pytx should set sane defaults for fields returned and use them by default about threatexchange

[wxsBSD]

As the person who ran into this and contacted @mgoffin about it, I'm in agreement. It was very counter-intuitive to see this behavior by default, and I think the suggested change is a good one.

To provide a bit of context around this, I was helping someone use pytx to search for all PE files, but it wasn't clear to me what the appropriate search term to use is. Do I use Malware.objects(sample_type='PE') or something else? To answer this question we searched using Malware.objects(md5='foo') (where foo was an MD5 of a PE file we knew was in TE). We expected to see the sample_type field populated with the appropriate value, but instead we got None. It wasn't until we specified fields=['sample_type'] in the call that we got the appropriate value returned to us.

As I said, this is a very counter-intuitive behavior and I would suggest changing it so _default_fields is some sane list of things that are returned for each object unless explicitly overridden by the caller.

[hammem]

This makes perfect sense to me. I often find myself doing things like fields=ThreatDescriptor._default_fields in my queries.

I'd say we make this change, as I don't think(?) this would cause fields that are currently returned by default to disappear on folks.