Re: Re: [PATCH] %default keyword expansion

On 17 May 2004, John Belmonte said:
> Just last week I used custom option types for the first time, and was 
> thinking "I guess that %default expansion thing I proposed needs to be 
> tossed out the window".  (See attached example.)
> 
> Actually, %default expansion is innocent here.  The problem is that 
> defaults are not run through the option's checker/converter.  For types 
> which are converted, why shouldn't the programmer be able to give the 
> same value for a default that the user would put on the command line, 
> rather than have to mentally convert to a normalized value?  I've run 
> into the same issue with ZConfig, and it seems like a design error to 
> me.  The %default expansion just makes the issue more obvious.

At first glance, you're absolutely right -- default values should be
treated like values from the command line.  Lets the developer supply a
default that a) looks good in the help and b) maps onto the user's
mental model for that option, which hopefully is the same as the
developer's mental model.

> I propose that Option and add_option take a new keyword "nice_default". 
>  Nice defaults are passed through the option checker just as if they 
> were passed on the command line.  In this case, %default will expand to 
> the pre-checked value.

Possibly simpler: always pass default values through the type-checking
and -conversion code.  For built-in types, eg. string and int, this is
no problem: int("37") == int(37) == 37.  It might be problematic for
people who write custom types though -- eg. you'd have to make sure that
your _check_duration() accepts both "5m" and 300.

I'll create a bug report on SF to track this.  It'll have to queue up
behind several other pending bugs and features.

        Greg
-- 
Greg Ward <[email protected]>                         http://www.gerg.ca/
When you make your mark in the world, watch out for guys with erasers.


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