[Python-Dev] PEP 409 - now properly formatted (sorry for the noise) (original) (raw)

Ethan Furman ethan at stoneleaf.us
Wed Feb 1 04:58:30 CET 2012

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PEP: 409 Title: Suppressing exception context Version: RevisionRevisionRevision Last-Modified: DateDateDate Author: Ethan Furman <ethan at stoneleaf.us> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 26-Jan-2012 Post-History: 30-Aug-2002, 01-Feb-2012

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:

Python does it (buggy code, missing resources, ending loops, etc.)
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 proprose 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, while retained, is not displayed by the default exception printing routines.

Language Details

Currently, __context__ and __cause__ start out as None, and then get set as exceptions occur.

To support from None, __context__ will stay as it is, but __cause__ will start out as False, and will change to None when the raise ... from None method is used.

The default exception printing routine will then:

If __cause__ is False 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.

This has the benefit of leaving the __context__ intact for future logging, querying, etc., while suppressing its display if it is not caught. This is important for those times when trying to debug poorly written libraries with bad error messages_.

Patches

There is a patch for CPython implementing this attached to Issue 6210_.

References

Discussion and refinements in this thread on python-dev_.

.. _bad error messages: http://bugs.python.org/msg152294 .. _Issue 6210: http://bugs.python.org/issue6210 .. _thread on python-dev: http://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:

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