Constraint checking on dictionary-class converter

[phdum]

Hi,

I wanted to run an idea by you to see if it would be interesting thing to implement.

I remember that when you have a convertor from python to cpp (via CPP2PY_ARG_AS_DICT), it does type checking of the input to make sure everything is well defined.

Would it also be possible to specify additional constraints / bounds / traits during this conversion?

For example, it would be nice to specify something that a parameter (e.g. temperature) should be > 0. This is then checked on conversion and gives an error message if it fails.

Or would this cause other problems?

Thanks!

[Wentzell]

Why would you want to make this part of the conversion and not check these things in the actual function body?

[phdum]

I'm sure are several design options here (including ones I've not thought of). I view this as a pre-condition (expects) / post-condition (ensures) question:

The way the current interface (CPP2PY_ARG_AS_DICT) seems designed with the idea of always passing a single parameters to a single function. Here it is natural a pre-condition (expects) check on that input in the function. I guess this is the way you phrased the question above.

However we could also think of the conversion as a (complicated) constructor from of the c++ class inputting a python dict. Then it makes sense to think about checking validity as a post-condition on the ctor (ensures). But right now, I don't know if the user can specify such a post-condition.

[Personally, I like to separate conversion (ctor) and passing the data to a separate function. This is also why I suggest the keyword changes in #30. But this is not so important here.]

Pre-condition/post-condition is in a sense a matter of taste.

• The advantage of pre-conditions for a function, is that is where the input is used and the code logic is.
• The advantage of post-conditions in the ctor is that if there is a failure due to invalid input, it fails very quickly, without anything else being invoked. Especially, if you pass a dictionary of data to the constructor of another class, you want to do the checks before resource allocation. For example, since there is no languages support for contracts, I actually don't know any nice way to call an expects function before allocating (potentially large) resources.