Latest update from Ethan, marking PEP 409 as accepted.
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Brett Cannon
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pep-0409.txt
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pep-0409.txt
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@ -3,17 +3,18 @@ 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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Status: Accepted
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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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Post-History: 30-Aug-2002, 01-Feb-2012, 03-Feb-2012
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Resolution: http://mail.python.org/pipermail/python-dev/2012-February/116136.html
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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
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One of the open issues from PEP 3134 is suppressing context: currently
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there is no way to do it. This PEP proposes one.
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@ -22,14 +23,14 @@ Rationale
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There are two basic ways to generate exceptions:
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1. Python does it (buggy code, missing resources, ending loops, etc.)
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1) Python does it (buggy code, missing resources, ending loops, etc.)
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2. manually (with a raise statement)
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2) manually (with a raise statement)
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When writing libraries, or even just custom classes, it can become
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necessary to raise exceptions; moreover it can be useful, even
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necessary, to change from one exception to another. To take an
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example from my dbf module::
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necessary, to change from one exception to another. To take an example
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from my dbf module::
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try:
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value = int(value)
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@ -74,58 +75,55 @@ 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
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setting the cause::
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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 not
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displayed by the default exception printing routines.
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but because the cause is ``None`` the previous context is not displayed
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by the default exception printing routines.
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Implementation Discussion
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=========================
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Currently, ``None`` is the default for both ``__context__`` and
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``__cause__``. In order to support ``raise ... from None`` (which
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would set ``__cause__`` to ``None``) we need a different default value
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for ``__cause__``. Several ideas were put forth on how to implement
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this at the language level:
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Currently, ``None`` is the default for both ``__context__`` and ``__cause__``.
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In order to support ``raise ... from None`` (which would set ``__cause__`` to
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``None``) we need a different default value for ``__cause__``. Several ideas
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were put forth on how to implement this at the language level:
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* Overwrite the previous exception information (side-stepping the
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issue and leaving ``__cause__`` at ``None``).
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* Overwrite the previous exception information (side-stepping the issue and
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leaving ``__cause__`` at ``None``).
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Rejected as this can seriously hinder debugging due to `poor error
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messages`_.
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Rejected as this can seriously hinder debugging due to
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`poor error messages`_.
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* Use one of the boolean values in ``__cause__``: ``False`` would be
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the default value, and would be replaced when ``from ...`` was used
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with the explicity chained exception or ``None``.
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* Use one of the boolean values in ``__cause__``: ``False`` would be the
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default value, and would be replaced when ``from ...`` was used with the
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explicity chained exception or ``None``.
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Rejected as this encourages the use of two different objects types
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for ``__cause__`` with one of them (boolean) not allowed to have the
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full range of possible values (``True`` would never be used).
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Rejected as this encourages the use of two different objects types for
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``__cause__`` with one of them (boolean) not allowed to have the full range
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of possible values (``True`` would never be used).
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* Create a special exception class, ``__NoException__``.
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Rejected as possibly confusing, possibly being mistakenly raised by
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users, and not being a truly unique value as ``None``, ``True``, and
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``False`` are.
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Rejected as possibly confusing, possibly being mistakenly raised by users,
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and not being a truly unique value as ``None``, ``True``, and ``False`` are.
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* Use ``Ellipsis`` as the default value (the ``...`` singleton).
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Accepted. There are no other possible values; it cannot be raised
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as it is not an exception; it has the connotation of 'fill in the
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rest...' as in ``__cause__`` is not set, look in ``__context__`` for
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it.
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Accepted. There are no other possible values; it cannot be raised as it is
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not an exception; it has the connotation of 'fill in the rest...' as in
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``__cause__`` is not set, look in ``__context__`` for it.
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Language Details
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================
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To support ``from None``, ``__context__`` will stay as it is, but
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``__cause__`` will start out as ``Ellipsis`` and will change to
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``None`` when the ``raise ... from None`` method is used.
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``__cause__`` will start out as ``Ellipsis`` and will change to ``None``
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when the ``raise ... from None`` method is used.
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============================================ ================== =======================================
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form __context__ __cause__
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@ -137,20 +135,26 @@ reraise from ``None`` | ``ChainedException`` previous exception ``None`` | exp
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The default exception printing routine will then:
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* If ``__cause__`` is ``Ellipsis`` the ``__context__`` (if any) will
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be printed.
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* If ``__cause__`` is ``None`` the ``__context__`` will not be
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* If ``__cause__`` is ``Ellipsis`` the ``__context__`` (if any) will be
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printed.
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* If ``__cause__`` is anything else, ``__cause__`` will be printed.
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* If ``__cause__`` is ``None`` the ``__context__`` will not be printed.
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* if ``__cause__`` is anything else, ``__cause__`` will be printed.
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In both of the latter cases the exception chain will stop being followed.
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Because the default value for ``__cause__`` is now ``Ellipsis``::
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raise Exception from Ellipsis
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is allowed and will (re)set ``__cause__`` to ``Ellipsis``.
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Patches
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=======
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There is a patch for CPython implementing this attached to `Issue
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6210`_.
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There is a patch for CPython implementing this attached to `Issue 6210`_.
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References
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@ -171,7 +175,7 @@ Copyright
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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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@ -180,3 +184,4 @@ This document has been placed in the public domain.
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fill-column: 70
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coding: utf-8
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End:
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