PEP: 409 Title: Suppressing exception context Version: $Revision$ Last-Modified: $Date$ Author: Ethan Furman Status: Final Type: Standards Track Content-Type: text/x-rst Created: 26-Jan-2012 Python-Version: 3.3 Post-History: 30-Aug-2002, 01-Feb-2012, 03-Feb-2012 Superseded-By: 415 Resolution: https://mail.python.org/pipermail/python-dev/2012-February/116136.html Abstract ======== One of the open issues from :pep:`3134` is suppressing context: currently there is no way to do it. This PEP proposes one. Rationale ========= There are two basic ways to generate exceptions: 1) Python does it (buggy code, missing resources, ending loops, etc.) 2) manually (with a raise statement) When writing libraries, or even just custom classes, it can become necessary to raise exceptions; moreover it can be useful, even necessary, to change from one exception to another. To take an example from my dbf module:: try: value = int(value) except Exception: raise DbfError(...) Whatever the original exception was (``ValueError``, ``TypeError``, or something else) is irrelevant. The exception from this point on is a ``DbfError``, and the original exception is of no value. However, if this exception is printed, we would currently see both. Alternatives ============ Several possibilities have been put forth: * ``raise as NewException()`` Reuses the ``as`` keyword; can be confusing since we are not really reraising the originating exception * ``raise NewException() from None`` Follows existing syntax of explicitly declaring the originating exception * ``exc = NewException(); exc.__context__ = None; raise exc`` Very verbose way of the previous method * ``raise NewException.no_context(...)`` Make context suppression a class method. All of the above options will require changes to the core. Proposal ======== I propose going with the second option:: raise NewException from None It has the advantage of using the existing pattern of explicitly setting the cause:: raise KeyError() from NameError() but because the cause is ``None`` the previous context is not displayed by the default exception printing routines. Implementation Discussion ========================= Note: after acceptance of this PEP, a cleaner implementation mechanism was proposed and accepted in :pep:`415`. Refer to that PEP for more details on the implementation actually used in Python 3.3. Currently, ``None`` is the default for both ``__context__`` and ``__cause__``. In order to support ``raise ... from None`` (which would set ``__cause__`` to ``None``) we need a different default value for ``__cause__``. Several ideas were put forth on how to implement this at the language level: * Overwrite the previous exception information (side-stepping the issue and leaving ``__cause__`` at ``None``). Rejected as this can seriously hinder debugging due to `poor error messages`_. * Use one of the boolean values in ``__cause__``: ``False`` would be the default value, and would be replaced when ``from ...`` was used with the explicitly chained exception or ``None``. Rejected as this encourages the use of two different objects types for ``__cause__`` with one of them (boolean) not allowed to have the full range of possible values (``True`` would never be used). * Create a special exception class, ``__NoException__``. Rejected as possibly confusing, possibly being mistakenly raised by users, and not being a truly unique value as ``None``, ``True``, and ``False`` are. * Use ``Ellipsis`` as the default value (the ``...`` singleton). Accepted. Ellipses are commonly used in English as place holders when words are omitted. This works in our favor here as a signal that ``__cause__`` is omitted, so look in ``__context__`` for more details. Ellipsis is not an exception, so cannot be raised. There is only one Ellipsis, so no unused values. Error information is not thrown away, so custom code can trace the entire exception chain even if the default code does not. Language Details ================ To support ``raise Exception from None``, ``__context__`` will stay as it is, but ``__cause__`` will start out as ``Ellipsis`` and will change to ``None`` when the ``raise Exception from None`` method is used. ============================================ ================== ======================================= form __context__ __cause__ ============================================ ================== ======================================= raise ``None`` ``Ellipsis`` reraise previous exception ``Ellipsis`` reraise from ``None`` | ``ChainedException`` previous exception ``None`` | explicitly chained exception ============================================ ================== ======================================= The default exception printing routine will then: * If ``__cause__`` is ``Ellipsis`` the ``__context__`` (if any) will be printed. * If ``__cause__`` is ``None`` the ``__context__`` will not be printed. * if ``__cause__`` is anything else, ``__cause__`` will be printed. In both of the latter cases the exception chain will stop being followed. Because the default value for ``__cause__`` is now ``Ellipsis`` and ``raise Exception from Cause`` is simply syntactic sugar for:: _exc = NewException() _exc.__cause__ = Cause() raise _exc ``Ellipsis``, as well as ``None``, is now allowed as a cause:: raise Exception from Ellipsis Patches ======= There is a patch for CPython implementing this attached to `Issue 6210`_. References ========== Discussion and refinements in this `thread on python-dev`_. .. _poor error messages: http://bugs.python.org/msg152294 .. _issue 6210: http://bugs.python.org/issue6210 .. _Thread on python-dev: https://mail.python.org/pipermail/python-dev/2012-January/115838.html Copyright ========= This document has been placed in the public domain. .. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 coding: utf-8 End: