Reverse A Dictionary in Python

Dictionaries in python are a great tool to store key-value mappings. In this article, we will discuss how to reverse a dictionary in python. We will do it using different examples so that we can understand the approaches in a better manner.

How to Reverse a Dictionary in Python?

When we reverse a given python dictionary, we change the order of values in the key-value pair. In each key-value pair, the current key becomes the value and the current value becomes the key in the new dictionary. For instance, look at the following dictionary.

myDict = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

After we reverse this dictionary, it would look like this.

reversedDict = {1: 1, 4: 2, 9: 3, 16: 4, 25: 5}

To reverse a dictionary in python, we can use a for loop or dictionary comprehension. Let us discuss both approaches one by one.

Reverse a Dictionary in Python Using a For Loop

To reverse a dictionary using for loop, we will first create an empty dictionary to store the reverted key-value pairs. After that, we will traverse over each key-value pair in the input dictionary. While traversal, we will make each value of the input dictionary a key in the output dictionary and the key associated with the value in the input dictionary will be made the associated value in the output dictionary as follows.

myDict = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
print("The input dictionary is:")
print(myDict)
reversedDict = dict()
for key in myDict:
    val = myDict[key]
    reversedDict[val] = key
print("The reversed dictionary is:")
print(reversedDict)

Output:

The input dictionary is:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
The reversed dictionary is:
{1: 1, 4: 2, 9: 3, 16: 4, 25: 5}

Using keys() and values() Method

You can also use the keys() and values() method with the for loop to reverse a dictionary. In this approach, we will first make a list of keys and a list of values of the input dictionary. After that, we will make each element of the list of values a key in the output dictionary. The associated element in the list of keys will be made the corresponding value in the output dictionary as follows.

myDict = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
print("The input dictionary is:")
print(myDict)
reversedDict = dict()
key_list = list(myDict.keys())
val_list = list(myDict.values())
n = len(key_list)
for i in range(n):
    key = val_list[i]
    val = key_list[i]
    reversedDict[key] = val
print("The reversed dictionary is:")
print(reversedDict)

Output:

The input dictionary is:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
The reversed dictionary is:
{1: 1, 4: 2, 9: 3, 16: 4, 25: 5}

Using items() Method

You can also use the items() method to reverse a dictionary. In this approach, we will take each item from the input dictionary one by one. After that, we will reverse the key-value pair as follows.

myDict = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
print("The input dictionary is:")
print(myDict)
reversedDict = dict()
for item in myDict.items():
    key = item[1]
    val = item[0]
    reversedDict[key] = val
print("The reversed dictionary is:")
print(reversedDict)

Output:

The input dictionary is:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
The reversed dictionary is:
{1: 1, 4: 2, 9: 3, 16: 4, 25: 5}

Using Dictionary Comprehension

Instead of using a for loop, we can reverse a dictionary in a single python statement using dictionary comprehension. Here, we will create the output dictionary by reversing the key and value in each key-value pair of the dictionary as follows.

myDict = {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
print("The input dictionary is:")
print(myDict)
reversedDict = {val: key for (key, val) in myDict.items()}
print("The reversed dictionary is:")
print(reversedDict)

Output:

The input dictionary is:
{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
The reversed dictionary is:
{1: 1, 4: 2, 9: 3, 16: 4, 25: 5}

Conclusion

In this article, we have discussed four ways to reverse a dictionary in python. To learn more about dictionaries in python, you can read this article on how to convert a dictionary to list of tuples. You might also like this article on list comprehension in python.