[stringtemplate-interest] Python StringTemplate: defining __len__ for class rendered by subtemplate gives empty result

[Terence Parr]

Hi. I'm CC'ing Benjamin, the python guy.
T
On Feb 10, 2010, at 6:59 PM, Wray, Paul wrote:

> Hello
> I've started to use Python StringTemplate (very nice, thank you!), and I
> ran into a surprise that took me a while to track down. Maybe it
> deserves a note in the documentation or perhaps even a rework of part of
> the code? In any case this note may help others.
> The essence of the problem is shown in the code below. Class B defines
> an instance (a) of Class A, and renders it in a (named or anonymous)
> subtemplate, passing (a) either implicitly or explicitly to the
> subtemplate.
> 
> The code works fine unless Class A defines a __len__ function for its
> own purposes, which happens to return 0. Then, depending on which form
> of call is used, either the subtemplate is not rendered at all, or the
> attribute reference within the subtemplate gives an empty result.
> 
> import stringtemplate3 as st3
> from stringtemplate3.language import DefaultTemplateLexer
> import StringIO
> 
> groupStr = '''
>    group testGroup;
>    Atemplate(it) ::= <<
>        $it.value$
>>> 
> 
>    Btemplate(object) ::= <<
>        $object.a.value$
>        $object.a:{ $it.value$ }$
>        $object.a: { itm | $itm.value$ }$
>        $object.a:Atemplate()$
>        $Atemplate(it=object.a)$
>>> 
> '''
> 
> class A(object):
>    def __init__(self):
>        self.value = 'value'
> 
>    # if subtemplate-rendered class defines __len__, templates are
> empty.
>    #def __len__(self):
>    #    return 0
> 
> class B(object):
> 
>    def __init__(self):
>        self.a = A()
> 
>    def render(self, group):
>        template = group.getInstanceOf('Btemplate')
>        template['object'] = self
>        return template.toString()
> 
> if __name__ == '__main__':
>    group = st3.StringTemplateGroup(file=StringIO.StringIO(groupStr),
> lexer=DefaultTemplateLexer.Lexer)    
>    b = B()
>    print b.render(group)
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