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

[rahul]

Hi,
i need to write a 'c extension function' in this function i need to
change argument value with which this function called.
ie,
if a python code like
import changeValue as c
arg="old value"
c.changeValue(arg)
print arg

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__");
PyObject_SetAttrString(m, argName, "new value");
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???

 

[John Machin]

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

 

[Arnaud Delobelle]

Hi,
i need to write a 'c extension function' in this function i need to
change argument value with which this function called.
ie,
if a python code like
import changeValue as c
arg="old value"
c.changeValue(arg)
print arg

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__");
PyObject_SetAttrString(m, argName, "new value");
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, you can't and it's a good thing, because when reading your code you
can find out very easily when a variable has been rebound: it's on those
lines of the form

name = expression

Instead, make your 'changeValue" function return the new value, then
write:

import changeValue as c
arg = "old value"
arg = c.changeValue(arg)
print arg

It'll work, with the added benefit that it'll be clearer that arg was
changed.

 

[rahul]

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

like
import changeValue as c
arg="old value"
c.changeValue(&arg)
print arg

if yes, then how pointer of this variable handle through extended c
function. and how we can change the value through pointer.

 

[John Machin]

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

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?

This means you are assuming/hoping this function will be called only
from the main script ...

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 ...

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

 

[Aaron Brady]

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

There is a limited subset of cases where a function can modify its
argument. The first of course is with mutable types, but the function
would not operate successfully on future types. If it only needs to
handle one mutable type, then it's straightforward.

The second is with the 'ctypes.byref' function, which only accepts
ctypes types and structures, but can perform the swap, again, if the
types are known.

The generic solution is to just retrieve your value from the same
place in a container every time, i.e. 'Globals.val1', which you can
read and write at will. The container and attribute can be determined
at run-time (untested):

def swap( namespace, name1, name2 ):
temp= getattr( namespace, name1 )
setattr( namespace, name1, getattr( namespace, name2 ) )
setattr( namespace, name2, temp )