See responses inline On Aug 14, 2014 3:28 PM, "Dennis Brakhane" <brakhane@googlemail.com> wrote:
1. Annotations can be used to communicate additional restrictions on values that must be checked on run time
Let's assume a simple Web service that is called via HTTP to register a user, and the underlying framework decodes the request and finally calls a simple controller function, it could look like this
(Java code, @ signifies an annotation)
public Response register(@Range(18,100) int age, @ValidEmail String email) { ... }
The framework would check the range of the age parameter and the validity of the email and if there are validation errors, refusing the request with a suitable error message without ever calling our function.
This is exactly the kind of syntax I want to avoid. Python should not attempt to just blindly become like Java or any other language. Though Dennis was just illustrating his point (this was not a suggestion of an alternate syntax), I feel like Python is moving further and further towards code like this. Python programmers should not be forcing as much as possible into each line. Explicit is better than implicit.
Even if we assume that mypy's type system will be incredibly complex and allowing to specify all kinds of restrictions on a type, it won't help because those checks have to be done at run time, and are not optional.
This is a great point. Regardless of the nature of the annotation, we can't let this become a way out for lazy programmers. Having annotations is no excuse for poor error checking.
2. They can give meta information to a framework
An admittedly contrieved example, let's expand our register function:
public Response register( int age, @Inject @Scope("session") UserService userService, @Authenticated User user) ....
This is too much to put on one line in Python. We should be aiming to make code cleaner and promote proper error checking and documentation.
The flexibility annotations give in Java makes programming much less frustrating. It also took quite some time before annotations were widly used (they were only available starting in Java 5) and people started finding more uses for them. I think this will also be true for Python, it will take time before people find useful ways for them. Redefining them now to be "not much more" than static type information feels wrong to me.
I agree that annotations can be useful, but I don't think they should be used for type checking at this scale. Sunjay _______________________________________________
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