Dictionary with index as value in Python

In this article we will learn how to create a dictionary from a Python list, where the list values become dictionary keys and their positions become the values. This is useful when you need to map elements to their index positions.

Using enumerate with Dictionary Comprehension

The enumerate() function adds a counter to each element in the list. We can use it with dictionary comprehension to create a dictionary where list values become keys and their positions become values ?

Example

days = ['Mon', 'Tue', 'Wed', 'Wed', 11, 11]

# Given list
print("Given list:", days)

# Converting to Dictionary
new_dict = {val: key + 1 for key, val in enumerate(days)}

# Print result
print("Dictionary created with index:", new_dict)

Running the above code gives us the following result −

Given list: ['Mon', 'Tue', 'Wed', 'Wed', 11, 11]
Dictionary created with index: {'Mon': 1, 'Tue': 2, 'Wed': 4, 11: 6}

Note that when there are duplicate elements, only the last occurrence is displayed with its higher index value from among the duplicates.

Using zip and range

Another approach is to use range() to create index values starting from 1, then use zip() to pair list elements with their positions ?

Example

days = ['Mon', 'Tue', 'Wed', 'Wed', 11, 11]

# Given list
print("Given list:", days)

# Converting to Dictionary
new_dict = dict(zip(days, range(1, len(days) + 1)))

# Print result
print("Dictionary created with index:", new_dict)

Running the above code gives us the following result −

Given list: ['Mon', 'Tue', 'Wed', 'Wed', 11, 11]
Dictionary created with index: {'Mon': 1, 'Tue': 2, 'Wed': 4, 11: 6}

Comparison

Handling Zero-based Indexing

If you want zero-based indexing instead of starting from 1 ?

days = ['Mon', 'Tue', 'Wed', 'Thu']

# Zero-based indexing
zero_based = {val: key for key, val in enumerate(days)}
print("Zero-based indexing:", zero_based)

# One-based indexing  
one_based = {val: key + 1 for key, val in enumerate(days)}
print("One-based indexing:", one_based)

Zero-based indexing: {'Mon': 0, 'Tue': 1, 'Wed': 2, 'Thu': 3}
One-based indexing: {'Mon': 1, 'Tue': 2, 'Wed': 3, 'Thu': 4}

Conclusion

Both enumerate() and zip() + range() methods effectively convert list elements to dictionary keys with their positions as values. The enumerate() approach is more Pythonic, while zip() + range() is straightforward and easy to understand.