Using flexible attribute in path definition

[PierreRust]

This is a follow-up from a discussion on the mailing-list.

When using flexible attributes in path definition, missing fields are not expanded, meaning that they appear in the result as they were defined in the path. For example :

beet ls -f '$test_attr' album:Madame

Could return someting like :

flexattr_value
$test_attr
$test_attr
flexattr_value

The current behavior has the following advantages :

• It provides a way to differentiate between a missing and empty field.
• It makes debugging path templates easier : you can easily see if you misspelled a field rather than just having it disappear.
• Symmetry with Python's built-in templating.

One the other hand, this can be quite annoying as you generally do not want the $'field_name' string to appear in your path and missing fields are very common when using flexible attributes.

They seems to be several option to make this simpler :

• create a new construct for path definition %ifdef{field,true,false}
• output an empty string for the missing field, one can always use the already existing %if construct to specify a default value when needed.

[sampsyo]

I'd be interested to hear other users' input on this decision.

[axujen]

I've only started using beets recently but i think this is definetly something to be considered, right now im using the plugin mpdstats to fill $play_count attributes but at the moment its practically useless since running something like beet ls -f '$title - $play_count' would simply return $play_count for most titles, it would be much better if i could hide that or better yet turn it into a 0, either %ifdef or %if methods could get it done but i think the %if method would require the least amount of effort (im not sure on this im not familiar with the codebase)

[sampsyo]

The easiest route is probably %ifdef, since making non-existent field names expand to the empty string would lead to some usability problems that we'd need to cook up solutions for.

[GuilhermeHideki]

I did a "quick fix" with this:
TL;DR this won't work if your value starts with '$'

class IfDefPlugin(BeetsPlugin):
    def __init__(self):
        super(IfDefPlugin, self).__init__()

        self.template_funcs['ifdef'] = self.ifdef_template_func

    def ifdef_template_func(self, text):
        return 0 if not text or text.startswith('$') else 1

since returns 0 or 1, usage should be like %if{%ifdef{$series},$series/}$album, for example.
this would be better the attribute could return none instead of "${attribute}"

edit: remove redundant code

[sampsyo]

Cool! FWIW, text is None or not text is redundant; it's exactly the same as not text.

[awesomer]

FTR, I'm happy with the %ifdef solution, though I think I agree with sampsyo on IRC :

< sampsyo> I guess I'd advocate for something like %ifdef{field} instead of %ifdef{$field}
< sampsyo> Which would skip the default behavior of expanding $field to its value and directly check whether it's defined.

Edit : For those playing along at home, a way to do this with the inline plugin is
track_and_or_track_alt: u'%02i (%s)' % (track,track_alt) if 'track_alt' in locals() else u'%02i' % (track)