Properties of Dictionary Keys in Python

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, the same is not true for dictionary keys.

There are two important points to remember about dictionary keys:

No Duplicate Keys Allowed

More than one entry per key is not allowed. When duplicate keys are encountered during assignment, the last assignment wins ?

# Duplicate keys - last value overwrites previous ones
student = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'}
print("student['Name']:", student['Name'])
print("Full dictionary:", student)

student['Name']: Manni
Full dictionary: {'Name': 'Manni', 'Age': 7}

As you can see, the first 'Name': 'Zara' entry was overwritten by 'Name': 'Manni'.

Keys Must Be Immutable

Dictionary keys must be immutable (hashable) objects. You can use strings, numbers, or tuples as keys, but mutable objects like lists are not allowed ?

Valid Immutable Keys

# Valid keys: strings, numbers, tuples
valid_dict = {
    'string_key': 'value1',
    42: 'value2',
    (1, 2): 'value3',
    3.14: 'value4'
}
print("Valid dictionary:", valid_dict)

Valid dictionary: {'string_key': 'value1', 42: 'value2', (1, 2): 'value3', 3.14: 'value4'}

Invalid Mutable Keys

Attempting to use a mutable object like a list as a key will raise a TypeError ?

# This will cause an error
invalid_dict = {['Name']: 'Zara', 'Age': 7}
print("This won't work")

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Why Keys Must Be Immutable

Python uses a hash table implementation for dictionaries. Hash tables require that keys produce a consistent hash value. If a key could change (like a list), its hash would change, making it impossible to retrieve the stored value.

# Demonstrating hash values
print("Hash of string:", hash('key'))
print("Hash of number:", hash(42))
print("Hash of tuple:", hash((1, 2, 3)))

# Lists don't have hash values
try:
    print("Hash of list:", hash([1, 2, 3]))
except TypeError as e:
    print("Error:", e)

Hash of string: -6208160604277157117
Hash of number: 42
Hash of tuple: 2528502973977326415
Error: unhashable type: 'list'

Comparison of Key Types

Conclusion

Dictionary keys must be unique and immutable. Use strings, numbers, or tuples as keys. Attempting to use mutable objects like lists will raise a TypeError due to their unhashable nature.