Is this an example of tail recursion?

Chris Angelico rosuav at
Wed Aug 5 19:10:00 CEST 2015

On Thu, Aug 6, 2015 at 2:51 AM, Rustom Mody <rustompmody at> wrote:
> And I continue to have no idea what Chris is talking about.
> Here is C printf
>>>> from ctypes import *
>>>> cdll.LoadLibrary("")
>>>> libc = CDLL("")
>>>> libc.printf(b"%s", b"Hello")
> 5
> Hello>>>
> As far as I can see printf is a C function and its behaving like (an
> ill-behaved) python function as well.
> Likewise for anything else written ina C extension module
> Or a C-builtin
> If its callable from within python its python
> That it may also be C seems to me beside the point
> [As I said I dont get the point]

Sure, if it's callable from within Python. Where is this implemented in CPython?

def f(x): return x+2


There's PyNumber_Add() in abstract.c, which looks for the nb_add slot.
That contains a pointer to long_add, which is defined in longobject.c.
Is that the same thing as (1).__add__? Not really, but that's kinda
what implements the underlying operation. Also, the function is
declared as 'static', so I don't think you can find it using ctypes.

Adding two Python objects is *not* a function call. It is an
operator-controlled action. It's very similar, in many ways, to a
method call, but it isn't exactly that, and it certainly isn't the
sort of thing that you could tail-call-optimize as the concept applies
only to cases where you can actually replace a stack frame.


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