[Python-checkins] cpython (merge 3.3 -> default): Merge from 3.3 (Issue #15209)

[nick.coghlan]

http://hg.python.org/cpython/rev/5854101552c2
changeset:   80752:5854101552c2
parent:      80750:a2af31adc747
parent:      80751:8ba3c975775b
user:        Nick Coghlan <ncoghlan at gmail.com>
date:        Sat Dec 08 22:24:23 2012 +1000
summary:
  Merge from 3.3 (Issue #15209)

files:
  Doc/library/exceptions.rst |  26 ++++++++++++++++----------
  Misc/NEWS                  |   3 +++
  2 files changed, 19 insertions(+), 10 deletions(-)


diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
--- a/Doc/library/exceptions.rst
+++ b/Doc/library/exceptions.rst
@@ -39,17 +39,23 @@
 new exception is not handled the traceback that is eventually displayed will
 include the originating exception(s) and the final exception.
 
-This 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__` implicitly sets the :attr:`__suppress_context__` to
-``True``. If :attr:`__cause__` is an exception, it will be displayed. If
-:attr:`__cause__` is present or :attr:`__suppress_context__` has a true value,
-:attr:`__context__` will not be displayed.
+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``.
 
-In either case, the default exception handling code will not display any of the
-remaining links in the :attr:`__context__` chain if :attr:`__cause__` has been
-set.
+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.
+
+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
+exception that was raised.
 
 
 Base classes
diff --git a/Misc/NEWS b/Misc/NEWS
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -460,6 +460,9 @@
 Documentation
 -------------
 
+- Issue #15209: Clarify exception chaining description in exceptions module
+  documentation
+
 - Issue #15990: Improve argument/parameter documentation.
 
 - Issue #16209: Move the documentation for the str built-in function to a new

-- 
Repository URL: http://hg.python.org/cpython