[issue6210] Exception Chaining missing method for suppressing context

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

Ah, nice idea of bringing the boolean constants into the mix so we don't need to invent a new sentinel value.

However, to preserve the current behaviour that "raise X from Y" is essentially just syntactic sugar for: "_var = X; _var.__cause__ = Y; raise Y", I suggest setting the default value for "__cause__" to "False" (indicating "not set"). Then:

  "__cause__ is False" means "no cause set, display context"
  "__cause__ is None" means "cause explicitly set to None, suppress context"
  Any other value means "display cause"

The default value for cause is controlled by get_cause and set_cause in exceptions.c [1]. The basic idea would be to replace the current usage of Py_None and Py_RETURN_NONE in that code with Py_False and Py_RETURN_FALSE, and then change the criteria for valid causes to "arg != Py_None && !PyExceptionInstance_Check(arg)".

In addition to the files already touched by the patch, Lib/traceback.py [2] and its tests will also require updating.

[1] http://hg.python.org/cpython/file/default/Objects/exceptions.c#l293
[2] http://hg.python.org/cpython/file/default/Lib/traceback.py#l117

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