How to define class field in Python that is an instance of a class?

Question

I want to get a specific class instance in the class's namespace. In C# this would look like this:

public struct Foo
{
    public readonly static Bar = new Foo();
}

The only idea I have is to assign an instance right after the class definition (monkeypatch):

class Foo:
    def __init__(self, spam):
        self.spam = spam
Foo.bar = Foo(42)

But I want to provide instance in class definition, like this:

class Foo:
    ...
    bar = Foo(42)

And interface like this:

from foo import Foo
bar = Foo.bar

The last line in definition gives a syntax error because Foo is not yet defined. Is there a way to overcome this constraint except monkeypatching class?

Answer 1

The requested functionality can be achieved with custom descriptor object (doc):

class static_property:
    def __init__(self, getter):
        self.__getter = getter

    def __get__(self, obj, objtype):
        return self.__getter(objtype)

    @staticmethod
    def __call__(getter_fn):
        return static_property(getter_fn)

class A:
    _bar = None

    def __init__(self, spam):
        self.spam = spam

    @static_property
    def bar(cls):
        if cls._bar is None:
            cls._bar = A(10)
        return cls._bar

print('A.bar={} A.bar.spam={}'.format(A.bar, A.bar.spam))

a = A(20)
print('a.bar={} a.bar.spam={} a.spam={}'.format(a.bar, a.bar.spam, a.spam))

Prints:

A.bar=<__main__.A object at 0x7f0ab5e41eb8> A.bar.spam=10
a.bar=<__main__.A object at 0x7f0ab5e41eb8> a.bar.spam=10 a.spam=20

Answer 2

If you want to use Metaclasses for this then you can use following. I indeed don't know why you want to achieve this, or whether the method I present is good, but nevertheless it works well.

class Meta(type):
    def __new__(cls, name, bases, dct):
        print(cls,name, bases, dct)
        x = super(Meta, cls).__new__(cls, name, bases, dct)
        x.bar = x(46) //create the bar for the Foo
        return x




class Foo:
    __metaclass__ = Meta
    def __init__(self, spam):
        self.spam = spam

What this does is simply create a bar named attribute for the Foo while creating the object for the class. I guess the code is self explanatory