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> Should we do the same for __cause__? Is it possible to create __context__ or __cause__ loop without assigning these attributes directly?
Yes, let's mirror the __context__ behaviour for __cause__. New patch attached.
Serhiy, Guido,
The new patch raises a TypeError in __cause__ and __context__ setters when a cycle was introduced, so in pure Python the following won't work:
 # will throw TypeError("cycle in exception context chain")
 ex.__context__ = ex chain")
 # will throw TypeError("cycle in exception cause chain")
 ex.__cause__ = ex
However, since PyException_SetContext and PyException_SetCause are public APIs, and their return type is 'void', I can't raise an error when a C code introduces a cycle, in that case, the exc->cause/exc->context will be set to NULL.
Thoughts?
I think that this approach is the only sane one here. We can certainly fix the infinite loop in PyErr_SetObject, but it will only be a matter of time until we discover a similar loop somewhere else.