Better access to logical operand predicates

Vega-Lite has a number of logical operands; e.g.

• alt.LogicalAndPredicate
• alt.LogicalOrPredicate
• alt.LogicalNotPredicate

These come up, for example, in constructing filter transforms. It would be useful to make these available to Altair users via Python’s logical operators, &, |, and ~, respectively.

There are two challenges:

1. How to implement them effectively… maybe create some sort of PredicateMixin for these classes? If we were constructing the wrapper classes by hand, it would make sense for alt.Predicate to be the base class of each of these logical predicates… perhaps we should change the code generation framework so that this kind of class hierarchy is (optionally) inferred in generated classes? Sounds hard to do well in a general way, though.

2. The interaction with selection operands (e.g. alt.SelectionOr, alt.SelectionAnd, alt.SelectionNot, and alt.NamedSelection) makes this a bit tricky, because NamedSelection objects serve two purposes in Altair (they act both as a alt.SelectionDef and a alt.SelectionPredicate depending on the context).

[jakevdp]

Yes, you can do them in Altair. But no, you can’t have or and not as keys within the same object… I’m not quite sure how to interpret your intent here.

Is this what you were intending to do?

{'and': [
        {'or': [
            alt.FieldRangePredicate(field='year', range=[1850, 1880]), 
            alt.FieldRangePredicate(field='year', range=[1960, 2000])
        ]},
        {'not': alt.FieldRangePredicate(field='year', range=[1870, 1880])}
    ]
} 

[jakevdp]

@breadbaron fixed in #1075, which will make it into the 2.2 release 😄