Question

Python supports chained comparisons: 1 < 2 < 3 translates to (1 < 2) and (2 < 3).

I am trying to make an SQL query using SQLAlchemy which looks like this:

results = session.query(Couple).filter(10 < Couple.NumOfResults < 20).all()

The results I got were not as expected. I’ve turned the engine’s echo=True keyword, and indeed – the generated SQL query only included one of the two comparisons.

I can’t find any documentation that explicitly says this is forbidden. I assumed that if this type of expression is supported in Python, it should be supported in SQLAlchemy as well.

Why doesn’t this work? I have one possible solution in mind (shared in answers), but will be glad to hear other opinions.

Answer 1

The reason is that Python actually evaluates something akin to this:

_tmp = Couple.NumOfResults
(10 < _tmp and _tmp < 20)

The and operator is unsupported in SQLAlchemy (one should use and_ instead). And thus – chained comparisons are not allowed in SQLAlchemy.

In the original example, one should write this code instead:

results = session.query(Couple).filter(and_(10 < Couple.NumOfResults, 
                                            Couple.NumOfResults < 20)).all()