`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

you can simulate something like that through the descriptor protocol, since it allows to define reading and setting a variable the way you wish.

class Foo(object):

  @property
  def myvar(self):
     # return value here

  @myvar.setter
  def myvar(self, newvalue):
     # do nothing if some condition is met

a = Foo()
print a.myvar
a.myvar = 5 # does nothing if you don't want to

Answer 2

http://code.activestate.com/recipes/576527/ defines a freeze function, although it doesn't work perfectly.

I would consider just leaving it mutable though.

Answer 3

Python indeed does not have a final type, it does have immutable types such as tuples but that is something else.

Some of the other Answers here make classes full of pseudo final variables and I prefer my class to only have a few Final types, so I suggest using an descriptor to create the final type:

from typing import TypeVar, Generic, Type

T = TypeVar('T')

class FinalProperty(Generic[T]):
    def __init__(self, value: T):
        self.__value = value
    def __get__(self, instance: Type, owner) -> T:
        return self.__value
    def __set__(self, instance: Type, value: T) -> None:
        raise ValueError("Final types can't be set")

If you use this class like so:

class SomeJob:
    FAILED = FinalProperty[str]("Failed")

Then you will not be able to set that variable in any instance of that class. Unfortunately as with the WriteOnceReadWhenever answer you can still set the class variable.

job = SomeJob()
job.FAILED = "Error, this will trigger the ValueError"
SomeJob.FAILED = "However this still works and breaks the protection afterwards"

Answer 4

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

Answer 5

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 6

Although this is an old question, I figured I would add yet another potential option: You can also use assert to verify a variable is set to what you originally intended it to be set to – a double checking if you will. Although this is not the same as final in Java, it can be used to create a similar effect:

PI = 3.14
radius = 3

try:
    assert PI == 3.14
    print PI * radius**2
except AssertionError:
    print "Yikes."

As seen above, if PI were for some reason not set to 3.14, an AssertionError would be thrown, so a try/except block would probably be a wise addition. Regardless, it may come in handy depending on your situation.