typeerror unhashable type list python dictionary with code examples

The TypeError: unhashable type: 'list' error occurs in Python when you try to use a list as a key in a dictionary. This is because lists are mutable objects, which means that their contents can change after they are created, and therefore cannot be used as keys in dictionaries.

In Python, the keys in a dictionary must be hashable, which means they must be of an immutable type, such as a string, integer, or tuple. Hashable objects are used as keys in dictionaries because they can be used to quickly and efficiently identify the value associated with a particular key.

Here is an example of code that will result in a TypeError: unhashable type: 'list' error:

>>> my_dict = {}
>>> my_list = [1, 2, 3]
>>> my_dict[my_list] = "Hello World"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

To fix this error, you need to use an immutable object as the key in your dictionary. For example, you could use a tuple instead of a list:

>>> my_dict = {}
>>> my_tuple = (1, 2, 3)
>>> my_dict[my_tuple] = "Hello World"
>>> print(my_dict)
{(1, 2, 3): 'Hello World'}

Alternatively, you could convert the list to a string or an integer and use that as the key:

>>> my_dict = {}
>>> my_list = [1, 2, 3]
>>> my_dict[str(my_list)] = "Hello World"
>>> print(my_dict)
{'[1, 2, 3]': 'Hello World'}

>>> my_dict = {}
>>> my_list = [1, 2, 3]
>>> my_dict[hash(tuple(my_list))] = "Hello World"
>>> print(my_dict)
{-9223372041618797783: 'Hello World'}

In conclusion, when using a dictionary in Python, it is important to make sure that the keys are hashable, meaning they are of an immutable type. The TypeError: unhashable type: 'list' error occurs when you try to use a mutable object, such as a list, as a key in a dictionary. To avoid this error, you should either use an immutable object as the key or convert the mutable object to an immutable type before using it as a key.
Another way to understand hashable objects is to think of them as objects that have a unique and fixed value, like a string or an integer. For example, the string "hello world" always has the same value and will never change, so it can be used as a key in a dictionary. On the other hand, a list such as [1, 2, 3] can be changed after it is created, making its value not fixed, so it cannot be used as a key.

Tuple is also an immutable type and can be used as keys in dictionaries. Tuples are similar to lists, but unlike lists, tuples cannot be changed once they are created. This makes them ideal for use as keys in dictionaries, as their values will not change, making it possible to always locate the same value in the dictionary.

Here is an example of a dictionary with a tuple as a key:

>>> my_dict = {}
>>> my_tuple = (1, 2, 3)
>>> my_dict[my_tuple] = "Hello World"
>>> print(my_dict)
{(1, 2, 3): 'Hello World'}

It's also worth mentioning that any object can be made hashable by defining its __hash__ method and its __eq__ method. These methods allow you to define how the object should be hashed and compared to other objects, respectively.

For example, you can define a class that represents a point in two-dimensional space, and use instances of this class as keys in a dictionary:

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __hash__(self):
        return hash((self.x, self.y))

    def __eq__(self, other):
        return (self.x, self.y) == (other.x, other.y)

>>> my_dict = {}
>>> p = Point(1, 2)
>>> my_dict[p] = "Hello World"
>>> print(my_dict)
{Point(x=1, y=2): 'Hello World'}

In conclusion, the TypeError: unhashable type: 'list' error occurs in Python when you try to use a list as a key in a dictionary. To avoid this error, you should use an immutable type, such as a string, integer, or tuple, as the key in your dictionary. Alternatively, you can convert the list to an immutable type or define the __hash__ and __eq__ methods for a custom object to make it hashable and usable as a key in a dictionary.

Popular questions

  1. What does the error TypeError: unhashable type: 'list' mean in Python?

Answer: The error TypeError: unhashable type: 'list' occurs in Python when you try to use a list as a key in a dictionary. A key in a dictionary must be hashable, which means it must have a unique and fixed value, like a string or an integer. Lists are mutable and their values can change, so they cannot be used as keys in dictionaries.

  1. What are hashable objects in Python?

Answer: Hashable objects in Python are objects that have a unique and fixed value, like a string or an integer. These objects can be used as keys in dictionaries because their values will not change, making it possible to always locate the same value in the dictionary.

  1. Can tuples be used as keys in dictionaries in Python?

Answer: Yes, tuples can be used as keys in dictionaries in Python. Tuples are immutable, meaning their values cannot change once they are created, making them ideal for use as keys in dictionaries.

  1. Can a list be converted to a hashable type in Python to be used as a key in a dictionary?

Answer: Yes, a list can be converted to an immutable type, such as a tuple, in order to be used as a key in a dictionary. For example, my_tuple = tuple(my_list) will convert a list my_list to a tuple my_tuple, which can be used as a key in a dictionary.

  1. Can a custom object be made hashable in Python to be used as a key in a dictionary?

Answer: Yes, any object can be made hashable in Python by defining its __hash__ method and its __eq__ method. These methods allow you to define how the object should be hashed and compared to other objects, respectively. This makes it possible to use instances of a custom class as keys in a dictionary.

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Dictionaries

As a senior DevOps Engineer, I possess extensive experience in cloud-native technologies. With my knowledge of the latest DevOps tools and technologies, I can assist your organization in growing and thriving. I am passionate about learning about modern technologies on a daily basis. My area of expertise includes, but is not limited to, Linux, Solaris, and Windows Servers, as well as Docker, K8s (AKS), Jenkins, Azure DevOps, AWS, Azure, Git, GitHub, Terraform, Ansible, Prometheus, Grafana, and Bash.

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