How to get arguments list of a built-in Python class constructor?

I'm trying to use the inspect module, however it seems I can't use it on a built-in (native?) class, or else I misunderstood.

I'm using Python 2.7 and tried with Python 3.2.

This is working:

>>> import inspect
>>> class C:
...     def __init__(self,a,b=4):
...         self.sum = a + b
... 
>>> inspect.getargspec(C.__init__)
ArgSpec(args=['self','a', 'b'], varargs=None, keywords=None, defaults=(4,))

This is not working:

>>> import inspect
>>> import ast
>>> inspect.getargspec(a开发者_开发问答st.If.__init__)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/inspect.py", line 813, in getargspec
    raise TypeError('{!r} is not a Python function'.format(func))
TypeError: <slot wrapper '__init__' of '_ast.AST' objects> is not a Python function

I am wondering if there is another technique to get these parameters automatically?

(In my case, I think about an alternative which would be to parse the Python grammar, the ASDL file which explain how to init AST nodes using some code I saw in PyPy Project's source, but I'm wondering if there is another way)

There is no way to get them, as they are a property of the C code behind the function, and not of the function itself.

Note that this code works just fine on PyPy (where there is no really distinction between those two function types).

(pypy)fijal@helmut:~$ python
class C(Python 2.7.1 (f1e873c5533d, Sep 19 2011, 02:01:57)
[PyPy 1.6.0-dev1 with GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
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>>>> class C:
....  def __init__(self, a, b=4):
....    pass
.... 
>>>> import inspect
>>>> inspect.getargspec(C.__init__)
ArgSpec(args=['self', 'a', 'b'], varargs=None, keywords=None, defaults=(4,))
>>>>