Added custom priorities and Cut() validatable to stop validation immediately.

[CptBinho]

Hi! Here's the summary of my fork, which I think could be useful for inclusion in schema.

Validatables can have custom priorities so that we can control exactly in what order keys are tested in a dictionary schema.

The Cut() class can be used to fail validation immediately when we don't want a certain key to be caught by a more general rule. An example is given in the docstring for the Cut() class, and a test case is included.

Also, all validatable classes now share a common base class (SchemaBase), and instead of checking for the presence of a "validate" attribute in an object (which may be present but not necessarily intended for validation with schema) we check if the object is an instance of SchemaBase. The user can still provide custom validatables by merely adding SchemaBase to their bases.

Anyway, the cut was the main reason why I did this fork, as it was useful to me in a CLI I was doing with the amazing docopt :)

Cheers,
Rui

[keleshev]

There's quite a lot of changes. I have not yet formed opinions on all of them, but one thing I'm not quite happy is the base class.

The problem with the base class is that it forces people to use multiple inheritance, in order to make their things "validatable":

class Player(db.Model, SchemaBase):

    def validate(self, data):
        ...

So this is one disadvantage of a base class, which is minor, but I don't see any advantages to it.

In other languages I would definitely make an interface Validatable or something like that, but not a base class. Since we are using Python I would rather rely on duck-typing. What do you think?

[CptBinho]

I understand what you're saying, and I agree with it, but the reason why I added the base class is because the "validate" attribute may be part of the object for some other reason that has nothing to do with schema, and so the library would probably give the user a strange error message. In the worst case, validation would succeed with data that should not be valid (?!? not sure about this).

I know that people frown upon multiple inheritance, but in this case it would be totally free in the sense that all you need to do is simply add SchemaBase (I too prefer the name Validatable) to your class' bases and magic would ensue.

Another thing I noticed now is that the 'priority' attribute that is in the base class would be lost as well. 'priority' is used by the priority() function, which allows us to specify custom priorities for validatables, but they have a fallback value of 4 in SchemaBase.

So with these arguments, I leave it up to you to decide which alternative is better, as I believe both have advantages and disadvantages.

[keleshev]

Yeah, I need to think about it.