New-style classes and special methods

[Lenard Lindstrom]

Raj B wrote:
>  > Yes, special methods populate the slots in the structures which Python
>  > uses to represent types.  Objects/typeobject.c in the Python source
>  > distribution does the hard work, particularly in function type_new
> 
> 
> 
> Thanks for that quick response. I am quite comfortable with C code and 
> am trying to understand exactly what happens when a new-style class is 
> created, and then instantiated.
> 
> I have been reading typeobject.c and type_new() inside it in detail, and 
> there are a few issues I am trying to figure out.
> 
> I can see a lot of *SLOT() macros in the file that seem to set the slots 
> to appropriate values. What I am having trouble figuring out is the 
> connection i.e. at what point during construction of the class object in 
> type_new() are those slots allotted? Is it the tp_alloc() function which 
> does this?

The place to start is the PyType_Type tp_new slot function type_new(). 
The second to last statement is a call to fixup_slot_dispatchers(). This 
function goes through the dictionary looking for special methods and 
adds the appropriate slot functions. This gets very involved. I had to 
use the Visual Studio debugger to follow what was happening when trying 
to figure out what happens when assigning a special method to a class 
after it is declared.

> 
> Is there some kind of descriptor or other mechanism connecting special 
> method names with their slots in the object representation? (e.g. 
> "__call__" with type->tp_call)

There is a special tp_call slot function, slot_tp_call(), that calls the 
user defined __call__. The same goes for other special methods.

Descriptors only come into play with extension types. In this case if a 
slot function is found a descriptor is added to make the slot function 
accessible from Python as a special method.

> 
> Also, what happens when a class inherits from multiple classes with 
> their own __call__ methods? Where and how  is it decided which __call__ 
> goes into the tp_call slot?
> 

As Alex Martelli mentioned, __call__ is found using the method 
resolution order. The tp_call slot function slot_tp_call() uses 
lookup_method(), a variation of PyObject_GetAttribute(), to finding the 
appropriate Python method. Its all documented in the C file.

> I'm sure I'll eventually figure it out if I stare at the code hard 
> enough, but would totally appreciate any help I can get :)
> 

Just ask.

--
Lenard Lindstrom
<len-l at telus.net>