BUG: composing multiple selections

[mattijn]

This PR has still some issues, but it adds some documentation + inline example to compose multiple selections as discussed in #1704.

I tried to emulate the Vega-Lite example from here, but preferred to hold the ctrl-key or shift-key to separately initiate the selections.

I don't know how to make the following Vega-Lite example with bitwise operators work in Altair:

"selection": {"not": {"and": ["alex", "morgan"]}}

I tried some (eg. ~alex & ~morgan and ~[alex & morgan]), but got none working.

[jakevdp]

I think ~(alex & morgan) should work (or equivalently, ~alex | ~morgan following De Morgan's law)

[jakevdp]

Actually, it looks like those compound operands are not fully implemented currently.

[jakevdp]

Relevant issue is #695

[mattijn]

I think I got the compound operators working:

class Foo(object):
    def __init__(self, name):
        self.name = name      
    
    def __invert__(self):
        return Foo({'not': self.name})
        
    def __repr__(self):
        return str(self.name)
    
    def __or__(self, other):
        return Foo({'or': [self.name, other]})
    
    def __and__(self, other):
        return Foo({'and': [self.name, other]})


x = Foo('s1')
y = Foo('s2')
z = Foo('s3')

x | ~y

{'or': ['s1', {'not': 's2'}]}

~x | y

{'or': [{'not': 's1'}, s2]}

~x | ~y

{'or': [{'not': 's1'}, {'not': 's2'}]}

~(x & y)

{'not': {'and': ['s1', s2]}}

~z | x & ~y

{'or': [{'not': 's3'}, {'and': ['s1', {'not': 's2'}]}]}

[jakevdp]

Yeah, that's how to implement them. The challenge is that the relevant classes are auto-generated, so we'll need to create wrappers in order to define the operators correctly. And there are a lot of relevant classes.

[mattijn]

I've changed the operators (__invert__, __and__, and __or__) to return instances of Selection. With this I'm able to chain the selections to invoke further operations on them.

I add some tests to verify, but (!) also had to change some tests to make all tests pass. Please have a look.