Mirror
/
cpython
mirror of https://github.com/python/cpython
4
1
Fork 0
Code Issues Releases Wiki Activity

Merge from 3.3 (issue #15209)

This commit is contained in:
Nick Coghlan 2012-12-09 16:22:17 +10:00
parent bb7da03f6e 0eee97cce6
commit b4b8f234d4
1 changed files with 16 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
Doc/library/exceptions.rst
View File

@ -39,19 +39,27 @@ When raising (or re-raising) an exception in an :keyword:`except` clause
new exception is not handled the traceback that is eventually displayed will
include the originating exception(s) and the final exception.
When raising a new exception (rather than using to bare ``raise`` to re-raise
the exception currently being handled), the implicit exception chain can be
made explicit by using :keyword:`from` with :keyword:`raise`. The single
argument to :keyword:`from` must be an exception or ``None``. It will be set
as :attr:`__cause__` on the raised exception. Setting :attr:`__cause__`
also implicitly sets the :attr:`__suppress_context__` attribute to
``True``.
When raising a new exception (rather than using a bare ``raise`` to re-raise
the exception currently being handled), the implicit exception context can be
supplemented with an explicit cause by using :keyword:`from` with
:keyword:`raise`::
raise new_exc from original_exc
The expression following :keyword:`from` must be an exception or ``None``. It
will be set as :attr:`__cause__` on the raised exception. Setting
:attr:`__cause__` also implicitly sets the :attr:`__suppress_context__`
attribute to ``True``, so that using ``raise new_exc from None``
effectively replaces the old exception with the new one for display
purposes (e.g. converting :exc:`KeyError` to :exc:`AttributeError`, while
leaving the old exception available in :attr:`__context__` for introspection
when debugging.
The default traceback display code shows these chained exceptions in
addition to the traceback for the exception itself. An explicitly chained
exception in :attr:`__cause__` is always shown when present. An implicitly
chained exception in :attr:`__context__` is shown only if :attr:`__cause__`
is not set and :attr:`__suppress_context__` is false.
is :const:`None` and :attr:`__suppress_context__` is false.
In either case, the exception itself is always shown after any chained
exceptions so that the final line of the traceback always shows the last