Error when serializing/deserializing object from Generic subclass

[NiklasRosenstein]

Expected behaviour: When (de)serializing a dataclass that subclasses from Generic[T] and it has a member that uses T, we should be able to handle the member annotated with T gracefully when a concrete type is given.

Actual behaviour: Databind does not understand the concrete type for a type variable and fails at trying to (de)serialize the field when a concrete type A is given but the type at that position is TypeHint(~T).

Steps to reproduce:

from dataclasses import dataclass
from typing import Generic, TypeVar
from databind.json import dump, load

T = TypeVar("T")

@dataclass
class Box(Generic[T]):
  value: T

print(dump(Box(42), Box[int]))         # no serializer for `TypeHint(~T)` ...
print(load({"value": 42}, Box[int]))   # no deserializer for `TypeHint(~T)` ...

Results in

databind.core.converter.NoMatchingConverter: no serializer for `TypeHint(~T)` and payload of type `int`

Trace:
    $: TypeHint(__main__.Box[int])
    .value: TypeHint(~T)

[rhaps0dy]

This bug seems fixed in the latest develop branch. Should we close?

[NiklasRosenstein]

I can't remember fixing it tbh., what Python version have you tested it on? Regardless, I would want to add a test case before closing this.

[rhaps0dy]

Python 3.10. Yeah, I agree there should be a test case.

Zooming out, have you considered using Hypothesis (https://hypothesis.readthedocs.io/en/latest/) to test python-databind? The usecase here is perfect: write a test-case-generator for types and instances, and then apply it to a round-trip test. Then apply more computation and find bugs.