Optional default can’t be empty list

[klmr]

I‘m unsure whether this is a bug report or a feature request (i.e. whether the current behaviour is by design).

My desired behaviour is this: I would like to specify an optional map element that can be either a non-empty sequence or a non-empty comma-separated list. If the element isn’t specified, it should default to an empty list.

To clarify, here are valid examples:

a:
  - foo
b: bar

b: bar

And this is an invalid example:

a:
b: bar

Currently, the following works (from strictyaml import *):

schema = Map({Optional('a', None): Seq(Str()), 'b': Str()})
list(map(lambda t: load(t, schema), ['a:\n  - x\nb:\n', 'b:\n']))

However, when changing the default to [], it fails:

schema = Map({Optional('a', []): Seq(Str()), 'b': Str()})

InvalidOptionalDefault: Optional default for 'a' failed validation:
Expected a non-empty list, found an empty list.
Use EmptyList validator to serialize empty lists.

Likewise, the following fails, with a different error, at a different time:

schema = Map({Optional('a', []): CommaSeparated(Str()), 'b': Str()})
list(map(lambda t: load(t, schema), ['a: x\nb:\n', 'b:\n']))

IndexError: string index out of range (in strictyaml/utils.py L96)

As suggested in the first error message above the following works, but it describes a different schema (namely, it allows an existing, empty a)!

schema = Map({Optional('a', list()): EmptyList() | Seq(Str()), 'b': Str()})

[crdoconnor]

IndexError: string index out of range (in strictyaml/utils.py L96)

This exception should definitely never be happening, thanks for reporting it. I've got a fix ready to roll out in the next release. That one should work seamlessly now.

As suggested in the first error message above the following works, but it describes a different schema (namely, it allows an existing, empty a)!

Can you explain what the issue you have with this is? As far as I can see it just allows two different ways of representing the same valid data - one implicit (optional; no key) and one explicit (key, with blank string). This was by design: optional default is supposed to be a default - not a value that isn't representable if the key is present.

I use something like this (EmptyList() | Seq()) quite a lot, actually - and if, like me, you want "empty list" to be a valid value for a... allowing an empty "a" makes sense, no?

I'm open to the idea of disallowing the blank string representation, but I'd like to hear a coherent justification why - i.e. realistic examples/use cases.

I'm also looking at a related issue #62 - so any input on that would be welcome, too. Two people having a similar kind of issue makes me wonder if there's something I'm missing.

[klmr]

optional default is supposed to be a default - not a value that isn't representable if the key is present.

That makes sense. Since EmptyList() | … is what I’m currently using as a “workaround” I guess there’s no harm sticking with it. 👍

[klmr]

By the way, I did see #62 but that’s unrelated.