2012-01-27 10:42:24 -05:00
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PEP: 409
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Title: Suppressing exception context
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Version: $Revision$
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Last-Modified: $Date$
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Author: Ethan Furman <ethan@stoneleaf.us>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 26-Jan-2012
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Python-Version: 3.3
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2012-01-27 10:45:00 -05:00
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Post-History: 2012-01-27
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2012-01-27 10:42:24 -05:00
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Abstract
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========
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One of the open issues from PEP 3134 is suppressing context: currently there is
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no way to do it. This PEP proposes one.
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Motivation
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==========
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There are two basic ways to generate exceptions: 1) Python does it (buggy code,
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missing resources, ending loops, etc.); and, 2) manually (with a raise
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statement).
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When writing libraries, or even just custom classes, it can become necessary to
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raise exceptions; moreover it can be useful, even necessary, to change from one
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exception to another. To take an example from my dbf module:
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try:
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value = int(value)
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except Exception:
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raise DbfError(...)
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Whatever the original exception was (ValueError, TypeError, or something else)
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is irrelevant. The exception from this point on is a DbfError, and the original
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exception is of no value. However, if this exception is printed, we would
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currently see both.
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Alternatives
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============
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Several possibilities have been put forth:
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- raise as NewException()
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Reuses the 'as' keyword; can be confusing since we are not really
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reraising the originating exception
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- raise NewException() from None
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Follows existing syntax of explicitly declaring the originating
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exception
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- exc = NewException(); exc.__context__ = None; raise exc
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Very verbose way of the previous method
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- raise NewException.no_context(...)
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Make context suppression a class method.
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All of the above options will require changes to the core.
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Proposal
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========
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I proprose going with the second option:
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raise NewException from None
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It has the advantage of using the existing pattern of explicitly setting
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the cause:
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raise KeyError() from NameError()
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but because the 'cause' is None the previous context is discarded. There is a
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patch to this effect attached to Issue6210 (http://bugs.python.org/issue6210).
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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