Lesson of the day

If you use sys.settrace to set a tracing function, and that function prints to sys.stdout`, then don't ever trash ``sys.stdout, even briefly. You will raise an invisible exception and your trace function will be removed.

(I don't know precisely what is supposed to happen when a trace function raises an exception, but it seems like it is being removed. Fair enough. But frustrating.)

This cost me a few days of intermittent repair attempts on figleaf.

Perhaps I will track down this behavior and document it in the Python docs...

--titus


Legacy Comments

Posted by Brett on 2007-10-26 at 17:18.

The stopping of the trace makes sense as a trace function is supposed
to return itself after each call.  Obviously an exception blocks that.
As for no traceback if sys.stdout is lost, that is also expected as
the C code actually grabs that attribute directly off of the module
and prints to it.  I am pretty sure that sys.<em>_stdout_</em> is only
there to make it easy to reset sys.stdout later if you happen to
replace it with something.

Posted by Titus Brown on 2007-10-26 at 20:54.

Right you are -- all expected behavior.  I forgot that the local trace
function also needs to return itself!    &lt;sigh&gt;

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