[Python-bugs-list] [ python-Bugs-485153 ] Erroneous Fail of PyEval_CallObject

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Bugs item #485153, was opened at 2001-11-24 11:46
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Category: Documentation
Group: Python 2.1.1
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Cl. Schmidt (clemm)
Assigned to: Fred L. Drake, Jr. (fdrake)
Summary: Erroneous Fail of PyEval_CallObject

Initial Comment:
If embedding python v2.1.1 in a Windows MFC project,
it shows the following behaviour:

When a call to e.g. PyArgParseTuple had failed,
a subsequent call to PyEval_CallObject fails. The 
parameters of the both calls are completely 
independent and have nothing to do with each other.
Trying to retrieve the error text of the failed 
PyEval_CallObject returns the error of the 
PyArgParseTuple, namely
"new style getargs format but argument list is not a 
tuple".
If the first (erroneous) call to PyArgParseTuple is 
commented, the PyEval_CallObject works without any 
problem.

The attached code snippet documents the error.

No attempts have been made to reproduce the behaviour 
in other environments.

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>Comment By: Fred L. Drake, Jr. (fdrake)
Date: 2001-12-03 09:57

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Clarified the section on exception handling the C API manual
in Doc/api/exceptions.tex revision 1.3.

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Comment By: Cl. Schmidt (clemm)
Date: 2001-11-28 03:25

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Maybe I violated a python coding rule. In my case, I am 
embedding python
in a windows C++ program. I am having neither a console 
window nor a caller to the method
where the error occurred. Thus, raising an exception would 
not have made any sense.
It is not correct to state that I ignore the error, because 
there is a return value from the function which is
of course retrieved.

It is at least disturbing when after having e.g. forgotten 
to clear an error,
three subsequent calls to the C API succeed (as in my 
case), and suddenly one fails, reporting something
completely irrelevant to that error. This behaviour is 
different in other cases. In another, very similar
scenario, the call to PyEval_CallObject succeeded.

I agree that it is a documentation issue. The doc should 
state something like this:
"If you do not raise an exception to python and do not call 
PyErr_Clear, the results are unpredictable."


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Comment By: Tim Peters (tim_one)
Date: 2001-11-27 13:14

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Changed to Documentation and reassigned to Fred:  Fred, I 
don't think we ever spell out that C API errors must be 
passed on or explicitly cleared (before calling another C 
API function).  The Exceptions section of the C API manual 
does not spell this out.  It's possible <wink> that 
the "Intermezzo: Errors and Exceptions" section of the 
Extending and Embedding manual is clear enough -- your call.

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Comment By: Tim Peters (tim_one)
Date: 2001-11-27 12:47

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I don't understand what "the bug" is here:  if you get an 
error return from a Python C API function, and you intend 
to ignore the error, you must call PyErr_Clear() before 
calling another Python C API function.  You're not doing 
that.

The attachment isn't executable as-is, so I can't say 
whether that fixes your particular problem -- but it's 
never legitimate to ignore an error in C API coding (you 
must either pass it on to your caller or explicitly clear 
it).

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