Unions and recursive type hints

[python-desert]

There are at least two issues going on with our handling of recursive types.

One is handling of forward references in type hints. Python's representation inside the typing module seems to be evolving, and the tools for dealing with them (typing_inspect and pytypes) may be still catching up. I think that's what's catching us in #88.

There is also a general issue about the robustness of our union fields.

• Theoretically, a greedy recursive descent should serialize fine in simple cases like the one, but the implementation we're using seems to be having problems. There may be some fixes for marshmallow-union that could address this.
• There are also more complicated situations where greedy strategies will necessarily yield ambiguous parses. We could look into fancier parser generators that could detect and report ambiguities.
• A more explicit union representation would eliminate the issue of ambiguity by simply declaring what types are being represented.

[sveinse]

In the context of serialization, unions must carry additional information to be able to differentiate between the union variant being used, either implicitly via type Union[str, int], or explicitly using a selector field. For this https://github.com/adamboche/python-marshmallow-union or https://github.com/Bachmann1234/marshmallow-polyfield/ exists.

I think it is error prone to rely on implicit type checks, so I think the first design decision would be if desert should extend into explicit union selectors. The downside of adding union selector fields is that the serialized stream adds information; There will be more fields than what the data class or schema suggests.

[altendky]

I think that's #36? It kind of got put on hold while I did some CI stuff for desert and now I'm trying to catchup pyqt5-tools... I did get a wrapper approach implemented as ExplicitPolyField over in Bachmann1234/marshmallow-polyfield#34.

[python-desert]

Yeah let's call this a duplicate of #36.

[altendky]

And #88 for the recursion piece mentioned here, I think.