PyRun_String

[Matthias]

Hello,

I encountered a strange problem today. I try to do this:

Log("Marker 1");
Py_XDECREF( PyRun_String( "print 'Hi!'", Py_single_input, Dict, Dict) );
Log("Marker 2");

The output looks like

Marker 1
Hi!
Hi!
Marker 2

Obviously Hi! is printed twice. I've extracted the above example of a more
complex piece of code where you can clearly see that the statement is
actually executed twice (not only printed). You can also see this with

Log("Marker 1");
Py_XDECREF( PyRun_String( "def func():\n\ti = 0\n\twhile 1:\n\t\tyield
i\n\t\ti += 1\no=func()", Py_file_input, Dict, Dict) );
Py_XDECREF( PyRun_String( "print o.next()", Py_file_input, Dict, Dict) );
Log("Marker 2");
Here the output goes

Marker 1
0
1
Marker 2
What is the reason for this very strange behaviour? Did I do something
wrong?

-Matthias

[Duncan Booth]

Matthias wrote:

Log("Marker 1");
Py_XDECREF( PyRun_String( "print 'Hi!'", Py_single_input, Dict, Dict) );
Log("Marker 2");

The output looks like

Marker 1
Hi!
Hi!
Marker 2

Obviously Hi! is printed twice.

Py_XDECREF is a C macro. If you check its definition you will see that it
does indeed evaluate the argument twice. This is a not uncommon problem
when using macros in C: if in doubt, do not call a macro with anything
as a parameter which has side effects.

#define Py_XDECREF(op) if ((op) == NULL) ; else Py_DECREF(op)

[Matthias]

Am Thu, 09 Mar 2006 20:06:54 +0100 hat Duncan Booth
<du**********@invalid.invalid> geschrieben:

Matthias wrote:

Log("Marker 1");
Py_XDECREF( PyRun_String( "print 'Hi!'", Py_single_input, Dict, Dict) );
Log("Marker 2");

The output looks like

Marker 1
Hi!
Hi!
Marker 2

Obviously Hi! is printed twice.

Py_XDECREF is a C macro. If you check its definition you will see that it
does indeed evaluate the argument twice. This is a not uncommon problem
when using macros in C: if in doubt, do not call a macro with anything
as a parameter which has side effects.

#define Py_XDECREF(op) if ((op) == NULL) ; else Py_DECREF(op)

Arrrghh, I've seen the evil side today; C macros are surely devil-sent. I
spent hours tracking down this bug, I didn't even consider the Py_XDECREF
and concentrated on PyRun_String. It's a pity C doesn't support inline
functions. Thanks a lot for pointing this out before I lost my head,
Duncan!

Maybe Py_XDECREF(op) could be replaced with

#define Py_XDECREF(op) {PyObject* obj = (op); if (obj == NULL) ; else
Py_DECREF(obj)}

but I guess this eats one or two clock cycles too much for other people
who use Py_XDECREF.

Thanks again!

-Matthias