TypeError: unhashable type: 'list'

TypeError: unhashable type: 'list' usually means that you are trying to use a list as an hash argument. This means that when you try to hash an unhashable object it will result an error. For ex. when you use a list as a key in the dictionary , this cannot be done because lists can't be hashed. The standard way to solve this issue is to cast a list to a tuple .

example

 
my_dict = {'name': 'John', [1,2,3]:'values'}
print(my_dict)

output

 
Traceback (most recent call last):
  File "sample.py", line 3, in <module>
    my_dict = {'name': 'John', [1,2,3]:'values'}
TypeError: unhashable type: 'list'

This error shows that the my_dict key [1,2,3] is List and List is not a hashable type in Python . Dictionary keys must be immutable types and list is a mutable type.

Fix: Cast list to a tuple

You'll have to change your list into tuples if you want to put them as keys in your dictionary .

 
my_dict = {'name': 'John', tuple([1,2,3]):'values'}
print(my_dict)

output

 
{'name': 'John', (1, 2, 3): 'values'}

The hash() is a built-in python method, used to return a unique number . This can be applied to any user-defined object which won’t get changed once initialized. This property is used mainly in dictionary keys .

Examples of hashable objects:

 
int, float, decimal, complex, bool, string, tuple, range, frozenset, bytes

Examples of Unhashable objects:

 
list, dict, set, bytearray, user-defined classes

Tuple and List

Though tuples may seem similar to lists, they are often used in different situations and for different purposes. Tuples are immutable , and usually contain an heterogeneous sequence of elements that are accessed via unpacking or indexing . Lists are mutable, and their elements are usually homogeneous and are accessed by iterating over the list.

Hashing

Hashing is a concept in computer science which is used to create high performance , pseudo random access data structures where large amount of data is to be stored and accessed quickly. Immutable objects , or objects that can't be altered, are hashable and they have a single unique value that will never change. A hashing function is a function which takes an object, say a string such as "Java" and returns a fixed-size code, assume the return value is an integer .

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