how to know argument name with which a function of extended c called

John Machin sjmachin at lexicon.net
Wed Apr 15 10:52:51 CEST 2009


On Apr 15, 6:13 pm, rahul <rahul03... at gmail.com> wrote:
> On Apr 14, 6:24 pm, John Machin <sjmac... at lexicon.net> wrote:
>
>
>
> > On Apr 14, 10:35 pm, rahul <rahul03... at gmail.com> wrote:
>
> > > Hi,
> > >   i need to write a 'c extension function' in this function i need to
> > > change argument value with  which this function called.
>
> > The appropriate way for a function to give output is to return a
> > value, or a tuple of values.
>
> > example:
>
> > def get_next_token(input_buffer, offset):
> >    """get next lexical token, starting at offset
> >       return (the_token, new offset)"""
> >    length = find_len_of_token_somehow(input_buffer, offset)
> >    new_offset = offset + length
> >    return input_buffer[offset:new_offset], new_offset
>
> > and you can call it by
> >    token, pos = get_next_token(buff, pos)
> >    return input
>
> > >   ie,
> > >          if a python code like
> > >             import changeValue as c
> > >             arg="old value"
> > >             c.changeValue(arg)
> > >             print arg
>
> > Fortunately, you can't construct such a thing in Python or in a C
> > extension. Consider the following:
>
> > print "two", 2
> > c.changeValue(2)
> > print "two maybe", 2
>
> > What would you want to it to print the second time?
> > two maybe new value?
>
> > >  then it print "new value"
>
> > >  i write code like this..
>
> > > static PyObject *changeValue(PyObject *self,PyObject *args){
> > >         PyObject *sampleObj, *m ;
> > >         char *argName;
>
> > >       if (!PyArg_ParseTuple(args, "O", &sampleObj)){
> > >                 return NULL;
> > >       }
>
> > >    m = PyImport_AddModule("__main__");
>
> > This means you are assuming/hoping this function will be called only
> > from the main script ...
>
> > >    PyObject_SetAttrString(m, argName, "new value");
>
> > Even if you know the name, you have the problem that it is changing
> > the __main__ module's globals ... but the arg could be local or it
> > could be an expression ...
>
> > >    return Py_BuildValue("");
>
> > > }
>
> > > But for this i need to know the argument name with which this function
> > > called .
> > > Is this possible to know argument name in extended c function? if yes,
> > > than how i can do it???
>
> > No, it's not possible to know the argument name (without help from the
> > caller e.g. keyword args), it may not even have a name, it may have
> > multiple names ... this is just another variation of the old "what is
> > the name of my object" FAQ.
>
> > Why don't you tell us what you are trying to achieve (a higher-level
> > goal than "I need to poke some value at/into some variable of doubtful
> > name and unknowable location"), and then we might be able to give you
> > some ideas.
>
> > HTH,
> > John
>
> Hi John,
>    thanks for your great full information,
>      But, In my project anyhow i have to change argument value in some
> cases.

I say again: You can't do that. If you care to explain AT A HIGHER
LEVEL than "change argument value" what you are trying to achieve,
then some help might be available.

If you are translating from C to Python and you have a function that
is expected to be called like func(in1, &out2, &inout3) then you will
have to write the function as
   def func(in1, inout3):
      # calculate out2
      # calculate new_inout3
      return out2, new_inout3
(or the equivalent in a C-extension) and the caller will need to be
changed to do
   out2, inout3 = func(in1, inout3)

Alternatively, leave the function in C and provide a wrapper function
in C that bridges the different argument-passing conventions. There
are various options for doing this -- the Cython/boost/ctypes/etc
gurus can help with this once you've explained what your real
requirement is.

> can we pass pointer  of an variable in extended c function.

Python doesn't have "variables". It has objects, which have zero, one,
or many names. The names exist in namespaces of various types: local
to a function/method, attribute of a class, attribute of a class
instance, global to a module, ... there is no unified "address"
concept; the concept "&arg" has no meaning and no means of realising
it as a vehicle for doing the equivalent of C's "*pointer =
new_value".

HTH,
John



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