Similar to the .case method on array expressions, create a case builder that accepts self-contained boolean expressions (as opposed to expressions which are to be equality-compared with a fixed value expression)
Use the .when method on the resulting object followed by .end to create a complete case.
Returns: case : CaseBuilder
- expr = (ibis.case()
- .when(cond1, result1) .when(cond2, result2).end())