New submission from STINNER Victor:
In Python 3, it becomes possible to chain two exceptions. It's one of the
killer feature of Python 3, it helps debugging.
In Python, exceptions are chained by default. Example:
try:
raise TypeError("old message")
except TypeError:
raise ValueError("new message")
Output:
Traceback (most recent call last):
File "x.py", line 2, in <module>
raise TypeError("old message")
TypeError: old message
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "x.py", line 4, in <module>
raise ValueError("new message")
ValueError: new message
In C, using the public PyErr_SetNone(), PyErr_Format(), PyErr_SetString(), ...
functions, exceptions are *not* chained by default.
You have to call explicitly the new private _PyErr_ChainExceptions() function
introduced in Python 3.4. It is not trivial to use it: you have to call
PyErr_Fetch() and check if an exception was already raised.
In Python, the following examples are bad practice because they may hide
important exceptions like MemoryError or KeyboardInterrupt:
try:
....
except:
pass
or
try:
....
except:
raise ValueError(...)
In C extensions, it's common to write such code, few functions check which
exception was raised.
Last months, I enhanced Python to detect exceptions ignored by mistake: I added
assert(!PyErr_Occurred()) in many functions which can clear the current
exception (ex: call PyErr_Clear()) or raise a new exception (ex: call
PyErr_SetString(...)). The last step is the issue #23571 which now implements
in release mode.
For the next step, I propose to explicitly clear the current exception before
raising a new exception.
I don't know yet if it would be a good idea to modify PyErr_*() functions to
automatically chain exceptions.
----------
messages: 239130
nosy: haypo
priority: normal
severity: normal
status: open
title: Chain exceptions in C
_______________________________________
Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue23763>
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