[capi-sig] Constructing variable arg tuples

[Hrvoje Niksic]

Todd Lyons <tlyons at ivenue.com> writes:

> Hi all, I'm embedding python into a c app (exim, the MTA).  I've
> gotten code in to initialize python, read in the embedded script, call
> various functions within that script, and appropriately return data
> from those functions back to the exim call.
>
> One thing that does not make sense yet though, is how to assemble the
> data to be passed to the python functions.  Imagine the following
> simple function:
>
> def simple_math(a,b):
>      return a+b
>
> In this case, a and b are integers or long integers.  How can I format
> that incoming data without knowing in advance that there are two
> arguments to this function and that are both are integers?

Obviously the scripts and your embedding module need to agree on an API;
for example, the script could look like this:

import exim

exim.modify_configuration(...)
x = exim.query_something(...)
exim.add_callback(my_function)
...

The "exim" module is provided by you as the embedder and serves for the
script to access exim's functionality.  (Otherwise the script would be
executed as just another Python script, which would not be too useful
from the embedding standpoint.)

In other words, you're not calling the functions from the script: the
script is calling your functions.  The only time that you are calling a
script's function is when the script passes a function to one of your
functions that accepts a callback argument -- but even then the
interface of the function is regulated by you, and it is you who
prescribe how many and what kind of arguments it will accept.

> If the reason for this question is not clear, imagine that the
> sysadmin goes and edits the python script so that it now is:
>
> def simple_math(name,a,b):
>      // do something to log "name" to a logfile
>      return a+b

What would be the point of such a function, before or after the edit?
You cannot just call any random function from a script: you need to
execute the script, providing a way for the script to call *you*.