[Python-Dev] Assigning to __debug__

[M.-A. Lemburg]

Guido van Rossum wrote:
> > ...
> > So anybody porting to Python 2.1 will essentially have to remove all
> > __debug__ blocks that were previously disabled by assigning 0 to
> > __debug__. I think this is undesirable.
> 
> Assigning to __debug__ was never well-defined.  You used it at your
> own risk.
> 
> > As I recall, in the original description of __debug__, being able to
> > assign to it was reported as one of its main features, so that you
> > still had a run-time option (unless the interpreter was running with
> > -O, which eliminates the __debug__ blocks).
> 
> The manual has always used words that suggest that there is something
> special about __debug__.  And there was: the compiler assumed it could
> eliminate blocks started with "if __debug__:" when compiling in -O
> mode.  Also, assert statements have always used LOAD_GLOBAL to
> retrieve the __debug__ variable.
> 
> > So in short, I think this change should be reverted.
> 
> It's possible that it breaks more code, and it's possible that we end
> up having to change the error into a warning for now.  But I insist
> that assignment to __debug__ should become illegal.  You can *use* the
> variable (to determine whether -O is on or not), but you can't *set*
> it.
> 
> > Regards,
> > Martin
> >
> > P.S. What was the motivation for that change, anyway?
> 
> To enforce a restriction that was always intended: __debug__ should be
> a read-only variable.

So you are suggesting that we change all our code to something like:

__enable_debug__ = 0 # set to 0 for production mode

...

if __debug__ and __enable_debug__:
   print 'debugging information'

...

I don't see the point in having to introduce a new variable
just to disable debugging code in Python code which does not
run under -O.

What does defining __debug__ as read-only variable buy us 
in the long term ?

-- 
Marc-Andre Lemburg
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