✨ Add error messages for constraint failures

[chrisjsewell]

closes #1011

@danwos I felt the name help was not very descriptive of its purpose, and having the config key as error_message and the needs field constraints_error was more understandable. What do you think?

Also perhaps:

• Add documentation
• A try/catch for errors in the jinja templating
• Including the message in the sphinx warning

---

I'd note that, in principle, I feel it would be more "correct" to lay out the configuration like this:

needs_constraints = {
   "name": {
       "checks": {
            "0": "'critical' in tags",
		},
		"severity": "CRITICAL",
        "error_message": "need {{id}} does not fulfill CRITICAL constraint, because tags are {{tags}}",
   }
}

i.e. having the checks under a namespace, rather than having "special" keys

[danwos]

Thanks for the PR. Have only one suggesting regarding performance optimization.

[danwos]

Initiating a jinja template takes some time, can we do it only once and execute only the rendering per need?

[chrisjsewell]

Initiating a jinja template takes some time, can we do it only once and execute only the rendering per need?

Thanks @danwos, this pushed me to do something I was already planning to do in #1018: process all constraints in a single pass, rather than per document.

Do you know any reason why it was per document in the first place?
In fact, from the snapshots, this seems to fix some issues, where some constraints were not getting processed 😄

[danwos]

No, can't say (anymore) why it was implemented like this.
@mawieland As the initial implementer of this feature, do you remember or see any constraints?