Re: How to call TPyException in Python?

Hello Wim,

Thank you for your detailed explanation. It makes sense. At the first time I read the HTML document, I thought that TPyException can be caught inside python. e.g.,

try:

    ROOT.throw_TPyException()
except ROOT.PyROOT.TPyException, e:

    # do something

That's is the reason why I was trying to access ROOT.PyROOT.TPyException in my test script.

According to your explanation, it seems that my own exception classes derived from std::exception are not useful because they are all turned into single class, Exception. Therefore, if I want to distinguish different type of errors (exceptions) from each other, I always have to check Exception.message in if statement instead of try statement. Am I correct?

Regards,

--
OKUMURA, Akira oxon@{astro.isas.jaxa.jp,stanford.edu}
Institute of Space and Astronautical Science (ISAS/JAXA)
Now at KIPAC/SLAC/Stanford
Varian Physics #306, 382 Via Pueblo Mall, MC 4060
Stanford, CA 94305-4060
TEL 650-736-0971/FAX 650-724-5065
Skype : okumura.akira

On 2011/05/03, at 22:51, WLavrijsen_at_lbl.gov wrote:



> Akira,

> 

>> What I want to do is to throw C++ exceptions from my C++ ROOT code, and
>> catch them in PyROOT.


> 

> that's fine, just throw any old exception, but preferably ones derived from

> std::exception b/c their what() can be called to transfer the error message.

> All C++ exceptions are turned into instances of Exception and can be caught.

> 

> For example:

> 

> % cat dothrow.C #include <stdexcept>

> 

> void dothrow() {

>   throw std::runtime_error( "hey there!" );

> }

> 

> % python

>>>> import ROOT
>>>> ROOT.gROOT.LoadMacro( 'dothrow.C+' )


> Info in <TUnixSystem::ACLiC>: creating shared library /Users/wlav/./dothrow_C.so

> 0

>>>> try:


> ...    ROOT.dothrow()

> ... except Exception, e:

> ...    error = e

> ... 

>>>> print error


> hey there! (C++ exception)

>>>> ^D


> 

> Perhaps the translation should be finer grained (e.g. std::runtime_error

> could have been RuntimeError), but no-one's ever asked for that. :)

> 

>> ==============
>> http://root.cern.ch/root/html/PyROOT__TPyException.html
>> Purpose: A C++ exception class for throwing python exceptions
>>        through C++ code.
>> ==============


> 

> The important bit is "through", rather than "from." TPyException is used

> to get _python_ exceptions through C++ back into python. For example, if

> a python callable is handed to a TF1 for plotting on a canvas, and the

> callable then raises. That raised exception (python) is turned into a

> thrown TPyException (C++), caught, and turned back into a python exception

> (which is no work, really, since it's already set, but the return value is

> what matters).

> 

>> Is there any tutorial of TPyException?


> 

> No: there's only one use that I'm aware of and that's deep inside PyROOT.

> 

> Best regards,

>           Wim

> -- 

> WLavrijsen_at_lbl.gov    --    +1 (510) 486 6411    --    www.lavrijsen.net