Override Python metaclass __getitem__

ghz 17hours ago ⋅ 8 views

Given the following code, I expect a return value of 'overridden' not 'value1':

class MyMetaclass(type):
  def __new__(cls, name, bases, attrs):
    attrs.update({'_my_dict': {'key1': 'value1', 'key2': 'value2'}})
    return super().__new__(cls, name, bases, attrs)
  def __getitem__(cls, value):
    return cls._my_dict[str(value)]


class MyBaseClass(metaclass=MyMetaclass):
  pass


class MyClass(MyBaseClass):
  @classmethod
  def __getitem__(cls, value):
    return 'overridden'


>>> MyClass['key1']
'value1'  # I expect: 'overridden'

What do I need to change?

Answer

The issue you're encountering stems from the way Python's metaclass mechanism works in your code. In Python, when you define a class with a metaclass, the metaclass is responsible for constructing the class. So, your MyMetaclass is setting up the _my_dict attribute on the class, and the __getitem__ method on MyClass is getting overridden, but the metaclass's __getitem__ method is still in effect. This results in the method from the metaclass being called instead of the one in MyClass.

To fix this and ensure that MyClass['key1'] returns 'overridden', you need to make sure that the metaclass's __getitem__ method does not override the __getitem__ method of MyClass. One way to achieve this is by checking if the class has its own __getitem__ method before falling back to the one in the metaclass.

Here’s how you can modify the code:

Solution

class MyMetaclass(type):
    def __new__(cls, name, bases, attrs):
        attrs.update({'_my_dict': {'key1': 'value1', 'key2': 'value2'}})
        return super().__new__(cls, name, bases, attrs)
    
    def __getitem__(cls, value):
        # Ensure that the class's own __getitem__ method is used first
        if '__getitem__' in cls.__dict__:
            return cls.__dict__['__getitem__'](value)
        return cls._my_dict[str(value)]


class MyBaseClass(metaclass=MyMetaclass):
    pass


class MyClass(MyBaseClass):
    @classmethod
    def __getitem__(cls, value):
        return 'overridden'


# Testing
print(MyClass['key1'])  # Should print 'overridden'

Explanation

  • __getitem__ check: Inside the __getitem__ method of MyMetaclass, it checks whether MyClass (or any class that uses MyMetaclass) has its own __getitem__ method. If it does, that method is used. If not, the metaclass's __getitem__ method defaults to accessing the _my_dict attribute.

With this change, when you call MyClass['key1'], it will invoke the __getitem__ method of MyClass, which returns 'overridden', as you expected.

#py