Hacker News Reader

Comment by rhaps0dy | Hacker News Reader

I disagree. From the end section of "real-world usage examples", there is this library https://github.com/mrocklin/multipledispatch/ that implements "Decorators for registration. It's used in at least one (probably many more) serious open-source project that I know of: GPflow https://github.com/GPflow/GPflow/blob/master/gpflow/expectat... .

"Decorators for preconditions" look pretty neat as well. It's a little sad that the preconditions themselves have to be written inside strings though.

View on HN

"Decorators for preconditions" indeed looks neat, reminds me of Clojure which has both preconditions and postconditions built in.

> It's a little sad that the preconditions themselves have to be written inside strings though.

Isn’t it possible to pass a lambda instead?

Yes. Lambdas in Python are awkward, but you can definitely use either a lambda or a named function.

In fact, you could define a number of useful precondition functions in a single module and use them throughout a project. A couple of higher-order functions could make the post's examples safer and nicer-looking too. For example:

    @precondition(starts_with('The year is '))

where the starts_with precondition is:

    def starts_with(param):
        def test(s):
            return s.startswith(param)
        return test

> Lambdas in Python are awkward

Can you elaborate on that, please?

Considering the examples given by the author, I think they would be a better option.

Not the OP but I think he was referring to the restrictions python puts on lambda expressions:

- they can only contain a single expression

- they cannot contain statements

Python has lambdas that have multiple expressions and statements.

They're called functions.

Python functions aren't anonymous

For precondition checking, shouldn’t that suffice?

Depends on the precondition.

I've used that multipledispatch, along my own homegrown version, in production systems.

See my comment above. And yes, the preconditions could be written with a lambda, which would be much cleaner than using eval.

Reply on news.ycombinator.com