Python Coding challenge - Day 267 | What is the output of the following Python Code?

 

Code Explanation:

my_dict = {}

my_dict[[1, 2, 3]] = "value"

print(my_dict)

Step-by-Step Explanation:

Dictionary Creation:

my_dict = {}

An empty dictionary my_dict is created. At this point, it contains no key-value pairs.

Attempt to Add a Key-Value Pair:

my_dict[[1, 2, 3]] = "value"

Here, you are trying to use a list [1, 2, 3] as a key in the dictionary.

In Python, dictionary keys must be hashable (immutable and capable of producing a consistent hash value). Lists, however, are mutable and therefore not hashable.

As a result, this operation raises a TypeError with the message:

TypeError: unhashable type: 'list'.

Print Statement:

print(my_dict)

This line is never executed because the code raises an error at the previous step.

Why Lists Cannot Be Keys:

Lists in Python are mutable, meaning their contents can change (e.g., adding/removing elements). If lists were allowed as dictionary keys, the hash value of the key could change after it was added to the dictionary, leading to unpredictable behavior.

To fix this, you could use an immutable tuple instead of a list as the key:

my_dict[(1, 2, 3)] = "value"

print(my_dict)

Output:

{(1, 2, 3): 'value'}

Final Outcome:

The original code raises a TypeError because lists are not hashable and cannot be used as dictionary keys.