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Tue Jul 13 16:57:31 CEST 2004 · https://mail.python.org/pipermail/python-dev/2004-July/046116.html

Never mind.

Tim pointed out to me that return values other than 0 for the
visit proc are used.

The documentation is still off I think, as it says that a
non-zero value indicates an error, but I don't think that this
is true.

Jim

Jim Fulton wrote:
> Tim Peters wrote:
> 
>> [Jim Fulton]
>> ...
>>
> 
> ...
> 
>> BTW, re-invented at least as often is a VISIT macro for use in
>> tp_traverse slots, like typeobject.c's (and several other files')
>>
>> #define VISIT(SLOT) \
>>     if (SLOT) { \
>>         err = visit((PyObject *)(SLOT), arg); \
>>         if (err) \
>>             return err; \
>>     }
> 
> 
> First, I don't like this macro, based on my own experience writing macros
> that hide returns. :)
> 
> Second, Aaargh!  I missunderstood the use of the visit function
> passed to traverse.  The source of my missunderstanding was
> 
>   1) visit is an int function and I normally expect int functions to
>      return a negative value to indicate an error and
> 
>   2) The documentation for the traversal slot says:
>      "If visit returns a non-zero value then an error has occurred
>      and that value should be returned immediately."
>      That is, the documentation says that the return value is an
>      error indicator.  I missed the bit about a non-zero return value
>      and needing to return it. My bad.
> 
> Last night, Tim explained to me that a non-zero return value is not an
> error indicator.  In fact, the GC system doesn't really allow errors.
> Rather, a non-zero return value provides a way for a visit proc to
> short-circuit the traversal process.  As far as Tim knows, this feature
> has never been used.  All visit functions in the core always return 0.
> 
> Alas, all my extensions that implement tp_traverse and the documentation
> I wrote on writing types is wrong and has to be changed. Sigh.
> 
> Should we call WHUI (we haven't used it!) on this feature that has been 
> around
> for 4 versions of Python, that complicates tp_traverse implementations and
> hasn't been used? It would be simpler if we said that the return value 
> could
> be ignored.
> 
> Jim
> 

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C API Macro to decref and set to NULL)