Methods of Ordered Dictionary in Python

An OrderedDict is a dict that remembers the order in that keys were first inserted. If a new entry overwrites an existing entry, the original insertion position is left unchanged. Deleting an entry and reinserting it will move it to the end. Ordered dictionary somehow can be used in the place where there is a use of hash Map and queue. It has characteristics of both into one. Like queue, it remembers the order and it also allows insertion and deletion at both ends. And like a dictionary, it also behaves like a hash map. 

Note: From Python 3.6 onwards, the order is retained for keyword arguments passed to the OrderedDict constructor, refer to PEP-468. 

Methods of ordered Dictionary

Let's look at various methods offered by the ordered dictionary. 

• popitem():

This method is used to delete a key from the beginning.

Syntax:

popitem(last = True)                      

If the last is False then this method would delete a key from the beginning of the dictionary. This serves as FIFO(First In First Out) in the queue otherwise it method would delete the key from the end of the dictionary.

Time Complexity: O(1).

For Better Understanding have a look at the code.

from collections import OrderedDict


ord_dict = OrderedDict().fromkeys('GeeksForGeeks')
print("Original Dictionary")
print(ord_dict)

# Pop the key from last
ord_dict.popitem()
print("\nAfter Deleting Last item :")
print(ord_dict)

# Pop the key from beginning
ord_dict.popitem(last = False)
print("\nAfter Deleting Key from Beginning :")
print(ord_dict)

Output:

Original Dictionary 
OrderedDict([('G', None), ('e', None), ('k', None), ('s', None), ('F', None), ('o', None), ('r', None)])
After Deleting Last item : 
OrderedDict([('G', None), ('e', None), ('k', None), ('s', None), ('F', None), ('o', None)])
After Deleting Key from Beginning : 
OrderedDict([('e', None), ('k', None), ('s', None), ('F', None), ('o', None)])

• move_to_end():

This method is used to move an existing key of the dictionary either to the end or to the beginning. There are two versions of this function - 

Syntax:

move_to_end(key, last = True)

If the last is True then this method would move an existing key of the dictionary in the end otherwise it would move an existing key of the dictionary in the beginning. If the key is moved at the beginning then it serves as FIFO ( First In First Out ) in a queue.

Time Complexity: O(1) 

from collections import OrderedDict


ord_dict = OrderedDict().fromkeys('GeeksForGeeks')
print("Original Dictionary")
print(ord_dict)

# Move the key to  end
ord_dict.move_to_end('G')
print("\nAfter moving key 'G' to end of dictionary :")
print(ord_dict)

# Move the key to beginning
ord_dict.move_to_end('k', last = False)
print("\nAfter moving Key in the Beginning :")
print(ord_dict)

Output:

Original Dictionary 
OrderedDict([('G', None), ('e', None), ('k', None), ('s', None), ('F', None), ('o', None), ('r', None)])
After moving key 'G' to end of dictionary : 
OrderedDict([('e', None), ('k', None), ('s', None), ('F', None), ('o', None), ('r', None), ('G', None)])
After moving Key in the Beginning : 
OrderedDict([('k', None), ('e', None), ('s', None), ('F', None), ('o', None), ('r', None), ('G', None)])

Working of move_to_end() function

Basically, this method looks up a link in a linked list in a dictionary self.__map and updates the previous and next pointers for the link and its neighbors. It deletes that element from its position and adds it to the end or beginning depending upon parameter value. Since all of the operations below take constant time, the complexity of OrderedDict.move_to_end()  is constant as well.