Passing expressions to functions?

Question

In SQLAlchemy, it appears I'm supposed to pass an expression to filter() in certain cases. When I try to implement something like this myself, I end up with:

>>> def someFunc(value): ...     print(value)  >>> someFunc(5 == 5) True 

How do I get the values passed to == from inside the function?

I'm trying to achieve something like this

 >>> def magic(left, op, right):  ...    print(left + " " + op + " " + right)   >>> magic(5 == 5)  5 == 5 

What about if one of the parameters was an object?

Answer 1

You can achieve your example if you make "op" a function:

>>> def magic(left, op, right): ...     return op(left, right) ... >>> magic(5, (lambda a, b: a == b), 5) True >>> magic(5, (lambda a, b: a == b), 4) False 

This is more Pythonic than passing a string. It's how functions like sort() work.

Those SQLAlchemy examples with filter() are puzzling. I don't know the internals about SQLAlchemy, but I'm guessing in an example like query.filter(User.name == 'ed') what's going on is that User.name is a SQLAlchemy-specific type, with an odd implementation of the __eq() function that generates SQL for the filter() function instead of doing a comparison. Ie: they've made special classes that let you type Python expressions that emit SQL code. It's an unusual technique, one I'd avoid unless building something that's bridging two languages like an ORM.