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<div class=Section1>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>It would seem that the current default mechanism is poorly
suited to providing default values for non-<span class=SpellE>immutables</span>.
For example:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><span class=GramE><font size=2 face=Arial><span
style='font-size:10.0pt;font-family:Arial'>class</span></font></span><font
size=2 face=Arial><span style='font-size:10.0pt;font-family:Arial'> <span
class=SpellE>IBar</span>( Interface ):<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span style='mso-spacerun:yes'> </span>a =
<span class=GramE>Object(</span> schema = <span class=SpellE>IFoo</span>,
default = <span class=SpellE>Foo</span>() )<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>But if a “<span class=SpellE>Foo</span>” is not
immutable this doesn’t make sense. (In my case, I want “a” to
be a collection providing <span class=SpellE>IFoo</span>, which defaults to an
empty collection. Each <span class=GramE>Bar</span> implementing <span
class=SpellE>IBar</span> should have its own instance of <span class=SpellE>Foo</span>.)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>A proposal to remedy: if the default is a callable object,
it is assumed to be a factory.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>How does this sound? (This would apply to <span
class=SpellE>missing_value</span> as well.)<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>A few further questions: <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>1) <span class=GramE>who</span> should be responsible for
setting defaults, and how should it be done?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>I have a base class from which I derive (e.g.) Bar, whose constructor
loops through fields of interfaces provided by the instance:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'> </span><span
class=GramE>for</span> <span class=SpellE>iface</span> in <span class=SpellE>zope.interface.providedBy</span>(
self ):<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span><span class=GramE>for</span> <span class=SpellE>fname</span>, field in <span
class=SpellE>zope.schema.getFields</span>( <span class=SpellE>iface</span> ).<span
class=SpellE>iteritems</span>():<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>It checks default & <span class=SpellE>missing_value</span>,
and sets them. However, if one field shadows another in the interfaces, there
is no guarantee that I hit them in the right order. Might it not be good to
have a “<span class=SpellE><span class=GramE>attributesProvidedBy</span></span><span
class=GramE>(</span> instance ) in <span class=SpellE>zope.interface</span>
that guarantees that it passes back the most derived versions (and resolves
consistently when there is no “most derived” )?<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Also, currently, when something doesn’t have a
definition, and is not required, I check first for “default” then
for “<span class=SpellE>missing_value</span>”, and use the first I
find as a missing value. Is this correct? How should “default” and “<span
class=SpellE>missing_value</span>” interact with each other? What makes
most sense to me is: (<span class=SpellE>pseudocode</span>):<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>If not specified:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span style='mso-spacerun:yes'> </span>If default
defined<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'> </span>Use default<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span style='mso-spacerun:yes'> </span>Else<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'> </span>If required<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span>Raise error<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'> </span>Else<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span>If missing value defined<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span>Use missing value<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span>Else<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><span
style='mso-spacerun:yes'>
</span>Raise error<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Finally, a missing <span class=SpellE>missing_value</span>
eventually gets “None” in the current Field constructor. Shouldn’t
it be left undefined when it wasn’t specified? <span class=GramE>(Same
for default.)</span><o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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</span></font></span></span></font><![endif]><span dir=LTR><font size=2
face=Arial><span style='font-size:10.0pt;font-family:Arial'>Shaun<o:p></o:p></span></font></span></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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