Python: Can a class forbid clients setting new attributes?

Question

I just spent too long on a bug like the following:

>>> class Odp():
    def __init__(self):
        self.foo = "bar"


>>> o = Odp()
>>> o.raw_foo = 3 # oops - meant o.foo

I have a class with an attribute. I was trying to set it, and wondering why it had no effect. Then, I went back to the original class definition, and saw that the attribute was named something slightly different. Thus, I was creating/setting a new attribute instead of the one meant to.

First off, isn't this exactly the type of error that statically-typed languages are supposed to prevent? In this case, what is the advantage of dynamic typing?

Secondly, is there a way I could have forbidden this when defining Odp, and thus saved myself the trouble?

Answer 1

You can implement a __setattr__ method for the purpose -- that's much more robust than the __slots__ which is often misused for the purpose (for example, __slots__ is automatically "lost" when the class is inherited from, while __setattr__ survives unless explicitly overridden).

def __setattr__(self, name, value):
  if hasattr(self, name):
    object.__setattr__(self, name, value)
  else:
    raise TypeError('Cannot set name %r on object of type %s' % (
                    name, self.__class__.__name__))

You'll have to make sure the hasattr succeeds for the names you do want to be able to set, for example by setting the attributes at a class level or by using object.__setattr__ in your __init__ method rather than direct attribute assignment. (To forbid setting attributes on a class rather than its instances you'll have to define a custom metaclass with a similar special method).