Python – Insertion at the beginning in OrderedDict

Given an ordered dict, write a program to insert items in the beginning of the ordered dict.

Examples:

Input: 
original_dict = {'a':1, 'b':2}
item to be inserted ('c', 3)

Output:  {'c':3, 'a':1, 'b':2}

Input: 
original_dict = {'akshat':1, 'manjeet':2}
item to be inserted ('nikhil', 3)

Output:  {'nikhil':3, 'akshat':1, 'manjeet':2}

Below are various methods to insert items in starting of ordered dict.

Method #1: Using OrderedDict.move_to_end()

Python3

# Python code to demonstrate
# insertion of items in beginning of ordered dict
from collections import OrderedDict

# Initialising ordered_dict
iniordered_dict = OrderedDict([('akshat', '1'), ('nikhil', '2')])

# Inserting items in starting of dict
iniordered_dict.update({'manjeet': '3'})
iniordered_dict.move_to_end('manjeet', last=False)

# Printing result
print("Resultant Dictionary : "+str(iniordered_dict))

Output

Resultant Dictionary : OrderedDict([('manjeet', '3'), ('akshat', '1'), ('nikhil', '2')])

Time complexity: O(n), where n is the number of values in the dictionary.
Auxiliary Space: O(1), constant extra space is required

Method #2: Using Naive Approach

This method only works in case of unique keys 

Python3

# Python code to demonstrate
# insertion of items in beginning of ordered dict
from collections import OrderedDict

# initialising ordered_dict
ini_dict1 = OrderedDict([('akshat', '1'), ('nikhil', '2')])
ini_dict2 = OrderedDict([("manjeet", '4'), ("akash", '4')])

# adding in beginning of dict
both = OrderedDict(list(ini_dict2.items()) + list(ini_dict1.items()))

# print result
print ("Resultant Dictionary :"+str(both))

Output

Resultant Dictionary :OrderedDict([('manjeet', '4'), ('akash', '4'), ('akshat', '1'), ('nikhil', '2')])

Method #3: Using OrderedDict.popitem() 

To add a new key-value pair at the beginning of an OrderedDict, we can use popitem() method with a new OrderedDict. This method returns and removes the last key-value pair in the OrderedDict. We can keep popping the last key-value pair and adding it to a new OrderedDict until we get the desired order of insertion.

Python3

from collections import OrderedDict

# Initialising ordered_dict
ini_dict = OrderedDict([('akshat', '1'), ('nikhil', '2')])

# Creating a iniordered ordered dict
iniordered_dict = OrderedDict()

# Inserting new key-value pair at the beginning of iniordered_dict
iniordered_dict.update({'manjeet':'3'})
iniordered_dict.move_to_end('manjeet', last = False)

# popitem() method to remove and insert key-value pair at beginning
while ini_dict:
    iniordered_dict.update({ini_dict.popitem(last=False)})

# Print result
print("Resultant Dictionary :" + str(iniordered_dict))

Output

Resultant Dictionary :OrderedDict([('manjeet', '3'), ('akshat', '1'), ('nikhil', '2')])

Time Complexity: O(n), where N is the number of key-value pairs in the ini_dict
Auxiliary Space: O(n), because we are iterating through all the key-value pairs in ini_dict once.