Why must dictionary keys be immutable in Python?

To understand why dictionary keys must be immutable. Let us related it to hash table. The hash table implementation of dictionaries uses a hash value calculated from the key value to find the key. If let’s say the key was a mutable object, its value could change, and thus its hash could also change.

As stated above, since whoever changes the key object can’t tell that it was being used as a dictionary key, it can’t move the entry around in the dictionary. Then,

• When you try to look up the same object in the dictionary it won’t be found because its hash value is different.

• If you tried to look up the old value it wouldn’t be found either, because the value of the object found in that hash bin would be different.

If you want a dictionary indexed with a list, follow these steps −

• Convert the list to a tuple first
• The function tuple(myList) creates a tuple with the same entries as the list myList.
• Tuples are immutable and can therefore be used as dictionary keys.

Convert List to Tuple

Example

To convert List to Tuple, use the tuple() −

# Creating a List
mylist = ["Jacob", "Harry", "Mark", "Anthony"]

# Displaying the List
print("List = ",mylist)

# Convert List to Tuple
res = tuple(mylist)
print("Tuple = ",res)

Output

List =  ['Jacob', 'Harry', 'Mark', 'Anthony']
Tuple =  ('Jacob', 'Harry', 'Mark', 'Anthony')

Tuple as Keys

Example

Now, we will see an example where first we convert list to tuple, then tuple is set as keys −

# Creating a List
mylist = ["Jacob", "Harry", "Mark", "Anthony"]

# Displaying the List
print("List = ",mylist)

# Convert List to Tuple
res = tuple(mylist)
print("Tuple = ",res)
dct = {2:"num","demo":"string"}
print("Dictionary = ")
print(dct)

# Overwrite existing value with a tuple
dct[2]=res
print("\nUpdated Dictionary (Tuple as Key)...")
print(dct)

Output

('List = ', ['Jacob', 'Harry', 'Mark', 'Anthony'])
('Tuple = ', ('Jacob', 'Harry', 'Mark', 'Anthony'))
Dictionary = 
{'demo': 'string', 2: 'num'}

Updated Dictionary (Tuple as Key)...
{'demo': 'string', 2: ('Jacob', 'Harry', 'Mark', 'Anthony')}