[IPython-dev] New display methods, should we fine-tune our naming decision?

[Robert Kern]

On 2/1/11 10:36 AM, Fernando Perez wrote:
> Hi folks,
>
> I was just thinking, before the new display infrastructure starts
> getting used too much, should we adjust our convention to use
> __repr_html__, __repr_svg__, __repr_*__ as a naming pattern instead of
> __html__, __svg__, etc?  My concern is two-fold:
>
> - the more generic names may be already in use by something else that
> doesn't quite match our 'protocol',

This is a pretty big problem particularly since there is currently no error 
recovery around the method call. There is a lot of room for error here. For 
example, the PlainTextFormatter, which uses pretty.py, specifies a 
print_method='__pretty__'. However, the __pretty__ method has a distinctly 
different signature that takes arguments. Fortunately, the __call__ was 
overridden to ignore the print_method and just use the pretty.py machinery 
directly. These kinds of mistakes are easy to make, and it bakes in a certain 
amount of brittleness into the system.

> or python may in the future
> (though this is probably needless paranoia) choose to add them.

Python reserves __foo__ names for itself and requests that third party packages 
do not make their own. Third party packages violate this pretty frequently, but 
that doesn't mean it's kosher. SAGE does something sensible and uses single 
underscores on either side.

Personally, I don't want to ever change one of my classes to get it printed 
nicely for IPython. That's an inversion of responsibilities that I find 
difficult to swallow (especially for HTML and JSON that are best done using 
particular libraries). Instead, I want the to write formatter functions and 
register them for the appropriate class. Maybe a conventional print_method could 
be a default.

In any case, the priority as implemented is inverted; we should check for the 
print_method *after* the registered functions are checked. I, the IPython user, 
should always be able to override what the class author thought was a good idea 
at the time. Or who implemented an __html__ method for a different system and a 
different calling convention.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco