Immutable vs. Mutable Types in Object-Oriented Programming: What's the Difference?

Understanding Immutable vs. Mutable Types

In object-oriented programming, objects can be categorized into two types based on their ability to change: immutable and mutable. The distinction between the two lies in their internal structure and how they handle assignment and modification.

Immutable Types: Preserving Value within Objects

Immutable types are objects whose values cannot be altered once created. Float, for instance, is considered immutable. Creating a new instance of the RoundFloat class, as seen in the example provided, does not affect the original float object. This is because the new instance is a distinct object with its own unique memory location.

Immutable types maintain their original state even when methods operate on them. For example, extending a string object (dict) or passing integers to the RoundFloat class does not modify the original object. Instead, a new object is created with the changes.

Mutable Types: Dynamic Changes within Objects

Mutable types, on the other hand, can be altered both internally and through assignment. As mentioned in the question, the example of SortedKeyDict_a illustrates an object that allows internal modification via its method example().

Passing a set to SortedKeyDict without invoking the method does not trigger an error because the original set remains unchanged. However, using integers in RoundFloat with new raises no errors because the class structure allows for modifications.

In-place Modification vs. Reassignment of References

The crucial difference between immutable and mutable types lies in their behavior during assignment and modification. Immutable types preserve their state by altering the object's reference, while mutable types modify the object itself.

Through examples using strings, lists, functions, and variables, the answer highlights that immutable types maintain their initial identity despite modifications to assigned variables, while mutable types exhibit in-place changes and affect assigned variables.

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