创建类的静态属性（属性是同一类的实例）

Question

Let's say I have a class defined as follow :

class Foo():
   baz = None
   def __init__(self, bar):
       self.bar = bar

Now, in that example Foo.baz is None . Now let's say that this class attribute needs to be an instance of Foo like below:

class Foo():
    baz = Foo("baz")
    def __init__(self, bar):
        self.bar = bar

How would I proceed?

Similarly, is there any way to create a "class property". I know I can assign a lambda function returning a new instance of the class to a class attribute, but I'd rather not have to write the parenthesis.

Answer 1

If you want to use the line baz = Foo("baz") inside a class even before defining Foo earlier; it's not possible ie Python will throw a NameError: name 'Foo' is not defined at you, but here's a workaround to achieve what you intend:

class Foo:
   def __init__(self, bar):
       self.bar = bar
Foo.baz=Foo('foo')

Now Foo.baz is an instance of the Foo class and also baz is a class attribute of the Foo class and hence will be inherited by all instances of the class as well:

myFoo = Foo('myFoo')

You can verify that myFoo.baz.bar is infact 'foo' and not 'myFoo'

print(myFoo.bar) 
# myFoo
print(myFoo.baz.bar)
# foo

Also note that this will cause Foo.baz , Foo.baz.baz , Foo.baz.baz.baz etc. all to be a Foo object because Foo.baz has value Foo('foo') ie an object of Foo class and baz is also a class attribute of Foo .