Question

I have a class like:

class MyClass:
     Foo = 1
     Bar = 2

Whenever MyClass.Foo or MyClass.Bar is invoked, I need a custom method to be invoked before the value is returned. Is it possible in Python? I know it is possible if I create an instance of the class and I can define my own __getattr__ method. But my scnenario involves using this class as such without creating any instance of it.

Also I need a custom __str__ method to be invoked when str(MyClass.Foo) is invoked. Does Python provide such an option?

Answer 1

__getattr__() and __str__() for an object are found on its class, so if you want to customize those things for a class, you need the class-of-a-class. A metaclass.

class FooType(type):
    def _foo_func(cls):
        return 'foo!'

    def _bar_func(cls):
        return 'bar!'

    def __getattr__(cls, key):
        if key == 'Foo':
            return cls._foo_func()
        elif key == 'Bar':
            return cls._bar_func()
        raise AttributeError(key)

    def __str__(cls):
        return 'custom str for %s' % (cls.__name__,)

class MyClass(metaclass=FooType):
    pass

# # in python 2:
# class MyClass:
#    __metaclass__ = FooType


print(MyClass.Foo)
print(MyClass.Bar)
print(str(MyClass))

printing:

foo!
bar!
custom str for MyClass

And no, an object can’t intercept a request for a stringifying one of its attributes. The object returned for the attribute must define its own __str__() behavior.

Updated 2023-02-20 for Python 3.x default implementation (python 2 as a comment).