lazy class variables

create_lazy_var.py

# first, subclass property:

class ClassProperty(property):
    def __get__(self, cls, owner):
        return self.fget.__get__(None, owner)()

# then, define create_lazy_var

def create_lazy_var(cls,name, func):
    private = '_'+name
    setattr(cls,private,None)

    def get_var(cls):
        if getattr(cls,private) is None:
            setattr(cls,private,func(cls))
        return getattr(cls,private)
    get_var = classmethod(get_var)
    setattr(cls,name,ClassProperty(get_var))

## example usage

class A(object): pass

create_lazy_var(A,'x',lambda cls: 1)

assert A._x is None
assert A.x == 1
assert A._x == 1