`final` keyword equivalent for variables in Python?

Question

I couldn't find documentation on an equivalent of Java's final in Python, is there such a thing?

I'm creating a snapshot of an object (used for restoration if anything fails); once this backup variable is assigned, it should not be modified -- a final-like feature in Python would be nice for this.

Answer 1

Having a variable in Java be final basically means that once you assign to a variable, you may not reassign that variable to point to another object. It actually doesn't mean that the object can't be modified. For example, the following Java code works perfectly well:

public final List<String> messages = new LinkedList<String>();  public void addMessage() {     messages.add("Hello World!");  // this mutates the messages list } 

but the following wouldn't even compile:

public final List<String> messages = new LinkedList<String>();  public void changeMessages() {     messages = new ArrayList<String>();  // can't change a final variable } 

So your question is about whether final exists in Python. It does not.

However, Python does have immutable data structures. For example, while you can mutate a list, you can't mutate a tuple. You can mutate a set but not a frozenset, etc.

My advice would be to just not worry about enforcing non-mutation at the language level and simply concentrate on making sure that you don't write any code which mutates these objects after they're assigned.

Answer 2

There is no final equivalent in Python. 

But, to create read-only fields of class instances, you can use the property function.

Edit: perhaps you want something like this:

class WriteOnceReadWhenever:     def __setattr__(self, attr, value):         if hasattr(self, attr):             raise Exception("Attempting to alter read-only value")          self.__dict__[attr] = value