How to use a list as a dictionary key in Python 3?

Dictionaries are one of the most powerful data structures in the Python programming language. This is a data structure consisting of key-value pairs. It has several advantages; for example, accessing values ​​has a time complexity of O(1), it is memory efficient, easy to update, delete, and iterate, and provides many built-in functions for fast operations.

Problems that occur when using lists directly

We focus on this topic because problems arise when we try to use lists as keys. Lists are mutable data types in Python. So we can delete, update and append values ​​inside the list. So if we generate a hash function from a list and list items, when the items of the list change, we will no longer be able to find the hash function because the hash function has changed.

Another potential problem is that different lists may have the same hash value. If the sum of the values ​​of two lists is the same, their hash values ​​will also be the same. In this case, if one of the lists is used as a key in the dictionary, and another list with the same hash is searched, the dictionary may give incorrect results.

Convert The List To Tuple

One way to convert a dictionary's list of keys into a tuple and use them as keys is to use this indirection. Note that although the values ​​remain the same, the data type is no longer a list

grammar

<variable name> = tuple(<list to convert into tuple>)

Here the tuple takes one parameter, which is the list's name.

Example

In the following code, we first created an empty dictionary named my_dict. Next, we created our list named my_list. We used the tuple method to convert the list into a Tuple object. We now used the Tuple object as the key as a string "hello, world!" as the value.

my_dict = {}
my_list = [1, 2, 3]
my_tuple = tuple(my_list)
my_dict[my_tuple] = "Hello, World!"
print(my_dict)

Output

{(1, 2, 3): 'Hello, World!'}

Convert list to string

Another way is to convert the list to string data type. String is also an immutable data type; therefore, we can use it as a key of a dictionary.

grammar

<variable name> = ''.join(str(e) for e in <name of the list>)

The statements here get individual elements from the list and combine them into a string. The Join function joins sequences of strings.

Example

In the following code, we first created a list named my_list. Next, we used the join method that creates a string of list elements. Since the string can be used as the dictionary key, we used this as te key of the dictionary my_list_str.

my_list = [1, 2, 3]
my_list_str = ''.join(str(e) for e in my_list)
my_dict = {my_list_str: 'value'}
print(my_dict)
print(my_dict[my_list_str])

Output

{'123': 'value'}
value

Convert list to JSON

We can also use the JSON module and built-in functions to convert the list first to string using the dumps method and later use this as the dictionary's key.

grammar

<name of variable< = json.dumps(<name of list<)

Here is the dumps method of the JSON library, which serializes Python objects into JSON-formatted strings. The dumps method accepts the name of the string as a parameter

Example

In the code below, we first import the JSON library. Next, we create a list called my_list. We created a serialized object from the list using the dumps method. Now, we have the serialized object as dictionary key my_dict.

import json
my_list = [1, 2, 3]
my_key = json.dumps(my_list)
my_dict = {my_key: 'value'}
print(my_dict)

Output

{'[1, 2, 3]': 'value'}

Conclusion

In this article, we learned how to use lists as the key of a dictionary in Python 3. We understood the difficulties we faced if we directly tried to make the lists as the keys in the Python dictionary. Therefore we first need to convert the list data type to tuples, strings, etc., which are immutable data types. We also learned how to use the JSON module to use lists as the dictionary key.

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