[Python] How to Create Immutable Dictionaries & Sets in Python?

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What are Immutable Dictionaries & Sets?

Before reading this article, you should have an understanding of mutable and immutable types in python. You can look at my article that explains them pretty easy: [CS fundamentals] Mutable vs. Immutable.

Dictotionaries and sets are mutable as we can change values. For example:

d = {'key1': 'value1'}
print(d) # {'key1': 'value1'}

d['key2'] = 'value2'
d.setdefault('key3', 'value3')
print(d) # {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}

s = {'Apple', 'Orange', "Apple", "Orange", 'Kiwi'}
print(s) # {'Kiwi', 'Orange', 'Apple'}

s.add('Melon')
print(s) # {'Melon', 'Kiwi', 'Orange', 'Apple'}

As you may see, the values of d and s can be changed, which means they are mutable.

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Immutable Dictionaries and Sets

Creating Immutable Dictionary:

from types import MappingProxyType

d = {'key1': 'value1'}

# Read-only
d_frozen = MappingProxyType(d)
print(d_frozen) # {'key1': 'value1'}

d_frozen['key2'] = 'value2' # error

You can make your dictionary as immutable by using MappingProxyType function in types module.

You are making your dictionary as read-only, so that your co-workers cannot edit the sensitive data stored in your immutable dictionary.

If you try to assign a new key-value pair, it will print an error.

Traceback (most recent call last):
  File "your_file", line 14, in <module>
    d_frozen['key2'] = 'value2'
TypeError: 'mappingproxy' object does not support item assignment

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Creating Immutable Set:

s = frozenset(['Apple', 'Orange', "Apple", "Orange", 'Kiwi'])
s.add('Melon') # error

It’s very simple. You can use frozenset() to make it immutable. Just like a dictionary, immutable set is also read-only, so that the data are not changed by someone.

If you try to modify it, it will print an error:

Traceback (most recent call last):
  File "your_file", line 29, in <module>
    s6.add('Melon')
AttributeError: 'frozenset' object has no attribute 'add'