constructor of subclass and base's tp_new()

[Greg Ewing (using news.cis.dfn.de)]

Maciej Kalisiak wrote:
>      static PyObject*
>      noddy_new_noddy(PyObject* self, PyObject* args)
>      {
>          noddy_NoddyObject* noddy;
>      
>          if (!PyArg_ParseTuple(args,":new_noddy"))
>              return NULL;
>      
>          noddy = PyObject_New(noddy_NoddyObject, &noddy_NoddyType);
>      
>          return (PyObject*)noddy;
>      }
> 
> What is the role of `self' in *this* factory function??

Nothing. It's there simply because it's part of the
signature of every C-implemented method or function.
In this case, it's unused (it's a function, not a
method, so it gets called with self == NULL).

> ,----
> | from State import State
> | class StateOfSize5(State):
> |     def __init__(self):
> |         State.__init__(self, 5)
> |     
> | s = State(5)		# this runs OK
> | s = StateOfSize5()	# ERROR
> | s = StateOfSize5(5)   # a workaround, but undermines the purpose of subclass
> `----
> 
> The error I get is:
>   TypeError: state_new() takes exactly 1 argument (0 given)

That's because everything you pass to the constructor gets
passed to __new__ as well as __init__.

One way to deal with this is to make your tp_new
accept any number of arguments, and ignore the ones it's
not interested in. That's what object.__new__ seems to
do, for instance.

Otherwise, whenever a subclass overrides __init__ to have
a different number of arguments, it will have to override
__new__ correspondingly.

You have to do this with some of the builtin types, e.g.


 >>> class S(str):
...  def __init__(cls, x, y):
...   print x, y
...
 >>> s = S(55, 66)
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
TypeError: str() takes at most 1 argument (2 given)

 >>> class S(str):
...  def __init__(cls, x, y):
...   print x, y
...  def __new__(cls, x, y):
...   return str.__new__(cls, x)
...
 >>> s = S(55, 66)
55 66
 >>> s
'55'

-- 
Greg Ewing, Computer Science Dept,
University of Canterbury,	
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg