Channel C++ warnings to Python's warning system

[ischoegl]

While exceptions raised in the C++ core are already captured and thrown by the Python API, the same is not true for the warning system. I had originally added this as a discussion topic, but it may be more appropriate to discuss this as an issue (feature request).

Discussed in #112

^{Originally posted by ischoegl August 18, 2021}
At the moment, the C++ code base issues warning messages via warn_deprecated and warn_user (the latter introduced in Cantera/cantera#741). In this context, @bryanwweber commented on a new deprecation warning proposed in Cantera/cantera#1084:

One other comment, and I suppose this applies to all the warn_deprecated() calls in C++, I wonder if it's possible to catch those in Python and turn them into real Python DeprecationWarnings so they can be controlled by Python's warning interfaces?

(see here; the same presumably applies to warn_user).

@speth's response was:

I think this would require something a bit like how we interface with Python for writing messages to stdout, that is, registering a handler in the Application class and in that handler making whatever the correct calls are to the Python C API.

Additional context:

• Handle deprecation warnings from all sources consistently in Python cantera#796
• Comment about Boost logger in Improve warning messages in C++  cantera#741

Other thoughts:

As there may be a relatively straight-forward route to this after all, it may make sense to convert this discussion to an enhancement request (i.e. issue).

[ischoegl]

@speth ... while I know how C++ outputs logging messages (fairly apparent in src/base/application.h) and have a hunch for how information flows (re-routed to wrappers.h and - I believe - captured with _logger in utils.pyx?), I'm not sure that I am following how those are actually caught and displayed in Python? It would be great if you could point me in the right direction.

[speth]

Right, the lines

cdef CxxPythonLogger* _logger = new CxxPythonLogger()
CxxSetLogger(_logger)

in utils.pyx create a new instance of PythonLogger (defined in wrappers.h) and assign that as the logwriter member of the global Application object. That PythonLogger object is then used for all calls to writelog().

I think for this purpose, what you would want is an additional method for the Logger class that's used to handle calls to warn_user instead of having that just call writelog. The implementation of this method in PythonLogger would then need to call something like PyErr_WarnEx (see https://docs.python.org/3/c-api/exceptions.html#issuing-warnings). For the base Logger class, I guess you would probably want to direct the warning to std::cerr.

[ischoegl]

🎉 Ha, that comment made all the difference! Thanks!! I guess the magic sauce in the current implementation is in PySys_WriteStdout and PyRun_SimpleString. This was hiding in plain sight! Exact same thing (i.e. PyRun_SimpleString) can be used for triggering warnings.