Operator Precedence/Boolean Logic

Antoon Pardon antoon.pardon at rece.vub.ac.be
Thu Jun 23 09:24:43 EDT 2016

Op 23-06-16 om 14:37 schreef Steven D'Aprano:

> So I'm not really sure what you are trying to describe. I guess it might be
> something like this:
> def spam(alist):
>     if alist:
>         process(alist)
>     else:
>         print("empty list")
> If you pass 1 instead of an actual list, then you don't get an error until
> somewhere inside process(). Potentially far, far away. So you want to
> write:
> def spam(alist):
>     if len(alist):
>         process(alist)  # requires an actual list
>     else:
>         print("empty list")
> and now calling spam(1) will raise immediately. Great.
> But that's not really anything to do with *bools* specifically. If you call
> spam() with a dict, or a set, or a tuple, or a RedBlackTree, any other
> object with a length, you're no better off. If you absolutely need a list,
> and nothing else, then you have to type-check.

Yes it has to do with the booly nature of python objects. The fact that your
examples still allows for bugs, doesn't contradict it already cathes a lot
of bugs. So yes I'm better of even if I am not completly save. Encouraging
the user to write explicit test, will detect bugs sooner and will make
it more probable the code behaves as expected even when the tests are done
one an unexpected kind of object.

> In practice, I don't see how truthy/falsey objects lead to more or worse
> bugs than any other form of dynamic typing.

Sure, this from a language that states that explicit is better than implicit.
The truthy/falsy objects encourage people to be implicit and thus allowing
a lot of things to pass that were originally not though off.

> To me, your position is equivalent to saying that dynamic typing and
> duck-typing is really great, except for len(). len() should absolutely only
> work on lists, and nothing else, so any time you want to get the length of
> an object, you must work with real lists, not listy sequences:
> if len(list(mystring)) > 20: ...
> print(len(list(mydict.keys())), "items")
> etc. Substitute "bool" for "len" and you might understand how I feel about
> your position.

IMO, bool is like you would give any object a lengthy nature and so having any
object also behave like a tuple with the object as it's only item. Then bool and
len would be similar. 


More information about the Python-list mailing list