sys.exc_info

[Thomas Jollans]

On 29/06/17 19:00, eryk sun wrote:
> 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.
> 

This is doubly silly, as since 1.5 the stdlib documentation has stated
that the value "is always a class instance if the exception type is a
class object" (... the formulation was changed in py3)