On Wed, Mar 23, 2016 at 08:19:34AM -0300, David Bremner wrote:
> W. Trevor King writes:
> >  from __future__ import print_function
> > @@ -232,6 +232,10 @@ class Page (object):
> >  class HtmlPage (Page):
> >      _slug_regexp = re.compile('\W+')
> >
> > +    def __init__(self, message_url_template, **kwargs):
> > +        self.message_url_template = message_url_template
> > +        super(HtmlPage, self).__init__(**kwargs)
> > +
>
> > @@ -395,6 +400,8 @@ _PAGES['text'] = Page()
> >  _PAGES['html'] = HtmlPage(
> >      header=header_template.format(**context),
> >      footer=footer_template.format(**context),
> > +    message_url_template=config['meta'].get(
> > +        'message-url', 'http://mid.gmane.org/{message-id}'),
> >      )
> >
>
> Maybe I'm missing some python knowledged, but it looks the
> constructor is defined to take a regular argument for
> message_url_template, but only passed in as a keyword. Does this
> really work?

Yup.  From [1]:

  positional-only: specifies an argument that can be supplied only by
    position. Python has no syntax for defining positional-only
    parameters. However, some built-in functions have positional-only
    parameters (e.g. abs()).

I can't find a similar Python 3 glossary, but see [2].  Whether we use:

  def __init__(self, message_url_template, **kwargs):
      …

or:

  def __init__(self, message_url_template='foobar', **kwargs):
      …

just controls whether the message_url_template has a default or not,
and not whether it can be set via positional or keyword arguments.

There is Python syntax for keyword-only arguments, and it would look
like [2]:

  def __init__(self, *, message_url_template, **kwargs):
      …

or:

  def __init__(self, *, message_url_template='foobar', **kwargs):
      …

in the former case, you'd have to call __init__ with a
message_url_template=… keyword argument or you'd get:

  TypeError: __init__() missing 1 required keyword-only argument: 'message_url_template'

With the latter case, calling __init__ without a message_url_template
argument would just get you the default value (‘foobar’ in these
examples).

Cheers,
Trevor

[1]: https://docs.python.org/2/glossary.html#term-parameter
[2]: https://docs.python.org/3/reference/compound_stmts.html#function-definitions

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