What are the consequences of using a list as a key in a dictionary?

In Python, dictionary keys must be hashable (immutable). Lists are mutable data types, so they are not hashable and cannot be used as keys. If you try to use a list as a key, a TypeError: unhashable type: 'list' will be raised.

Example:

my_dict = {}
key = [1, 2, 3]
my_dict[key] = "value"  # TypeError

To use a composite key, you can apply a tuple, which is immutable and hashable:

my_dict = {}
key = (1, 2, 3)
my_dict[key] = "value"  # Works correctly