Using Python Metaclasses to Limit the Number of Attributes

Question

I trying to define a Python metaclass to limit the number of attributes that a class may contain, starting at creation time. I am constrained to use Python 3.7 due to system limitations

I managed to do it without a metaclass:

class MyClass:
    def __init__(self):
        self.param1 = 7
        self.param2 = 8

    def __setattr__(self, key, value):
        if key not in self.__dict__ or len(self.__dict__.keys()) < 2:
           self.__dict__[key] = value

        raise ValueError("Excess of parameters.")

test = MyClass()

test.param1 = 3
test.param2 = 5

print(test.param1)      # Prints 3
print(test.param2)      # Prints 5

test.param3 = 9         # RAISES AN ERROR, AS IT SHOULD!

The problem that I am having is that I want to do this through a metaclass. Sure, I bet that there can be better options, but I find that, in this case, the best option is to do it through a metaclass (?)

I have read several posts and this is the closest Q&A I have found to solving the problem.

Question

Is there a way to limit the number of attributes of a class to a specific number, at creation time, using metaclasses?

Maybe use the dunder method __settattr__ to use the super class constructor?

Answer 1

If you really want to do it using metaclasses, you can, but I would recommend against it, because a class decorator suffices, and you can chain class decorators in future, whereas you cannot do so with metaclasses. Using a class decorator also frees up the possibility of using a metaclass in future.

With that in mind, here are the solutions I've cooked up. All code is tested and confirmed to function as intended on Python 3.7.3.

# A solution using a class decorator (recommended solution)
def limited_attributes(number_attributes):
    def __setattr__(self,key,value):
        if key not in self.__dict__ and len(self.__dict__.keys())>=number_attributes:
            raise ValueError('Too many attributes')
        self.__dict__[key]=value
    def decorator(cls):
        cls.__setattr__=__setattr__
        return cls
    return decorator

@limited_attributes(2)
class Foo:
  def __init__(self):
    self.param1=4
    self.param2=5

test = Foo()
test.param1=3 # no error
test.param2='bar' # no error

test.param3='baz' # raises ValueError

# A solution using a function as a metaclass
def limited_attributes(number_attributes):
    def __setattr__(self,key,value):
        if key not in self.__dict__ and len(self.__dict__.keys())>=number_attributes:
            raise ValueError('Too many attributes')
        self.__dict__[key]=value
    def metaclass(name,bases,attrs):
        cls=type(name,bases,attrs)
        cls.__setattr__=__setattr__
        return cls
    return metaclass

class Foo(metaclass=limited_attributes(2)):
  def __init__(self):
    self.param1=4
    self.param2=5

test = Foo()
test.param1=3 # no error
test.param2='bar' # no error

test.param3='baz' # raises ValueError