PyEval_EvalCodeEx return value

[Mateusz Loskot]

Hi,

I'm trying to dig out details about what exactly is the return
value the of PyEval_EvalCodeEx function in Python 3.x
The documentation is sparse, unfortunately.

Perhaps I'm looking at wrong function.
My aim is simple, I need to execute Python code using Python interpreter
embedded in my C++ application.
The Python code is a simple script that always returns single value.
For example:

#! /usr/bin/env python
def foo(a, b):
return a + b
f = foo(2, 3)

But, f can be of different type for different script: one returns
numeric value, another returns a sequence, so the type is not
possible to be determined in advance.

I know how to capture Python stdout/stderr.

I also know how to access the "f" attribute using
PyObject_GetAttrString and then I can convert "f" value to C++ type
depending on PyObject type.

However, I guess there shall be a way to access "f" value
directly from PyEval_EvalCode return object:

PyObject* evalRet = :yEval_EvalCode(...);

But, I can't find any details what the "evalRet" actually is.

Any pointers would be helpful.

Best regards,

 

[John Pinner]

Hi,

I'm trying to dig out details about what exactly is the return
value the of PyEval_EvalCodeEx function in Python 3.x
The documentation is sparse, unfortunately.

Perhaps I'm looking at wrong function.
My aim is simple, I need to execute Python code using Python interpreter
embedded in my C++ application.
The Python code is a simple script that always returns single value.
For example:

#! /usr/bin/env python
def foo(a, b):
    return a + b
f = foo(2, 3)

But, f can be of different type for different script: one returns
numeric value, another returns a sequence, so the type is not
possible to be determined in advance.

I know how to capture Python stdout/stderr.

I also know how to access the "f" attribute using
PyObject_GetAttrString and then I can convert "f" value to C++ type
depending on PyObject type.

However, I guess there shall be a way to access "f" value
directly from PyEval_EvalCode return object:

PyObject* evalRet = :yEval_EvalCode(...);

But, I can't find any details what the "evalRet" actually is.

I assume that you have read the documentation at http://docs.python.org/c-api/veryhigh.html,
and I agree that it's sparse, but the entry for the next entry,
PyEval_EvalCodeEx, tells you a little more, as does that for
PyEval_EvalFrameEx.

Obviously, it's returning a pointer to a PyObject, and Looking at the
source code, that may be NULL, to throw an exception, or an execution
frame, or a generator,etc, etc, depending on the code it's been given.
I guess that you should be able to inspect the PyObject to see what it
is.

What I'm wondering is, why are you eval-ing code ? A favourite device
of C programmers coming to Python (and mea culpa in the past), it's
fraught with potential problems and even security risks, and is
difficult to debug, and maybe you should be using introspection
instead.

And maybe you should be working in Python, and not C++ at this point,
do the dynamic stuff in the language best suited, and then return to C+
+ when needed.

Best wishes,

John
--

 

[Mateusz Loskot]

I assume that you have read the documentation at
http://docs.python.org/c-api/veryhigh.html,
and I agree that it's sparse, but the entry for the next entry,
PyEval_EvalCodeEx, tells you a little more, as does that for
PyEval_EvalFrameEx.

It does help, but only a bit.

Obviously, it's returning a pointer to a PyObject, and Looking at the
source code, that may be NULL, to throw an exception, or an execution
frame, or a generator,etc, etc, depending on the code it's been given.
I guess that you should be able to inspect the PyObject to see what it is.

Yes, that's one possibility. I tried to investigate others.

What I'm wondering is, why are you eval-ing code ? A favourite device
of C programmers coming to Python (and mea culpa in the past), it's
fraught with potential problems and even security risks, and is
difficult to debug, and maybe you should be using introspection instead.

My application accepts Python scripts from user and executes using embedded
Python interpreter.
So, I use this particular API.

Could you elaborate on the "using introspection"?

And maybe you should be working in Python, and not C++ at this point,
do the dynamic stuff in the language best suited, and then return to C++
when needed.

My application in C++ interacts with Python programs provided by users.
Depending on what these programs request, various PyObject objects are
created and returned back, etc.

I understand it's difficult to discuss it without long essays or without
providing the code, etc.
So, I can only discuss and investigate building blocks I use.
Anyway, thanks for your comments.

Best regards,