sys.exc_info

[eryk sun]

On Thu, Jun 29, 2017 at 6:50 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> try:
>     something
> except:
>     exc_type, exc, tb = sys.exc_info()
>     print(traceback.extract_tb(tb))
>     raise
>
> Why does it return the exception type separately from the exception, when
> the type can be derived by calling `type(exc)`?

I think normally it is redundant. While the type and value do have to
be tracked separately for the thread's hot exception state (i.e.
curexc_type, curexc_value, curexc_traceback), sys.exc_info() returns a
caught exception (i.e. exc_type, exc_value, exc_traceback), which is
normalized by PyErr_NormalizeException.

In CPython you can trivially force the exception value to be None by
calling PyErr_SetExcInfo. For example:

    import sys
    import ctypes

    PyErr_SetExcInfo = ctypes.pythonapi.PyErr_SetExcInfo
    PyErr_SetExcInfo.restype = None
    PyErr_SetExcInfo.argtypes = (ctypes.py_object,) * 3

    PyErr_SetExcInfo(TypeError, None, None)

    >>> sys.exc_info()
    (<class 'TypeError'>, None, None)

Of course this is a silly example, because C.