[I] Add explicit truthiness semantics for constant BooleanExpressions [iceberg-python]

["kevinjqliu (via GitHub)" <gi...@apache.org>]

kevinjqliu opened a new issue, #3543:
URL: https://github.com/apache/iceberg-python/issues/3543

   While this PR can use expression negation via `__invert__,` I think it may be worth supporting Python truthiness for the constant expressions in a follow-up:
   ```
   class AlwaysTrue:
       def __bool__(self) -> bool:
           return True
   
   class AlwaysFalse:
       def __bool__(self) -> bool:
           return False
   ```
   
   That would make patterns like this behave intuitively:
   ```
           if not self.visit_starts_with(term, literal):
               return AlwaysTrue()
   ```
   
   When adding this, we should also be careful that Python truthiness is for control flow, not expression construction: `not expr` returns a Python `bool`, while `~expr` returns a negated BooleanExpression. It would probably be safest for non-constant `BooleanExpression.__bool__` to raise, with only `AlwaysTrue` and `AlwaysFalse` overriding it. The follow-up should also audit optional-expression checks like `expr or AlwaysTrue()` and REST residual/filter handling so explicit `AlwaysFalse()` values are not treated as missing.
   
   _Originally posted by @kevinjqliu in https://github.com/apache/iceberg-python/issues/3503#issuecomment-4763176953_
               


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Re: [I] Add explicit truthiness semantics for constant BooleanExpressions [iceberg-python]

["rambleraptor (via GitHub)" <gi...@apache.org>]

rambleraptor commented on issue #3543:
URL: https://github.com/apache/iceberg-python/issues/3543#issuecomment-4763343643

   This scares me bit truthfully (pun not intended). 
   
   It seems like we're trying to get to a place where certain Boolean operators act on the object's presence, while others act on their name. 
   
   This only is really a problem for AlwaysFalse()


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