In Python, how can I make unassignable attributes (like ones marked with `final` in Java)?

Python has no equivalent of “final”. It doesn’t have “public” and “protected” either, except by naming convention. It’s not that “bondage and discipline”.

There is no such thing. In general, the Python attitude is “if you don’t want this modified, just don’t modify it”. Clients of an API are unlikely to just poke around your undocumented internals anyway.

You could, I suppose, work around this by using tuples or namedtuples for the relevant bits of your model, which are inherently immutable. That still doesn’t help with any part of your model that has to be mutable of course.

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

I would consider just leaving it mutable though.

There is no final equivalent in Python. To create read-only fields of class instances, you can use the property function, or you could do 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

Also note that while there’s @typing.final as of Python 3.8 (as Cerno mentions), that will not actually make values final at runtime.

An assign-once variable is a design issue. You design your application in a way that the variable is set once and once only.

However, if you want run-time checking of your design, you can do it with a wrapper around the object.

class OnePingOnlyPleaseVassily(object):
    def __init__(self):
        self.value = None
    def set(self, value):
        if self.value is not None:
            raise Exception("Already set.")
        self.value = value

someStateMemo = OnePingOnlyPleaseVassily()
someStateMemo.set(aValue) # works
someStateMemo.set(aValue) # fails

That’s clunky, but it will detect design problems at run time.

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

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.

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"

As of 2019 and PEP 591, Python has a Final type. It won’t be available in the standard library until the release of Python 3.8, but until then you can use it via the typing-extensions library. It won’t work as final works in Java though as Python is still a dynamically typed language. But if you use it together with a static type checker like mypy it will give you very similar benefits.

There is also a final decorator that can be applied to mark class methods as final and preventing from being overridden. Again this is only checked at “compile-time”, so you’d need to include a static type checker in your workflow.

Python 3.8 (via PEP 591) adds Final variables, functions, methods and classes. Here are some ways to use it:

@final Decorator (classes, methods)

from typing import final

@final
class Base:
    # Cannot inherit from Base

class Base:
    @final
    def foo(self):
        # Cannot override foo in subclass

Final annotation

from typing import Final

PI: Final[float] = 3.14159     # Cannot set PI to another value
KM_IN_MILES: Final = 0.621371  # Type annotation is optional

class Foo:
    def __init__(self):
        self.bar: Final = "baz"   # Final instance attributes only allowed in __init__

Please note that like other typing hints, these do not prevent you from overriding the types, but they do help linters or IDEs warn you about incorrect type usage.

As of Python 3.8, Python does have a keyword "final". However, it is a hint and is not enforced at runtime.