Re: Having to "print" before method invocation?
- From: "Jeremy L. Moles" <jeremy@xxxxxxxxxxxxxxxx>
- Date: Wed, 08 Mar 2006 10:17:08 -0500
Hey Fredrik, thanks for responding. :) Your posts are always helpful and
informative!
On Wed, 2006-03-08 at 15:41 +0100, Fredrik Lundh wrote:
Jeremy L. Moles wrote:
I have an object (written as part C extension, part pure Python) called
foo that I've been using without much fuss for a few months now.
However, in my latest project (a rather large one involving
multi-threading, pygtk, etc.), I'm seeing some really strange behavior
with a particular instance of my foo object.
About midway through my program, any attempt to use the instance fails;
however, if I add print statements before trying to invoke methods on
it, the foo object instance works fine.
fails in what way?
Unfortunately, very silently. There aren't any exceptions thrown or
anything. The be more specific, the C part of the object is a wrapper
around some socket Send/Recv functions (that interface w/
wpa_supplicant). The problem I'm getting is that unless I add that print
statement, wpaobject.Recv() returns "", whereas with the print statement
it returns what it should (a large string representing the response from
wpa_supplicant).
More strange is that this is the first time I've had to do this. Up
until this point, every unittest and app has succeeded without such
trickery.
if you get a spurious exception, it's very likely that your C extension sets the
exception state (either directly or because some API function it uses fails), but
forgets to report this back to Python.
e.g. if you have a C function that does something like
PyErr_SetString(PyExc_AttributeError, "blah blah"):
Py_INCREF(Py_None);
return Py_None;
instead of
PyErr_SetString(PyExc_AttributeError, "blah blah"):
return NULL;
the interpreter won't raise the exception immediately (since it expected you to
return NULL if something went wrong), but the exception may still be raised at
a later time, if you run interpreter code that does something like
do something
if (PyErr_Occurred())
... /* this will catch your error even if "something" succeeds */ ...
*or* it may be masked, by code that does
PyErr_Clear();
do something
the actual exception might give you additional clues (e.g. if you get a KeyError,
look for unchecked dictionary accesses in your code, etc).
hope this helps!
</F>
.
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