Parsing deserialize validation errors

[niamhdougan]

I'd like to be able to give a better error message with code snippet for ValidationErrors.
Using the 'Class as union of subclasses' example:

from dataclasses import dataclass
from typing import Any, Optional, Union

from apischema import deserialize, deserializer, identity, serializer
from apischema.conversions import Conversion

class Base:
    _union: Any = None

    # You can use __init_subclass__ to register new subclass automatically
    def __init_subclass__(cls, **kwargs):
        super().__init_subclass__(**kwargs)
        # Deserializers stack directly as a Union
        deserializer(Conversion(identity, source=cls, target=Base))
        # Only Base serializer must be registered (and updated for each subclass) as
        # a Union, and not be inherited
        Base._union = cls if Base._union is None else Union[Base._union, cls]
        serializer(
            Conversion(identity, source=Base, target=Base._union, inherited=False)
        )

@dataclass
class Foo(Base):
    foo: Optional["Foo"]

assert deserialize(Base, {"foo": {"foo": {"foo": 0}}}) == Foo(Foo(Foo(0)))

In VSCode editor this gives a red squiggly line under the 0 in the assertion Foo(Foo(Foo(0))).
The ValidationError in the terminal is:

Traceback (most recent call last):
  File "/dls/science/users/sfx44126/devel/ibek/src/ibek/apischemaerrorexample.py", line 29, in <module>
    assert deserialize(Base, {"foo": {"foo": {"foo": 0}}}) == Foo(Foo(Foo(0)))
  File "/venv/lib/python3.10/site-packages/apischema/utils.py", line 400, in wrapper
    return wrapped(*args, **kwargs)
  File "/venv/lib/python3.10/site-packages/apischema/deserialization/__init__.py", line 891, in deserialize
    return deserialization_method(
  File "apischema/deserialization/methods.pyx", line 655, in apischema.deserialization.methods.ConversionMethod.deserialize
  File "apischema/deserialization/methods.pyx", line 656, in apischema.deserialization.methods.ConversionMethod.deserialize
  File "apischema/deserialization/methods.pyx", line 1176, in apischema.deserialization.methods.ConversionMethod_deserialize
  File "apischema/deserialization/methods.pyx", line 1305, in apischema.deserialization.methods.DeserializationMethod_deserialize
  File "apischema/deserialization/methods.pyx", line 1078, in apischema.deserialization.methods.ObjectMethod_deserialize
apischema.validation.errors.ValidationError: ValidationError: [{'loc': ['foo'], 'err': 'expected type null, found object'}, {'loc': ['foo', 'foo'], 'err': 'expected type null, found object'}, {'loc': ['foo', 'foo', 'foo'], 'err': 'expected type object, found integer'}, {'loc': ['foo', 'foo', 'foo'], 'err': 'expected type null, found integer'}]

Is there a way of extracting the culpable code snippet and validation error from this to give a clearer error message?