On Jan 21, 11:24 am, Arnaud Delobelle <arno...@googlemail.com> wrote:
> Falcolas <garri...@gmail.com> writes:
> > I'm running into an issue with closures in metaclasses - that is, if I
> > create a function with a closure in a metaclass, the closure appears
> > to be lost when I access the final class. I end up getting the text
> > 'param' instead of the actual tags I am expecting:
>
> > ALL_TAGS = ['a', 'abbr', 'acronym', 'address', 'applet', 'b', 'bdo',
> > 'big'] #snip
>
> > def _tag_meta(name, bases, dict_):
> > for tag in ALL_TAGS:
> > def generic_tag(*args, **kwargs):
> > return Tag._generate_tag(tag, *args, **kwargs)
> > #generic_tag = eval("lambda *args, **kwargs: Tag._generate_tag
> > ('%s', *args, **kwargs)" % tag)
> > dict_[tag] = staticmethod(generic_tag)
> > return type(name, bases, dict_)
>
> This is almost a FAQ and has nothing to do with metaclasses. The
> simplest solution usually involves adding a default argument 'tag=tag'
> to the function you define (here, generic_tag), but you can't do this
> here because you have a **kwargs argument. Instead, you can use a
> closure and do this for example:
>
> def factory(tag):
> def generic_tag(*args, **kwargs):
> return Tag._generate_tag(tag, *args, **kwargs)
> return generic_tag
>
> def _tag_meta(name, bases, dict_):
> for tag in ALL_TAGS:
> dict_[tag] = staticmethod(factory(tag))
> return type(name, bases, dict_)
I see - I was thinking it would preserve the closure from the for
statement, but I can see now why that would be wrong.
>
> However, I am usure about why you are using a metaclass.
>
> HTH
>
> --
> Arnaud
It was the easiest way I found to add a lot of static methods to the
Tag class without writing each one out. __getattr__ was not working
for this application. This is for a very simple application, and I
didn't want to add a lot of complexity to it's use. I'm always open
for other options OTOH.
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