Expose bcf_get_value_count on the record object

[multimeric]

bcf_get_value_count seems to calculate how many values a given INFO/FMT field should have, on a given record. This is a useful feature, if you want to edit a VCF record but don't know how many values each field should have. Could this be added to the VariantRecord object, e.g. as VariantRecord#get_value_count(type: str, id: str)?

I'll probably try writing this myself at some point, but I wanted to keep track of this as an issue

[bioinformed]

Good idea! It will be a few weeks before I have time to work on this, but it will be extremely useful.

[multimeric]

Any chance of merging this?

[bioinformed]

Having to specify the type of attribute is too clunky. It would be better if we added:

rec.info.value_count(tag)
rec.samples[i].value_count(tag)

with some attention paid to encoding of variable length fields as None?

Specifically, I'd also expect value_count(FMT/GT) to always be None, rather than returning the current cardinality, attempting to match other samples within the same record or other records. All of those other use cases can be handled by testing len(gt).

Does this make sense?

[multimeric]

Totally agree with the interface of putting value_count inside the sample/info object. I forgot that they were actually C objects and not just dictionaries, and so can have methods.

I'm not sure I follow in terms of GT though. It's a field that always has a Number of R, effectively (although | or / separated, rather than comma separated). You can calculate the length of the field with len(gt), but I see no reason not to allow this function to cover all fields in a general way.