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What is an OrderedDict in Python?

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An OrderedDict is a dictionary subclass in which the order of the content added is maintained.

import collections

ordered_dict = collections.OrderedDict()
ordered_dict['one'] = 1
ordered_dict['two'] = 2
ordered_dict['three'] = 3

print(ordered_dict)
  • When we add a new element, it will be inserted at the end.

  • When we delete an element, the element below it will move to the deleted element’s position.

  • When we change the value of an element, the element will stay in same order.

import collections

ordered_dict = collections.OrderedDict()
ordered_dict['one'] = 1
ordered_dict['two'] = 2
ordered_dict['three'] = 3
print(ordered_dict)

print("\n-------\nAdding new element")
ordered_dict['four'] = 4
print(ordered_dict)


print("\n-------\nDeleting an element")
ordered_dict.pop('two');
print(ordered_dict)


print("\n-------\nReplacing value of an element")
ordered_dict['one'] = "one";
print(ordered_dict)

We can use move_to_end to move an element either to the first or the last element of the dictionary.

The second argument of the move_to_end method determines the position.

So, if we pass true it will reposition the element to the last position; however, if we pass false, then the element is repositioned to the first position.

import collections

ordered_dict = collections.OrderedDict()
ordered_dict['one'] = 1
ordered_dict['two'] = 2
ordered_dict['three'] = 3
print(ordered_dict)

print("\n---------\nMoving 'one' as last element")
ordered_dict.move_to_end('one');
print(ordered_dict);

print("\n---------\nMoving 'three' as first element")
ordered_dict.move_to_end('three', False);
print(ordered_dict);

To iterate the OrderedDict in reverse order, use reversed method.

import collections

ordered_dict = collections.OrderedDict()
ordered_dict['one'] = 1
ordered_dict['two'] = 2
ordered_dict['three'] = 3
print(ordered_dict)

print("\nReversed order")
for prop in reversed(ordered_dict):
    print(prop);

When checking for dict equality:

  • A regular dict looks at its contents

  • An OrderedDict looks at the order and the contents

import collections;

d1 = {'1': 1, '2': 2}
d2 = {'2': 2, '1': 1}

print("Cheking if two normal dic are equal => ", d1 == d2);

ordered_dict_1 = collections.OrderedDict({'1': 1, '2': 2})
ordered_dict_2 = collections.OrderedDict({'2': 2, '1': 1})

print("Cheking if two ordered dic are equal => ", ordered_dict_1 == ordered_dict_2);

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