regular dictionary is used.

Let’s look at some code examples of Python OrderedDict.

Creating OrderedDict object

from collections import OrderedDict
# creating a simple dict
my_dict = {'kiwi': 4, 'apple': 5, 'cat': 3}
# creating empty ordered dict
ordered_dict = OrderedDict()
print(ordered_dict)
# creating ordered dict from dict
ordered_dict = OrderedDict(my_dict)
print(ordered_dict)

Output:

OrderedDict()
OrderedDict([('kiwi', 4), ('apple', 5), ('cat', 3)])

Adding, Replacing, Removing items from OrderedDict

# adding elements to dict
ordered_dict['dog'] = 3
# replacing a dict key value
ordered_dict['kiwi'] = 10
print(ordered_dict)
# removing and adding a value
ordered_dict.pop('kiwi')
print(ordered_dict)
ordered_dict['kiwi'] = 4
print(ordered_dict)

Output:

OrderedDict([('kiwi', 10), ('apple', 5), ('cat', 3), ('dog', 3)])
OrderedDict([('apple', 5), ('cat', 3), ('dog', 3)])
OrderedDict([('apple', 5), ('cat', 3), ('dog', 3), ('kiwi', 4)])

OrderedDict move_to_end example

# moving apple to end and dog to start
ordered_dict.move_to_end('apple')
ordered_dict.move_to_end('dog', False)
print(ordered_dict)

Output:

OrderedDict([('dog', 3), ('cat', 3), ('kiwi', 4), ('apple', 5)])

OrderedDict popitem example

# pop last item
item = ordered_dict.popitem(True)
print(item)
print(ordered_dict)

Output:

('apple', 5)
OrderedDict([('dog', 3), ('cat', 3), ('kiwi', 4)])

OrderedDict Reverse Iteration

# reversed iteration
for item in reversed(ordered_dict):
    print(item)

Output:

kiwi
cat
dog

OrderedDict Equality Tests Example

# equality tests
d1 = {'a': 'A', 'b': 'B'}
d2 = {'b': 'B', 'a': 'A'}
od1 = OrderedDict({'a': 'A', 'b': 'B'})
od2 = OrderedDict({'b': 'B', 'a': 'A'})
print(d1 == d2)
print(od1 == od2)
print(d1 == od1)

Output:

True
False
True

You can download the complete example code from our GitHub Repository.

Reference: Python Docs