Wandering Thoughts archives


One reason why Python doesn't let you overload the boolean AND and OR operations

Recently I read Kurt Rose's DISappearing and (via Planet Python), where Kurt noted that Python doesn't have __...__ methods that let you override boolean and and or operations on your class objects. As it happens, there's a really good reason for this, which is that Python would require a new fundamental data type in order to make it really work.

Boolean and and or have the extremely valuable property of short-circuiting evaluation, where if you write, say, 'a() and b()' and a() evaluates to false, Python will not even call b(). Let's imagine a hypothetical world in which Python allows you to do this overriding and the boolean operators still preserve this short circuiting. As usual, if you write 'a and b', this will (at least some of the time) translate into a call to the override method on a, let's call it __band__, and the __band__ method will receive an additional argument that represents the right hand side:

class AClass:
  def __band__(self, right):

Now here is the big question: what's the type of right in this method?

In binary __and__, right is the value we get from evaluating the right hand side expression; if you write 'a & b()', this is roughly the same as a.__and__(b()). However this can't be the case for __band__, because that would mean no more short-circuiting; if a had a __band__ method, writing a and b() would call b() all of the time. To preserve short-circuiting, right has to be some type that represents the right hand side expression in an un-evaluated form.

However, Python has no such type today. Closures sort of come close, but they create additional effects and do things like appear in Python exception backtraces. This means that adding override methods for boolean operations would require either discarding short-circuiting (and making right be the evaluation result) or figuring out and introducing a new, relatively complex type in Python just to support this.

(Continuations are sort of what you'd need but I think they're not quite what you want, or at least you need a continuation that captures only the right side expression.)

The other problem of such a right type is that you'd want to be able to peer inside it relatively easily. After all, the entire purpose of implementing your own __band__ method is so that you can do something different from a plain boolean and when the right hand side is some special thing. If all you're going to do is:

def __band__(self, right):
  if not bool(self):
    return False
    return right.eval()

then there's not really any point in having a __band__ at all, especially given the general complexity involved in Python as a whole.

(This is of course not necessarily the only reason for Python to fence off boolean operations as things that you absolutely can't override. You can certainly argue that they should be inviolate and not subject to clever redefinitions simply as a matter of principle.)

python/WhyNoAndOverloading written at 00:33:01; Add Comment

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