python: how to refer to the class from within it ( like the recursive function)

Question

For a recursive function we can do:

def f(i):
  if i<0: return
  print i
  f(i-1)

f(10)

However is there a way to do the following thing?

class A:
  # do something
  some_func(A)
  # ...

Answer 1

If you want to refer to the same object, just use 'self':

class A:
    def some_func(self):
        another_func(self)

If you want to create a new object of the same class, just do it:

class A:
    def some_func(self):
        foo = A()

If you want to have access to the metaclass class object (most likely not what you want), again, just do it:

class A:
    def some_func(self):
        another_func(A) # note that it reads A, not A()

Answer 2

Most code in the class will be inside method definitions, in which case you can simply use the name A.

Answer 3

What do you want to achieve? It's possible to access a class to tweak its definition using a metaclass, but it's not recommended.

Your code sample can be written simply as:

class A(object):
    pass
some_func(A)

Answer 4

There isn't a way to do that within the class scope, not unless A was defined to be something else first (and then some_func(A) will do something entirely different from what you expect)

Unless you're doing some sort of stack inspection to add bits to the class, it seems odd why you'd want to do that. Why not just:

class A:
    # do something
    pass

some_func(A)

That is, run some_func on A after it's been made. Alternately, you could use a class decorator (syntax for it was added in 2.6) or metaclass if you wanted to modify class A somehow. Could you clarify your use case?

Answer 5

Nope. It works in a function because the function contents are executed at call-time. But the class contents are executed at define-time, at which point the class doesn't exist yet.

It's not normally a problem because you can hack further members into the class after defining it, so you can split up a class definition into multiple parts:

class A(object):
    spam= 1

some_func(A)

A.eggs= 2

def _A_scramble(self):
    self.spam=self.eggs= 0
A.scramble= _A_scramble

It is, however, pretty unusual to want to call a function on the class in the middle of its own definition. It's not clear what you're trying to do, but chances are you'd be better off with decorators (or the relatively new class decorators).

Answer 6

It's ok to reference the name of the class inside its body (like inside method definitions) if it's actually in scope... Which it will be if it's defined at top level. (In other cases probably not, due to Python scoping quirks!).

For on illustration of the scoping gotcha, try to instantiate Foo:

class Foo(object):
    class Bar(object):
        def __init__(self):
            self.baz = Bar.baz
        baz = 15
    def __init__(self):
        self.bar = Foo.Bar()

(It's going to complain about the global name 'Bar' not being defined.)

---

Also, something tells me you may want to look into class methods: docs on the classmethod function (to be used as a decorator), a relevant SO question. Edit: Ok, so this suggestion may not be appropriate at all... It's just that the first thing I thought about when reading your question was stuff like alternative constructors etc. If something simpler suits your needs, steer clear of @classmethod weirdness. :-)

Answer 7

You can't with the specific syntax you're describing due to the time at which they are evaluated. The reason the example function given works is that the call to f(i-1) within the function body is because the name resolution of f is not performed until the function is actually called. At this point f exists within the scope of execution since the function has already been evaluated. In the case of the class example, the reference to the class name is looked up during while the class definition is still being evaluated. As such, it does not yet exist in the local scope.

Alternatively, the desired behavior can be accomplished using a metaclass like such:

class MetaA(type):

    def __init__(cls):
        some_func(cls)

class A(object):
    __metaclass__=MetaA
  # do something
  # ...

Using this approach you can perform arbitrary operations on the class object at the time that the class is evaluated.

Answer 8

If you want to do just a little hacky thing do

class A(object):
    ...
some_func(A)

If you want to do something more sophisticated you can use a metaclass. A metaclass is responsible for manipulating the class object before it gets fully created. A template would be:

class AType(type):
    def __new__(meta, name, bases, dct):
        cls = super(AType, meta).__new__(meta, name, bases, dct)
        some_func(cls)
        return cls

class A(object):
    __metaclass__ = AType
    ...

type is the default metaclass. Instances of metaclasses are classes so __new__ returns a modified instance of (in this case) A.

For more on metaclasses, see http://docs.python.org/reference/datamodel.html#customizing-class-creation.

Answer 9

In Python you can NOT reference to the class in the class body, in languages like Ruby you can do that.

In python instead you could use class decorator but that will be called once the class has initialized, Other way could be to use metaclass but it all depends on what you are trying to achieve.

But short answer is you can not reference to the class in the class body.