Constant instance variables?

Question

I use @property to ensure that changes to an objects instance variables are wrapped by methods where I need to.

What about when an instance has an variable that logically should not be changed? Eg, if I'm making a class for a Process, each Process instance should have a PID attribute that will frequently be accessed but should not be changed.

What's the most Pythonic way to handle someone attempting to modify that instance variable?

• Simply trust the user not to try and change something they shouldn't?

• Use property but raise an exception if the instance variable is changed?

• Something else?

Answer 1

Prepend name of the variable with __, and create read-only property, Python will take care of exceptions, and variable itself will be protected from accidental overwrite.

class foo(object):
    def __init__(self, bar):
        self.__bar = bar

    @property
    def bar(self):
        return self.__bar

f = foo('bar')
f.bar         # => bar
f.bar = 'baz' # AttributeError; would have to use f._foo__bar