getattr for static/class variables

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?

Asked By: Tuxdude

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Source

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).

Answered By: Matt Anderson

Answer 2

For the first, you’ll need to create a metaclass, and define __getattr__() on that.

class MyMetaclass(type):
  def __getattr__(self, name):
    return '%s result' % name

class MyClass(object):
  __metaclass__ = MyMetaclass

print MyClass.Foo

For the second, no. Calling str(MyClass.Foo) invokes MyClass.Foo.__str__(), so you’ll need to return an appropriate type for MyClass.Foo.

Answered By: Ignacio Vazquez-Abrams

Answer 3

Surprised no one pointed this one out:

class FooType(type):
    @property
    def Foo(cls):
        return "foo!"

    @property
    def Bar(cls):
        return "bar!"

class MyClass(metaclass=FooType):
    pass

Works:

>>> MyClass.Foo
'foo!'
>>> MyClass.Bar
'bar!'

(for Python 2.x, change definition of MyClass to:

class MyClass(object):
    __metaclass__ = FooType

)

What the other answers say about str holds true for this solution: It must be implemented on the type actually returned.

Answered By: Jonas Schäfer

Answer 4

(I know this is an old question, but since all the other answers use a metaclass…)

You can use the following simple classproperty descriptor:

class classproperty(object):
    """ @classmethod+@property """
    def __init__(self, f):
        self.f = classmethod(f)
    def __get__(self, *a):
        return self.f.__get__(*a)()

Use it like:

class MyClass(object):

     @classproperty
     def Foo(cls):
        do_something()
        return 1

     @classproperty
     def Bar(cls):
        do_something_else()
        return 2

Answered By: shx2

Answer 5

Depending on the case I use this pattern

class _TheRealClass:
    def __getattr__(self, attr):
       pass

LooksLikeAClass = _TheRealClass()

Then you import and use it.

from foo import LooksLikeAClass
LooksLikeAClass.some_attribute

This avoid use of metaclass, and handle some use cases.

Answered By: geckos

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